THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF BIOMEDICAL ENGINEERING STRUCTURAL FACTORS AFFECTING ANKLE STRENGTH HANNAH PUTNAM Spring 2015 A thesis submitted in partial fulfillment of the requirements for baccalaureate degrees in Biomedical and Mechanical Engineering with honors in Biomedical Engineering Reviewed and approved* by the following: Stephen Piazza Professor of Kinesiology Thesis Supervisor Peter Butler Professor of Biomedical Engineering Honors Adviser * Signatures are on file in the Schreyer Honors College.
i ABSTRACT Much previous research has been dedicated to analyzing the musculoskeletal structural variables that determine muscle strength. Most of this work has identified muscle volume as the main determinant, with less attention given to joint structure and other muscle architecture values. For this thesis, my primary goal was to identify individuals whose maximal ankle muscle strength could not be explained by the size of their muscles. By specifically seeking farmer strong subjects with smaller plantarflexor muscle volumes and comparing their ankle strength to that of average sized subjects, we attempted to show that those who display superior strength with smaller plantarflexor muscles are able to do so because they have longer Achilles tendon moment arms. Based on previous work in our laboratory, we anticipated that a third of the small-calf group would display strength superior to the mean of the control group. After comparing the results from six small calf subjects and 12 controls, one subject from the small calf group showed higher isometric torque values than the control average. This subject also had a longer moment arm than the mean of the control group. There was also a strong correlation in the small calf group between moment arm and isometric torque (R 2 = 0.674, p <0.001) showing ankle moment arm has a significant effect on torque production among these subjects. As expected, there were strong correlations between muscle size, as measured by PCSA (R 2 = 0.336, p = 0.005) and calf-circumference (R 2 = 0.692, p < 0.001), and maximal ankle torque. We did not, however, find significant correlations between size and strength for the control group, possibly because certain artifacts affected dynamometer measurements of strength. We are currently investigating this possibility and making plans to address this limitation with additional testing.
ii TABLE OF CONTENTS List of Figures... iii List of Tables... iv Acknowledgements... v Chapter 1 Introduction... 1 Chapter 2 Methods... 7 Subjects... 7 Measurement of Moment Arm Length... 9 Ultrasound Image Analysis of Muscle Volume... 10 Approximate Physiological Cross-Sectional Area... 13 Maximum Torque Testing... 14 Statistical Analysis... 17 Chapter 3 Results... 19 Chapter 4 Discussion... 28 Key Results... 28 Comparison to Previous Results... 29 Implications... 31 Limitations... 31 Chapter 5 Conclusion... 33 Appendix A Results... 34 Appendix B MATLAB Code... 38 BIBLIOGRAPHY... 40 ACADEMIC VITA... 42
iii LIST OF FIGURES Figure 1: Magnetic resonance image of human ankle with reference to ankle joint center and length of moment arm....2 Figure 2: Example image of subject's foot. Points 1-5 represent the points used to measure moment arm and heel length.......10 Figure 3: Skeletal structure of lower leg. Limb length was measured as the distance between the lateral condyle of the tibia and the most prominent point on the lateral malleolus...11 Figure 4: Ultrasound image of lateral gastrocnemius, soleus, and tibialis posterior. Total plantarflexor muscle thickness measurement was taken from the muscle-adipose tissue border to the muscle-bone border (red line with double arrows)...12 Figure 5:Thickness (t) and pennation angle ( ) of the gastrocnemius lateralis. Values used to calculated fascicle length and subsequently approximate PCSA for the GL of all subjects.13 Figure 6: Biodex set-up for isometric and isokinetic testing. Foot plate is attached an in place with the subject fully strapped into place and ready to begin testing...14 Figure 7: Laser alignment of Biodex axis of rotation and lateral/medial malleolus (rotational axis of ankle joint) of subject...15 Figure 8: Maximum isometric (left) ankle torque and maximum isokinetic (right) ankle torque plotted versus moment arm for control subjects (filled circles) and small calf subjects (open circles). A significant correlation was found between both isometric and isokinetic torque and moment arm for small calf subjects but no significant correlation was observed for controls...20 Figure 9: Maximum isometric (left) ankle torque and isokinetic (right) ankle torque plotted versus calf circumference for control subjects (filled circles) and small calf subjects (open circles). A moderate correlation was found between torque and circumference for the small calf group but the correlation between the control group and calf circumference was not significant...21 Figure 10: Maximum isometric torque (left) and maximum isokinetic torque (right) versus muscle volume. No significant correlation can be found in either the small calf (open circles) or control (filled circles) can be found...21 Figure 11: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus GL muscle thickness for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found for either group...22 Figure 12: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus GL pennation angle for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found for either group...23
iv Figure 13: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus approximate GL PCSA for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found for either group...24 Figure 14: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus fascicle length of GL for control subjects (filled circles) and small calf subjects (open circles). Negative correlations were found for the small calf group. No correlation was found for the control group...24 Figure 15:Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus height for control subjects (filled circles) and small calf subjects (open circles). Negative correlations were found for the small calf group. Significant positive correlations were found for the small calf group. No significant correlation was found for the controls.26 Figure 16: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus weight for control subjects (filled circles) and small calf subjects (open circles). Negative correlations were found for the small calf group. Significant positive correlation was found for the control group with isokinetic torque...26 Figure 17: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus body mass index for control subjects (filled circles) and small calf subjects (open circles). Strong positive correlations were found for the small calf group. No correlation was found for the control group...27 Figure 18: Maximum isometric torque versus heel length for control subjects (filled circles) and small calf subjects (open circles). Positive correlation was found between the small calf heel length and isometric torque...34 Figure 19: Height versus moment arm length for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found between either group...35 Figure 20: Height versus muscle volume length for control subjects (filled circles) and small calf subjects (open circles). A small, positive but insignificant correlation was found for the small calf subjects. No significant correlation was found for the control group...35 Figure 21: Weight versus moment arm for control subjects (filled circles) and small calf subjects (open circles). Moderate correlation was found for the small calf group. No correlation was found for the control subjects...36 Figure 22: Weight versus muscle volume for control subjects (filled circles) and small calf subjects (open circles). Moderate correlation was found for both the control and small calf group. The significance is also very close between the two subject categories...36 Figure 23: Pennation angle versus PCSA for control group subjects (filled circles) and small calf subjects (open circles). Strong correlation was found between pennation angle and PCSA for small calf group. No significant correlation was found for the control group...37
Figure 24: Calf circumference versus PCSA for control group subjects (filled circles) and small calf subjects (open circles). A very strong correlation was found for the small calf group and a moderate correlation was found for the control group...37 v
vi LIST OF TABLES Table 1: Subject characteristics for all subjects and for the small and control groups. Mean values with standard deviations in parentheses.... 8
vii ACKNOWLEDGEMENTS First, I would like to thank Dr. Piazza for giving me the opportunity to pursue research in the Functional Biomechanics Laboratory and work under him this year. I would also like to thank Becky Rogers for showing me how to use both the ultrasound machine and the dynamometer. Also, Herman Van Werkhoven for graciously lending his assistance with the computer measurements of moment arm. Another thank you to Courtney Rockwell, Kevin Roberts, Caroline Vink, and Josh Swenson for being my assistants throughout this process and always being excited to help. Lastly, I would like to thank my parents for always supporting me and pushing me to do my best in all aspects of life.
1 Chapter 1 Introduction Bodybuilding competitions across the globe suggest that you must have big muscles to be strong. But is this truly the case? Are there people who are just as strong as those with large muscles but do not outwardly display such strength? If those people do not look strong, what is it that makes them strong? It is possible that they have a hidden physiological advantage that allows them to produce the same amount of force as their more buff compatriots without the requirement of large muscle size? It is these unique subjects that we seek, those sometimes identified as farmer strong a phrase that describes someone who doesn t look strong, but is extremely strong (Farmer). While many studies have shown that muscle size to be a determinant of human strength, not many have examined the contribution of muscle leverage, or muscle moment arm, to strength. More attention may have been given to muscle size because it is more obviously apparent, or perhaps because size is more easily modified through resistance training or disuse. In order to proceed with this research, we must first examine the basic research that has already been performed in this area. With this study we seek to build on this previous work and expand its scope by targeting a specific sample of subjects who are strong despite having small muscles.
The ankle joint permits movement in a variety of directions but the greatest ankle joint 2 moments are produced during plantar flexion because the plantarflexor muscles (calf muscles) are the most massive and the Achilles tendon those muscles insert upon has a sizeable plantarflexion lever arm about the ankle joint center. The primary plantarflexor muscles are the two heads of gastrocnemius (lateral and medial), the soleus, and the tibialis posterior. If we think of the foot as a rigid lever with the fulcrum Figure 1: Magnetic resonance image of human ankle with reference to ankle joint center and length of moment arm. of the lever being the center of the ankle joint, the lever arm of the plantarflexor muscle force becomes approximately equal to the length of the heel (Figure 1). When this lever arm is longer, less force is required to generate the same muscular ankle joint moment. This lever arrangement implies that muscle moment arm about the ankle joint as well as plantarflexor force affects the ankle joint moment that the plantarflexors can produce. Many previous investigators have examined muscle size as a determinant of strength. Correlation between muscle size or volume has been demonstrated when size has been assessed by various means including magnetic resonance imaging on the elbow flexors and plantarflexors, (Akagi et al.., 2009; Bamman, Newcomer, Larson-Meyer, Weinsier, & Hunter, 2000; Fukunaga et al.., 2001; Jeng et al.., 2012), ultrasound (Fukunaga et al.., 2001), and computer modeling (Nagano, Yoshioka, Komura, & Fukashiro, 2007). In most studies using MRI, the muscle
3 volume was calculated using the cross-sectional area of the muscle taken from the image slices (Akagi et al.., 2009; Bamman et al.., 2000; Fukunaga et al.., 2001). Miyatani et al. examined both MRI measurements of muscle volume as well as ultrasound measurements and found ultrasound measurements of plantarflexor muscle thickness combined with limb length to be a good predictor of plantarflexor muscle volume (Miyatani, Kanehisa, Ito, Kawakami, & Fukunaga, 2004). When ultrasound imaging was used to measure the thickness of the middle trapezius muscle, Bentman et al. found that the reliability of the test intra-rater between-days and inter-rater were both within the 95% confidence interval (Bentman, O Sullivan, & Stokes, 2010). This shows that not only is ultrasound testing reliable with the same rater, but it can also be reliable if there are multiple experimenters taking measurements. Ultrasound has been found to be reliable and accurate when measuring muscle thickness, and these values can be used to estimate total muscle volume without the use of MRI. Fortunately, ultrasound measurement of muscle thickness can be converted using an equation to find muscle volume. The values from the equations of Miyatani s study determined that muscle thickness from an ultrasound probe was a good predictor of total muscle volume when compared to measurements of volume taken from MRI (Miyatani et al.., 2004). While each of these investigations used a variety of means to measure the torque or force production of the desired joint and all found a correlation between muscle volume and maximal effort, no other factors potentially affecting strength were considered. It has been shown many times that large muscles are associated with force and torque production, but few studies have investigated factors beyond muscle size to examine other reasons for superior strength. Blazevich et al. published one of the first studies of the influence of lever arm on strength (Blazevich, Coleman, Horne, & Cannavan, 2009a). The authors examined the effects of knee
4 extensor moment arm on knee joint torque production, finding a weak correlation between knee extensor moment arm and quadriceps muscle moment. This analysis covered a variety of factors that could affect muscle strength including: knee extensor moment and quadriceps muscle volume, anatomical and physiological cross-sectional area, muscle architecture, and moment arm (Blazevich, Coleman, Horne, & Cannavan, 2009b). Blazevich et al. found that for the knee, the patellar tendon moment arm was not an important predictor of knee extensor moment. This was due to a moderate correlation of moment at a speed of 30 degrees per second but no correlation at 300 degrees per second. Because the final results of this study showed a much stronger correlation between quadriceps muscle volume and isometric moment as compared to moment arm and isometric moment, the linear regression stage of analysis did not include moment arm (Blazevich et al.., 2009a). Baxter and Piazza reported a correlation between maximal plantarflexor torque and plantarflexor moment arm(baxter & Piazza, 2014). Utilizing MRI to calculate muscle volume and dynamometer measures of ankle torque, Baxter and Piazza were able to find a strong correlation between muscle volume and maximal plantar flexor torque, as others had, but also found a strong correlation between moment arm length and torque production. Another interesting discovery by Baxter and Piazza was that, while muscle volume correlated with body mass and stature, moment arm did not (Baxter & Piazza, 2014). These examinations of moment arm have not been performed on joints other than the ankle and the knee. Based on the difference in significance between Blazevich et al. and Baxter and Piazza, it is likely the effect of moment arm would depend on the joint. One subject from the study of Baxter and Piazza was found to have an intriguing combination of characteristics. This subject s strength was one standard deviation above average,
5 but his calf circumference and muscle volume were about one standard deviation below average, it may have been the case that this subject s strength was attributable to leverage, because his Achilles tendon moment arm was one standard deviation above average. The measurements made for this single subject provide a basis for one of our hypotheses, that some individuals will exhibit abnormally high strength that is attributable to muscle because of his moment arm despite small muscle size. Had Baxter and Piazza found more subjects with these characteristics among the 20 they tested, they may have been able to show that those with higher strength and small muscles will have a longer muscle moment arm. In contrast with previous studies that recruited subjects without selecting muscle size, we will attempt to identify farmer strong individuals with high strength and small muscles. This novel approach gives us a better chance to examine the special characteristics of individuals who are strong despite smaller muscles. We expect that the dominant factor explaining their greater strength will be a longer muscle moment arm. Leg dominance is thought to be important in tests of strength, and most previous investigators have tested the dominant leg in their studies. While some studies have considered either the right or left leg, the dominant leg (be it right or left) has been shown to display higher strength as compared to the non-dominant leg (Sadeghi, Allard, & Duhaime, 1997). Obtaining the highest strength measurement is important when the determinants of maximum joint moment, or strength, are the focus of the investigation. It would be against the purpose of the study to force a subject to use their non-dominant leg in a strength test and could confound the results. Sadeghi et al. was not focused on strength and the difference was found as auxiliary information in a study surrounding functional gait asymmetry.
6 The purpose of this thesis is to test a small sample of control subjects as well as subjects in a small-calf group to find small-calf subjects who have higher strength than the mean of the control group. After identifying these individuals, we will examine the moment arms of the seemingly farmer strong subjects to confirm whether or not their superior strength stems from a moment arm advantage. We make two hypotheses: first, that among those with small muscles, there will be some subjects in the small-calf group with strength greater than the mean of the average group. The hypothesis is based on the existence of the subject tested by Baxter and Piazza (described above) who had superior strength but small calf circumference and small muscle volume. Second, we hypothesize that these subjects with small calves and superior strength will also have moment arms larger than the average of the control group.
7 Chapter 2 Methods Subjects A total of 18 male subjects were recruited between the ages of 18 and 30. The subjects were divided into two groups; the first was labeled as the average group. Subjects were recruited by word of mouth and posted flyers approved for IRB STUDY00001050. This group consisted of 12 of the 18 subjects who would be used as the control population for the data analysis. These subjects were between 170 cm and 185 cm in height, recreationally active, with a body mass index (BMI) of less than 30. This BMI restriction was chosen to account for those subjects with larger muscle mass whose BMI would translate to slightly overweight. The average group also had a calf circumference greater than 35.6 cm. This limit for small calves was taken from the previous data collected by Baxter. The cutoff is one standard deviation lower than the mean calf circumference(baxter & Piazza, 2014). This data was chosen over that of the National Health Statistics report due to it s the similar subject pool as compared to the general population (McDowell, Fryar, Ogden, & Flegal, 2008). While the report accounts for all American males between the ages of 20 and 29, the pool of subjects for this study and in Baxter s is more specific than the wide range of subjects taken from the American population. The second group is also within the same height range but this group is identified as the small calf group. The small calf group, while subject to the same activity and BMI restrictions, also has a calf circumference of less than one standard deviation from the mean and therefore below or equal to
35.6 cm. The small calf group was analyzed to see which subjects were strong compared to the 8 average group despite their small calf size. Calf circumference is correlated with muscle size, so this can be used as a filter for our subject pool. By pre-screening subjects base on calf circumference, we can control the muscle size of our research population and filter them into the small and average groups after testing is complete. By controlling the height and weight of the subjects, we can assure that body mass and stature will not affect the measurements. It is certainly possible that a 5 5 extremely muscular man will have the same muscle volume as a 6 5 skinnier man because the fascicle length and muscle itself of the 6 5 man will be longer and will therefore appear less large. For this reason, all subjects will have the same height range. The body mass index limit assures that the subjects are all relatively lean. Because calf circumference is an inclusion criterion, BMI restriction helps in avoiding obtaining subjects with larger calf circumference but less muscle mass due to excess fat. By setting these restrictions, we are able to collect an average control group that we suspect will have very similar strengths with low standard deviation and a group of small-calf subjects of whom we suspect about one third will have the desired unique physiological characteristics. Table 1: Subject characteristics for all subjects and for the small and control groups. Mean values with standard deviations in parentheses. Units Subjects All (n = 18) Small (n = 6) Control (n = 12) Age yr 20.8(1.47) 20.5(0.55) 21(1.76) Height cm 177.3(4.32) 178.5(4.32) 176.7(4.38) Weight kg 78.8(8.04) 74.1(4.62) 81.2(8.46) BMI 25.1(2.60) 23.3(1.54) 26(2.58) Calf Circumference cm 37.2(2.17) 34.8(0.80) 38.4(1.53)
9 After expressing interest in the study, subjects were asked to submit their weight and calf circumference. Once selected, subjects were asked to report to a one hour testing session wearing pants that could be rolled up past the knee. This was necessary for testing because the lateral malleolus and the lateral condyle of the tibia needed to be visible during testing. The subject was asked to properly hydrate before testing to assure the muscle is acting at peak performance and provide another level of control for the ultrasound images. All subjects gave written informed consent prior to any testing, which was approved by the Institutional Review Board (STUDY00001050) at The Pennsylvania State University, and was conducted in accordance with the Declaration of Helsinki. Measurement of Moment Arm Length Each subject s dominant leg was tested. This leg was established by asking the subject to kick a ball placed in front of them (Holden, Boreham, Doherty, Wang, & Delahunt, 2014). The ball was carefully placed at a point equidistant from each foot so the subject did not feel an inclination towards one foot or the other due to convenience. The dominant leg was chosen for this study because it has been shown that maximum power comes from the dominant leg (Sadeghi et al.., 1997). After the dominant leg is identified, a photograph was taken of the subject s foot to measure the length of the heel, which indicates the lever arm (or moment arm) of the Achilles tendon about the ankle. The participant stood on a wooden block with a millimeter-scale tape measure attached to provide a length reference (Figure 2). Lateral-view photographs were taken of the subject s ankle and foot using a digital camera. After each test
session, the resulting image files were transferred to a secure computer. A custom-written 10 computer program was used to analyze the image by scaling the image pixels to centimeters and correcting for distortions due to camera angle as well as distance adjustments (Appendix B). Without use of this program, it is likely the moment arm measurements would be affected by a large margin of error due to natural human variation in how the image was captured. Figure 2: Example image of subject's foot. Points 1-5 represent the points used to measure moment arm and heel length. Ultrasound Image Analysis of Muscle Volume A method published by Miyatani et al. (2003) was used to calculate muscle volume from muscle thickness (measured from an ultrasound image) and limb length. Equation 1 used for calculations can be seen below where Y is muscle volume, X1 is muscle thickness and X2 is limb length: Equation 1: Y = 218.1X 1 + 30.7X 2 1730.4
The ultrasound image is obtained by a B-mode ultrasound machine (Aloka 1100; 11 transducer: SSD-625, 7.5 MHz; Wallingford, CT). The subject was first asked to stand with arms and legs relaxed. The limb length was determined as the length of the lower leg (Figure 3). The length of the lower leg was measured with a tape measure as the distance between the lateral malleolus of the fibula and the lateral condyle of the tibia while the subject stood in a relaxed position. From this length, a point 30% of the total lower leg length was marked downwards from the lateral condyle of the tibia on the leg for approximate location of greatest muscle thickness of the plantar flexor muscles (Abe, Kondo, Kawakami, & Fukunaga, 1994). It is at this location that the subject s calf circumference was measured. Figure 3: Skeletal structure of lower leg. Limb length was measured as the distance between the lateral condyle of the tibia and the most prominent point on the lateral malleolus. A line was also drawn along the longitudinal axis of the lateral gastrocnemius. These two marks provided a crosshair to guide application of the ultrasound probe. The subject was asked to continue to stand relaxed in order to ensure the muscle is active yet relaxed while testing proceeded. After the ultrasound gel was applied and the probe was pressed to the skin with the probe centered on the crosshair mark and aligned with the longitudinal axis. Care was taken to avoid applying too much pressure to the probe. If too much pressure is applied, the muscle will be compressed which would affect the final measurement of the muscle thickness post-testing. When the image of the proper region was displayed on the ultrasound screen, depth and brightness were adjusted to obtain the clearest image of the calf muscles. The resulting image
12 was captured and analyzed for overall muscle thickness, gastrocnemius lateralis thickness, and pennation angle. The image of the calf is captured on the ultrasound and simultaneously projected on the ScionImage software of the computer Muscle Thickness monitor. The image was saved as Subject ID_MDDYY_Leg, e.g. if the test was performed with Figure 4: Ultrasound image of lateral gastrocnemius, soleus, and tibialis posterior. Total plantarflexor muscle thickness measurement was taken from the muscle-adipose tissue border to the muscle-bone border (red line with double arrows). Subject 1 on October 16 on the left leg for the first time the image would be titled 1_101614_L. The thickness of the muscle is measured as the distance between the adipose tissue-muscle interface to the muscle-bone interface and was measured in centimeters on the ultrasound display (Miyatani et al.., 2004). This distance can be seen in Figure 4. Following completion of testing, this image was uploaded to a secure computer and the muscle thickness value was confirmed using ImageJ software.
Approximate Physiological Cross-Sectional Area 13 Following the total plantar flexor muscle thickness measurement and subsequent volume calculation, the gastrocnemius lateralis (GL) was analyzed to approximate the physiological cross-sectional area (PCSA) of the GL. This analysis used Equations 2 and 3 below where lf is GL fascicle length, t is GL thickness, V is the volume of GL, and is pennation angle of GL: Equation 2: Equation 3: l f = t sin θ PCSA = V l f The thickness of the GL was measured as the distance from the superficial to the deep aponeuroses. The pennation angle was taken as the angle between the deep aponeurosis and the muscle fascicles on the ultrasound image using ImageJ software (Lee & Piazza, 2012). After fascicle length was calculated, an approximate PCSA of the GL was calculated using the estimated volume of GL (assumed to be Figure 5:Thickness (t) and pennation angle ( ) of the gastrocnemius lateralis. Values used to calculated fascicle length and subsequently approximate PCSA for the GL of all subjects. about 16% of the total volume of the plantarflexors as measure above) and the fascicle length of the GL.
14 Maximum Torque Testing Participants were positioned in the seat of an isokinetic dynamometer (Biodex System 3, Biodex Medical Systems, Shirley, NY) with the dominant leg in full knee extension, the axis of the motor spindle aligned with rotational axis of the lateral malleolus, and foot strapped to the foot platform. A flexible rubber heel support holds the heel against the foot plate. Additionally a strap goes across the top of the ankle and a padded foot strap is secured across the top of the foot and toes. An upper thigh extension was attached to the seat Figure 6: Biodex set-up for isometric and isokinetic testing. Foot plate is attached an in place with the subject fully strapped into place and ready to begin testing. of the dynamometer to ensure the knee does not hyper extend and the upper thigh is supported during testing. The seat and foot plate were then adjusted to align the center axis of the ankle with the axis of the dynamometer. This is performed by using a laser pointer mounted on a flexible arm to project a straight line at the marked center rotational axis of the dynamometer (see Figure 7). The foot is then placed in the foot plate and the positioning is confirmed by aligning the lateral malleolus (for left leg) or corresponding point on the medial malleolus (for right leg) with the laser pointer. Adjustments are made by moving the footplate along the arm of the dynamometer until the two are perfectly aligned and testing proceeds.
15 The alignment of the ankle and dynamometer axis is an important step in the measurement of the subject s maximum torque. If the lateral malleolus of the ankle lies below the axis of the dynamometer, the subject will have less leverage and in essence a shorter moment arm. This produces maximal torque values that are smaller than what the subject is actually generating with the ankle muscles. In contrast, if the lateral malleolus rests above the axis, the subject will appear to have higher torque values. Figure 7: Laser alignment of Biodex axis of rotation and lateral/medial malleolus (rotational axis of ankle joint) of subject. After alignment, all distances and locations were recorded to reduce set-up time in the event the subject was tested a second time. The subject was then strapped into the seat using a waist strap, a cross-body strap, and a strap over the thigh to hold all parts of the body in place. There is also a strap across the ankle and a padded restraint placed over the toes of the subject. The cross-body strap prevents the subject from leaning forward and giving them an unfair torque advantage. The purpose of the strap stretching across the foot and ankle was to hold the foot in the place, preventing the heel from lifting while the subject is applying torque. Once again, if the heel was allowed to rise, the subject would be able to produce more torque and create inaccurate readings on the dynamometer. As a warm-up, subjects performed several plantar flexion contractions while seated in the testing machine (a similar warm-up protocol has been used by Baxter and Piazza). Ankle dorsiflexion range of motion was tested by manually rotating the foot into dorsiflexion until participants feel slight discomfort. This angle was used during isokinetic testing to assure the footplate did not push the subject past the point of discomfort.
16 The subjects were tested using two protocols. The first protocol was for isometric (static) torque measurement and the second was for isokinetic (controlled ankle angular velocity) toque measurement. The isometric test was used to find the stationary maximal torque of the subject. The dynamometer was set to five degrees dorsiflexion so the ankle would be in a neutral position as assessed by the goniometer. Prior to testing, the foot plate was set to an anatomical angle of 90 degrees (confirmed using a level) from vertical and the limb weight was measured. During testing, the ankle torque due to weight of the limb is subtracted from the total torque to provide an accurate reading of muscular ankle torque alone. To test the strength and power of the plantar flexors, each subject was asked to point your toes away from you as hard as you possibly can while the footplate either remains stationary or rotates, moving the foot into plantar flexion. During these tests, the motor never rotated the ankle past the manually determined angle of discomfort. The joint rotation speed of 120 degrees per second used in isokinetic tests has been previously employed by other investigators who tested subjects of similar age and functional capacity (Baxter & Piazza, 2014). Between each contraction, subjects were provided with at least one minute to rest. Each subject was given a handheld button that allows him to start and begin the test when he was ready. After the command was given and the subject was prompted by the dynamometer display to apply pressure, the subject pressed against the footplate for two seconds. The subject was then given five seconds of rest before performing the test again. This test was repeated three times and the highest value from the three trials was selected as the subjects maximum torque production. The time course of the torque production measured by the dynamometer were saved to disk for future reference.
17 The isokinetic measurements were taken following the isometric measurements. For the isokinetic test, range of motion was from maximum dorsiflexion to 30 degrees plantar flexion. The previously determined angle of maximum dorsiflexion was used as the maximum angle toward the subject with a total range of motion of 40 degrees; this provided a plantar flexion angle of around 30 degrees. This test was set to run three times for a total of five seconds with a rest period of five seconds. The dynamometer was set to a speed of 120 degrees per second. The starting angle was at maximum dorsiflexion. The subject was told to begin pushing prior to releasing the footplate with the provided button and continue to push with maximal effort throughout the test. The torque value recorded was the value as the foot plate crossed through zero degrees plantarflexion/dorsiflexion. If the dynamometer did not record a value at zero degrees, the average was taken between the values recorded at -1 and 1 degree. Statistical Analysis After all testing was completed for each group and all the data were compiled in an MS Excel spreadsheet, statistical testing began. The control group was separated from the smallcalf group and maximum torque was plotted against a variety of data including: muscle volume, moment arm, calf circumference, and muscle thickness. These scatter plots were used to visually represent similar trends in data for each group as well as any positive or negative correlations. Simple linear regressions for each group were run for statistical testing of the significance of various data in connection with strength. To test for the existence of farmer strong subjects, members of the small calf group with greater than average moment arm lengths were compared to the mean moment arm
18 length of the average group. The strengths of each category were compared to each other to attempt to test the hypothesis that those with small muscle size but large moment arm will be just as strong if not stronger than their larger muscled counterparts.
19 Chapter 3 Results There is a strong correlation between isometric torque values and Achilles tendon moment arm in the small calf group (R 2 = 0.674, p<0.001) (Figure 8). There was also a strong correlation between isokinetic torque and Achilles tendon moment arm in the small calf group (R 2 = 0.846, p< 0.001) (Figure 8). The results also showed a significant correlation between heel length versus isometric toque for the small calf group (R 2 = 0.747, p < 0.001) (Figure 18 in Appendix A). There was no correlation between isometric torque and Achilles tendon moment arm for the control group. Among those in the small-calf group, only one subject tested thus far was found to be stronger than the mean of the controls (indicated by red circle on Figure 8). The Achilles tendon moment arm of
20 this subject was substantially longer than that of any of the other subjects in the small-calf group, and also was longer than the mean moment arm of the control group. - Figure 8: Maximum isometric (left) ankle torque and maximum isokinetic (right) ankle torque plotted versus moment arm for control subjects (filled circles) and small calf subjects (open circles). A significant correlation was found between both isometric and isokinetic torque and moment arm for small calf subjects but no significant correlation was observed for controls. Strong correlations were also found between the calf circumference and isometric torque for the small calf subjects (R 2 = 0.692, p < 0.001) (Figure 9). A less strong but still significant correlation was found between small-calf group circumference and isokinetic torque (R 2 = 0.403, p = 0.001) (Figure 9).
21 There was no significant correlation found between isometric (R 2 = 0.104, p = 0.144) or isokinetic ankle moment torque and muscle volume in the control group (R 2 = 0.011, p = 0.647) (Figure 10). Figure 9: Maximum isometric (left) ankle torque and isokinetic (right) ankle torque plotted versus calf circumference for control subjects (filled circles) and small calf subjects (open circles). A moderate correlation was found between torque and circumference for the small calf group but the correlation between the control group and calf circumference was not significant. Figure 10: Maximum isometric torque (left) and maximum isokinetic torque (right) versus muscle volume. No significant correlation can be found in either the small calf (open circles) or control (filled circles) can be found.
22 The data showed no significant correlation for either group for isometric or isokinetic torque versus GL muscle thickness (Figure 11). There was also no significant correlation between pennation angle and isometric and isokinetic torque (Figure 12). No significant correlation was found for both torques and approximate PCSA (Figure 13). The small calf group also displayed a negative correlation between Figure 11: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus GL muscle thickness for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found for either group.
23 fascicle length and both isometric (R 2 = 0.268, p < 0.001) and isokinetic (R 2 = 0.535, p = 0.013) (Figure 14). Figure 12: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus GL pennation angle for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found for either group.
24 Figure 13: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus approximate GL PCSA for control subjects (filled circles) and small calf subjects (open circles). No significant correlation was found for either group. Figure 14: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus fascicle length of GL for control subjects (filled circles) and small calf subjects (open circles). Negative correlations were found for the small calf group. No correlation was found for the control group.
25 Strong correlations were also found between body size and ankle moment torque. The small calf group showed a negative correlation between isometric and isokinetic ankle moment torque with height (R = - 0.531, -0.372; p = 0.011, 0.089) (Figure 15). The control group showed a very strong positive correlation between height and isokinetic ankle moment torque (R = 0.738, p < 0.001) (Figure 15). Weight and isometric and isokinetic torque were positively correlated for the small calf group with a stronger isokinetic correlation (R = 0.452, 0.546; p = 0.035, 0.009) (Figure 16). The control group also had a positive correlation between weight and isokinetic torque (R = 0.371, p = 0.089) (Figure 16). The small
calf group also had a positive and significant correlation between isometric and isokinetic torque and 26 body mass index (R = 0.840, 0.820; p < 0.001, 0.001) (Figure 17). Figure 15:Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus height for control subjects (filled circles) and small calf subjects (open circles). Negative correlations were found for the small calf group. Significant positive correlations were found for the small calf group. No significant correlation was found for the controls. Figure 16: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus weight for control subjects (filled circles) and small calf subjects (open circles). Negative correlations were found for the small calf group. Significant positive correlation was found for the control group with isokinetic torque.
27 Figure 17: Maximum isometric ankle torque (left) and isokinetic ankle torque (right) plotted versus body mass index for control subjects (filled circles) and small calf subjects (open circles). Strong positive correlations were found for the small calf group. No correlation was found for the control group. No significant correlations were found between height and moment arm nor height and muscle volume. There were also no significant correlations between weight and moment arm and weight and volume. The control and small calf subjects had a similar moderate correlation between weight and muscle volume. These graphs can be found as Figures 18 through 24 in Appendix A.
Chapter 4 28 Discussion Key Results Although the number of small-calf subjects tested so far is low, there were some intriguing correlations found within this group. As expected, larger muscle size correlated with strength, but there was also a strong correlation found between moment arm and strength. Correlations between plantarflexor strength and Achilles moment arm have been reported in only a handful of studies to date. The data showed a significant correlation between the moment arm length and maximum isometric torque for the small calf group. With an R value of 0.821, moment arm appears to have a strong positive effect on strength production. The isolation of small muscled subjects in this study allowed for a closer look at the effects of moment arm. By minimizing muscle size as a factor, this study attempted to highlight the effects of moment arm on strength in those who do not have the advantage of larger muscle mass. The data also showed a strong positive correlation between maximum isometric torque and calf circumference for the small calf subjects (R = 0.832). The two together have strong enough R values that they could be somewhat related in their effect on strength. One of the small calf subjects exhibited characteristics consistent with our hypotheses. The purpose of this thesis was to find farmer strong subjects with smaller calves that were stronger than those with average sized calves. We theorized that by isolating those with smaller calf circumference (smaller muscle size) we would be able to find more subjects who were farmer strong by examining whether they were stronger than the average of the controls as measured by isometric torque. We anticipated that such subjects would have a greater than average moment arm length and for the subject in question this was indeed true. This subject (Subject 15) also possessed a heel length (which should be similar to moment arm) that was longer than the control average. Based on these data, Subject 15 fit the
profile of someone who is farmer strong. Subject s with smaller calf circumference did not all have 29 lower ankle moment torque; therefore, there had to be another factor affecting the torque output of the subject. We believe this factor is likely moment arm. Although the subject did not have a large calf circumference overall, he did possess the largest of the small calf group. As stated earlier, in the case of the small calf group, both moment arm and calf circumference have almost equivalent positive correlations, showing they both may have some effect on strength. There was also a negative correlation between torque and fascicle length. The assumption would be that (all things equal), longer fascicles would create higher force for the isokinetic tests because the shortening is taken up by more sarcomeres. The results showed the opposite, indicating there may be some co-variation between fascicle length and PCSA. Comparison to Previous Results Blazevich et al. (2009) found a weak correlation between knee extensor moment arm and torque that was similar to the strong correlation between moment arm and torque found in the present study for small-calf subjects. In our study, there was also a strong correlation with isometric torque between PCSA and other structural factors such as pennation angle and fascicle length that corresponded to correlations found Blazevich et al. While this research focused on the PCSA of the lateral gastrocnemius, Blazevich examined the much larger vastus lateralis. So the PCSA values cannot be compared between studies. The correlation of anatomical factors also supports the hypothesis that some people are naturally predisposed to be stronger. The Blazevich et al. results showed a strong correlation between PCSA and isometric torque for the knee extensor moments. Baxter and Piazza (2014) provided the inspiration for this study and their study showed similar correlations between strength and ankle moment arm that were found in this study and had previously been found by Blazevich et al. The methods for both Baxter and Piazza s study and this study are
30 extremely similar and therefore the results were expected to be similar for data such as isometric torque versus muscle volume. The mean values found by Baxter and Piazza for muscle volume using magnetic resonance imaging are approximately 2.7 times larger (1348.2 ± 218.6 vs 496.6 ± 132.0) than those found for the present study, in which an ultrasound based measurement was used. Fukanaga and Hodgson found the mean soleus volume for healthy men was around 500 cm 3 (Fukunaga & Hodgson, n.d.). This value almost makes up the difference between the measured volume in this research and Baxter s, showing the soleus may have been partially excluded in our measurements and the muscle volume calculation provides a better prediction of medial and lateral gastrocnemius volume. The mean GL pennation angle was almost twice as large in this study as compared to that found by Lee and Piazza (2012) (22.2 ± 5.5 vs. 12.6 ± 1.7). The mean GL muscle thickness for the subjects tested in this study was within one standard deviation of the mean of the subjects examined by Lee and Piazza (2012) (15.0 ± 3.0 vs. 12.3 ± 3.0). The fascicle length of Lee and Piazza s subject s was longer than that of the subjects in this study (56.0 ± 6.8 vs. 41.9 ± 12.3). Lee and Piazza studied elderly men with an average age of 75.5 while the average age of our subjects was 20.8. It is interesting that the only part of the muscle significantly larger than Lee and Piazza s was pennation angle but muscle thickness was within one standard deviation and fascicle length was lower. The muscle thickness of the younger subjects was expected to be thicker. PCSA of younger subjects is expected to be larger as well and pennation angle is strongly correlated with PCSA. The mean maximum isometric torque for the subjects in this study is over one standard deviation lower than that found by Baxter and Piazza (104.0 ± 36.3 vs. 169.4 ± 52.9) as was the mean isokinetic torque (40.2 ± 27.5 vs. 92.0 ±35.4). The mean calf circumference was also one standard deviation lower for these subjects so the maximal torque would be expected to be lower. But, we also did not find correlations between torque and muscle size and torque and moment arm for the control subjects while Baxter and Piazza and other researchers found strong correlations. This brings into question the effectiveness of the dynamometer testing, which will be examined in further testing.