The Effect of Trunk Flexion in Healthy Volunteers in Rear Whiplash-Type Impacts

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1 The Effect of Trunk Flexion in Healthy Volunteers in Rear Whiplash-Type Impacts SPINE Volume 30, Number 15, pp , Lippincott Williams & Wilkins, Inc. Shrawan Kumar, PhD, DSc, FErg, SFRS(C),* Robert Ferrari, MD, FRCPC, and Yogesh Narayan, BSc (EE), PEng* Study Design. Twenty young, healthy volunteers in a laboratory were subjected to rear-end impacts 4.4, 7.9, 10.9, and 13.1 m/s 2 acceleration with head rotation to right and left. Objectives. The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity rear impacts when the head is rotated at the time of impact. Summary of Background Data. A previous study of rear impacts with head in neutral posture suggests that the burden of impact is borne primarily by the sternocleidomastoid muscles bilaterally. To improve automobile designs to prevent whiplash injury, we need to understand the response of the cervical muscles to whiplashtype perturbations in other conditions that mimic road collisions, such as when the head is rotated to the right and left at the time of rear-end impact. Methods. Triaxial accelerometers recorded the acceleration of the sled, torso at the shoulder level, and head of the participant, while bilateral electromyograms (EMGs) of the sternocleidomastoids, trapezii, and splenii capitis were also recorded on 20 subjects (10 males and 10 females, mean age of years) Results. For participants experiencing a rear-end impact, whether having the head rotated to the left or right at the time of impact, the muscle responses increased with increasing levels of acceleration (P 0 01). The time to onset and time to peak EMG for all muscles progressively decreased with increasing levels of acceleration (P 0.01). Which muscle responded most to a whiplashtype neck perturbation was determined by the direction of head rotation. With the head rotated to the left, the right sternocleidomastoid generated 88% of its maximal voluntary contraction EMG (at least triple the response of other muscles). In comparison, the left sternocleidomastoid, both trapezii, and the splenii capitis generated on average only 10 to 30% of their maximal voluntary contraction EMG with head rotated to the left. On the other hand, with the head rotated to the right, the left sternocleidomastoid generated 94% of its maximal voluntary contraction EMG (again, at least triple the response of other muscles). From the *Department of Physical Therapy, Faculty of Rehabilitation Medicine, and Department of Medicine, University of Alberta, Edmonton, Alberta, Canada. Acknowledgment date: June 4, First revision date: August 4, Second revision date: August 23, Acceptance date: August 23, The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and requests for reprints to Shrawan Kumar, 3-75 Corbett Hall, Department of Physical Therapy, Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada T6G 2G4. shrawan.kumar@ualberta.ca Conclusions. If the head is rotated out of neutral posture at the time of rear impact, the injury risk tends to be greater for the sternocleidomastoid muscle contralateral to the side of rotation. Measures to prevent whiplash injury may have to account for the asymmetric response because many whiplash victims are expected to be looking to the left or right at the time of collision. Key words. cervical muscles, electromyography, acceleration, motor vehicle collisions, rear impacts, whiplash injury. Spine 2005;30: Because of the widespread problem of rear-end collisions and the significant economic and health burden, 1 researchers have over a period of decades gradually approached the understanding of the complex response to whiplash-type impacts, beginning with Severy s early collision experiments with volunteers in the 1950s, 2 to a large body of work involving animals, cadavers, dummies, and volunteers. 3 Biomechanical studies of automobiles and impact studies with dummies have suggested that, in theory, being in a trunk-flexed posture at the time of rear impact may increase the risk and severity of an acute injury. 4 6 This is based on the finding with dummies that when the occupant s torso is in full contact with the seat, seat deformation may reduce neck injury risk by virtue of less torso-head lag and also that when there is a lack of full contact, the torso may ramp up the seat back and more neck extension may occur. This conclusion has not yet been confirmed by dedicated impact experiments with volunteers or through epidemiological surveys. We do know, however, based mainly on research involving volunteers, 7 25 that if the subject is in a neutral, upright posture the first movement of the body following collision is forward movement of the torso, which places the lower cervical segments in extension and the upper cervical segments in flexion because of the inertia. As viewed from the lateral aspect, this produces an elongated S-shaped curve to the cervical spine rather than simply the lordotic C curve that one would expect with neck extension alone. 21,22,25 The entire cervical spine does not go into extension. It then begins to return to its neutral position, after which there may be flexion of the upper cervical spine before it again returns to neutral. During the initial translation of the head backwards to form this S-shaped curve, the muscles of the neck are being stretched and a reflex contraction occurs during this stretch, again beginning at around 180 milliseconds after impact. This is an eccentric contraction (i.e., contraction occurring during stretching) and a possible injury mechanism. However, facet joint capsular strain has 1742

2 Effect of Trunk Flexion in Healthy Volunteers Kumar et al 1743 also been proposed as an injury mechanism through work with cadaveric models. 23,25 Other whiplash experiments, however, bring additional complexities to the problem; e.g., Castro et al 26 have shown that symptoms are not necessarily a reliable surrogate to injury, as acute whiplash symptoms may be produced in volunteers who are made to believe they have been in a collision when in reality they have not. Nevertheless, if one is to accept that injuries do occur in low-velocity rear-impacts, it may not be possible to apply the research from previous rear-impact studies to conditions where the subject s position is not a standard, neutral position. The reality is that vehicle occupants are often not positioned in the neutral position at the time of impact. We have previously reported in this journal that the cervical muscle responses are dramatically altered simply by having the head rotated at the time of impact and that expecting an impact also alters the cervical muscles response Our previous studies were accomplished using surface electromyography (EMG) combined with the use of regression techniques modeled on very low velocity collisions. This methodology is designed so as not to risk injury to the experiment subjects, but to derive data from which further extrapolations can be made. To address a void in current knowledge, we thus undertook a study to assess the cervical muscle response for rear-end impacts where the subject s trunk is flexed forward and to either the left or right. The study presented here is one section of a larger study where multiple directions were investigated by random assignment of volunteers to different conditions to minimize risk of injury, if any, because of multiple exposures. Materials and Methods Sample. Twenty healthy, normal subjects (10 males and 10 females) with no history of whiplash injury and no cervical spine pain during the preceding 12 months volunteered for the study. The 20 subjects had a mean age of years, a mean height of cm, and a mean weight of kg. The subjects were all right-hand dominant. The study was approved by the Health Research Ethics Board. Tasks. Active surface electrodes with 10 times on-site amplification were placed on the belly of the sternocleidomastoids, upper trapezius at C4 level, and splenius capitis in the triangle between sternocleidomastoids and trapezii bilaterally. The fully-isolated amplifier had additional gain settings up to 10,000 times with frequency response DC-5 khz and common mode rejection ratio of 92 db. Before calibrated sled acceleration, the cervical strength of the volunteers was measured to develop force-emg calibration factor. 30 The seated and stabilized subjects exerted their maximum isometric effort in attempted flexion, extension, and lateral flexion to the left and the right for force-emg calibration, as described by Kumar et al 30,31 The acceleration device consisted of an acceleration platform and a sled, the full details of the device and the electromyography data collection being published previously by Kumar et al After the experiment was discussed and informed consent obtained, the age, weight, and height of each volunteer was Figure 1. Diagrammatic representation of subject positioning on impact sled. recorded. The volunteers then were seated on the chair with a lap seat belt only so they could then be positioned out of neutral posture. Subjects were then outfitted with triaxial accelerometers (model CXL04M3, Crossbow Technology, Inc., San Jose, CA) on their glabela and the first thoracic spinous process. Another triaxial accelerometer was mounted on the sled. The accelerometers had a full scale nonlinearity of 0.2%, dynamic range of 5 g, with a sensitivity of 500 mv/g, resolution of 5 mg within a bandwidth of DC-100 Hz, and a silicon micromachined capacitive beam that was quite rugged and extremely small in die area. Subjects were then exposed to rear-impacts with their trunk flexed forward and to either their left and right at accelerations of 4.4, 7.7, 10.9, and 13.8 m/s 2 generated in a random order by a pneumatic piston. The subjects were asked to assume a position of trunk flexion (forward and lateral) and to look down at their right or left foot. This results in minimal neck and trunk rotation. We did not attempt to completely eliminate rotation as this would be an unnatural position for vehicle occupants to hold. We also did not attempt to have the subjects completely relaxed with the neck fully flexed (i.e., slumped posture), because we expected this would not be typical of road collisions. We positioned each of the volunteers in 45 flexion and 45 rotation either to the left or to the right using a goniometer (Figure 1). The accelerations involved in this experiment are again low enough that injury is not expected, but the acceleration impulse is delivered in a way that mimics the time course seen in motor vehicle collisions and occurs fast enough to produce eccentric muscle contractions. Otherwise, the accelerations are on the order of 2 g and amount to impact velocities of 6 km/hour. According to Allen et al, 32 such accelerations, though they are often the case in low-velocity whiplash claims, are within the range of other life experiences such as sneezing. Data Analysis. The data on the peak and average accelerations in all three axes of the sled, shoulder, and head for all four levels of accelerative impacts were measured. In the analysis, the sample of volunteers was collapsed across gender because preliminary analysis showed no statistically significant differences in the EMG amplitudes between the men and women. The sled velocity and its acceleration subsequent to the pneumatic piston impact and the rubber stopper impact were measured. All timing data were referred to the solenoid firing. The time of the peak acceleration was measured. Also, the time

3 1744 Spine Volume 30 Number Figure 2. Trunk flexed to right and left. Head acceleration in the x, y, and z axes of one subject in response to the level of applied acceleration. The z-axis is parallel, the x-axis orthogonal, and the y-axis vertical to the direction of travel. Head X, head acceleration in the x-axis; Head Y, head acceleration in the y-axis; Head Z, head acceleration in the z-axis. relations of the onset and peak of the EMG were measured and analyzed. The time to onset was determined when the EMG perturbation reached 2% of the peak EMG value to avoid false positives because of tonic activity. This method was chosen to avoid any false positives because of tonic EMG. This method was in agreement with projection of the line of slope on the baseline. EMG amplitudes were normalized against the subjects maximal voluntary contraction EMG. The ratio percentage of the EMG amplitude versus the maximal contraction normalized EMG activity for that subject allowed us to determine the force equivalent generated because of the impact for each muscle. The force equivalents were determined from data on maximum isometric effort in attempted flexion, extension, and lateral flexion to the left and the right for force-emg calibration, as described by Kumar et al. 30,31 Statistical analysis was performed using the SPSS statistical package (SPSS Inc., Chicago, IL) to calculate descriptive statistics, correlation analysis between EMG and head acceleration, analysis of variance of the time to EMG onset, time to peak EMG, average EMG, and the force equivalents. Additionally, a linear regression analysis was carried out for the kinematic variables of head displacement, head velocity and head acceleration, and EMG variables. Initially, all regressions were carried out to the level of exposure and subsequently they were extrapolated to twice the level of acceleration used in the study. Results The subjects reported no symptoms to suggest injury following the experiment and up to 6 months later. Head Acceleration The kinematic response of the head to the four levels of applied acceleration are shown in Figure 2. As antici-

4 Effect of Trunk Flexion in Healthy Volunteers Kumar et al 1745 Figure 3. Trunk flexed to right and left. Normalized peak and average EMG (percentage of isometric maximal voluntary contraction), force equivalent of EMG (N), and applied acceleration. LSCM, left sternocleidomastoid; RSCM, right sternocleidomastoid; LSPL, left splenius capitis; RSPL, right splenius capitis; LTRP, left trapezius; RTRP, right trapezius. pated, an increase in applied acceleration resulted in an increase in excursion of the head and accompanying accelerations. EMG Amplitude In a rear impact, with the trunk flexed to the right or left, the trapezii muscles showed the greatest EMG response compared with the remaining muscles (P 0.05), suggesting that the forward-flexed posture primes these neck extensors for action more so than other muscles. The mean peak (normalized) EMG amplitude of the cervical muscles tested in this experiment at each applied acceleration level are presented in Figure 3. As the level of applied acceleration in the rear impact increased, the magnitude of the EMG recorded from most muscles increased, though this was not a statistically significant finding. More importantly, the normalized EMG showed that the percentage of the EMG was very low for either trunk flexed to the left or right, the trapezii generating 28% or less of the maximal voluntary EMG. In terms of force equivalents, these were also quite low, at a level much lower than the maximal voluntary contraction capability of each muscle. To place the magnitude of the EMG responses in perspective, in Figure 4 we show the normalized EMG per-

5 1746 Spine Volume 30 Number studies (using 7 and 20 subjects, respectively). 27,29 Compared to the state of the head and trunk in neutral posture, both head rotation and also trunk flexion significantly reduce the EMG response (P 0.01). Figure 4. Trunk flexed to right and left. Normalized peak EMG (percentage of isometric maximal voluntary contraction) at the highest level of acceleration for three conditions: head and trunk in neutral posture, head rotated to right or left with trunk in neutral posture, and trunk flexed forward and laterally right or left. The example shown here is for the right and left sternocleidomastoid muscles. centage of sternocleidomastoid (SCM) for 3 conditions (all data from impacts where the impact was expected): head and trunk in neutral posture, head rotated to right or left with trunk in neutral posture, and trunk flexed forward and laterally right or left. The data for the head in neutral posture and head rotated are from previous Timing The time to onset of the sled, shoulder, and head acceleration onset in the z-axis (axis along impact direction) and the EMG signals of the six muscles examined for trunk flexed to the left or right are presented in Table 1. The time to onset was measured from the firing of the solenoid of the pneumatic piston. The time to onset of the head acceleration showed a trend to decrease with increased applied acceleration, as did the time to onset of EMG. The mean times at which peak EMG occurred for all the experimental conditions are presented in Table 2. The times at which peak EMG occurred showed a trend to being reduced with increasing acceleration, but this did not reach statistical significance. The relationship between the force equivalent EMG response of each muscle and the head acceleration are shown in Table 3. The kinematic responses show that very-low velocity impacts produce less force equivalent than the maximal voluntary contraction for the same subject, and thus this experimental approach allows us to gather valuable data without exposing subjects to any foreseeable injury. Statistical Analyses The applied acceleration, and the muscles examined had significant main effects on the peak EMG activity only for the trapezii (P 0.05). We used a linear regression model to plot the available data and extrapolate from the experimental accelerations to accelerations on the order of 30 m/s 2. Initially, regression analyses were performed only up to 13 m/s 2 using a linear function. The kinematic variables of head displacement and velocity were calculated from acceleration data by double and single integration, respectively (see Figure 1). Additionally, we also regressed the EMG magnitudes on acceleration. The Table 1. Rear Impact with Trunk Flexed to Right and Left Muscle Acceleration Onset Sternocleidomastoid Splenius Capitis Trapezius Impact Acceleration (m/s 2 ) Sled Shoulder Head Left Right Left Right Left Right Trunk flexed to right (15) 72 (27) 90 (28) 146 (26) 307 (155) 172 (81) 257 (49) 180 (86) 218 (33) (12) 64 (14) 70 (28) 146 (19) 281 (73) 136 (77) 253 (51) 178 (102) 215 (83) (11) 61 (16) 66 (21) 139 (14) 198 (109) 122 (68) 247 (89) 173 (75) 208 (75) (13) 52 (17) 65 (25) 132 (20) 178 (100) 121 (50) 238 (111) 158 (94) 194 (72) Trunk flexed to left (25) 77 (34) 89 (49) 156 (73) 162 (82) 255 (47) 142 (108) 221 (52) 185 (80) (12) 63 (24) 68 (32) 147 (51) 136 (43) 246 (61) 133 (58) 218 (81) 184 (70) (10) 54 (18) 65 (28) 134 (44) 123 (38) 227 (82) 131 (54) 211 (96) 169 (81) (14) 53 (18) 63 (17) 119 (68) 112 (38) 218 (102) 120 (96) 207 (93) 163 (75) Note. Mean time to onset (millisecond) of acceleration and of muscle EMG from the firing of the solenoid of the pneumatic piston. Times for the sled, shoulder, and head represent the time at which acceleration in z-axis (direction of travel) began. Times for the cervical muscles represent the onset time for EMG activity; values in parentheses represent 1 SD.

6 Effect of Trunk Flexion in Healthy Volunteers Kumar et al 1747 Table 2. Rear Impact with Trunk Flexed to Right or Left Muscle EMG Sternocleidomastoid Splenius Capitis Trapezius Acceleration (m/s 2 ) Left Right Left Right Left Right Trunk flexed to right (42) 709 (425) 490 (387) 608 (430) 595 (414) 410 (83) (21) 709 (366) 454 (279) 532 (188) 448 (221) 397 (82) (20) 427 (272) 447 (229) 504 (181) 439 (210) 382 (136) (21) 320 (135) 331 (126) 476 (159) 476 (159) 372 (203) Trunk flexed to left (522) 366 (163) 577 (123) 334 (88) 460 (133) 508 (247) (277) 226 (37) 565 (184) 325 (117) 424 (167) 404 (152) (331) 221 (35) 562 (246) 317 (129) 393 (140) 387 (129) (409) 217 (32) 505 (223) 301 (137) 378 (212) 381 (138) Note. Mean time (millisecond) at which peak EMG occurred after the firing of the solenoid of the pneumatic piston. Values in parentheses represent 1 SD. responses of the left and right muscle groups were extrapolated to more than twice the applied acceleration value (Figure 5), and one is impressed that the EMG magnitudes remain low over this range compared with previous studies with the head and trunk in neutral posture. 27 Discussion In rear impacts, whiplash victims may be reaching down for an object on the vehicle floor or leaning over as a result of watching for traffic or speaking with other occupants, etc. This affects the neck muscle response to accelerations. In the current study, where we have used EMG measurements to study the cervical muscle when the trunk is flexed to the right or left at the time of a rear impact, we find that the trapezii generate the greatest EMG response, but in general all muscles show low EMG magnitudes. Thus, trunk flexion greatly mitigates cervical muscle activity. That is, in a previous study we found that both sternocleidomastoids respond to a rear impact with EMG activity that is up to 179% as great as normal maximal static muscle contraction strength when the impact is unexpected, and the subject is looking forward (neutral position). When the impact is expected, this EMG activity drops to about 80% of maximal muscle contraction strength. In the current study, the subjects were also expecting the impact, so this explains partly why there was less muscle activity observed. Yet, there was so much less muscle activity for all muscles when the trunk was flexed that expectation alone cannot explain the variance. As seen in this experiment, even the most active muscles do not exceed 28% of their maximal EMG contraction magnitude. The fact that the trapezii are more active than the sternocleidomastoids is a finding that differs from previous studies with the head and trunk in neutral position 27 and also with the head in rotation. 29 It is suggested that the forward flexed trunk posture, with the subject looking down, likely stretches the trapezii and primes them for more activity than other muscles for this reason. This may explain why they were the most active. The regression findings suggest that these results hold into the low-velocity range of impacts. It is recognized that these are extrapolations and all extrapolations are approximations of the truth. Further studies will be required to determine if the liner regression findings hold in the higher velocity ranges. Table 3. Rear Impact with Trunk Flexed to Right and Left Force Equivalents for Muscle (N) Sternocleidomastoid Splenius Capitis Trapezius Chair Acceleration (m/s 2 ) Head Acceleration (m/s 2 ) Left Right Left Right Left Right Trunk flexed to right (0.8) 5 (4) 4 (4) 22 (10) 15 (5) 15 (6) 17 (5) (1.5) 7 (5) 5 (4) 25 (8) 16 (7) 15 (6) 18 (6) (1.7) 8 (4) 7 (5) 27 (10) 18 (7) 19 (7) 22 (7) (1.5) 12 (5) 8 (5) 34 (16) 21 (8) 20 (5) 24 (7) Trunk flexed to left (0.7) 4 (3) 6 (5) 21 (8) 16 (8) 19 (6) 16 (6) (1.3) 5 (5) 7 (3) 25 (6) 19 (8) 20 (6) 17 (6) (1.6) 8 (6) 8 (4) 26 (10) 21 (8) 22 (8) 20 (6) (2.0) 8 (7) 10 (6) 26 (11) 24 (7) 22 (6) 21 (6) Note. Mean force equivalents and mean head accelerations at time of maximal EMG in direction of travel. Values in parentheses represent 1 SD.

7 1748 Spine Volume 30 Number Figure 5. Trunk flexed to right and left. Extrapolated regression plots of the effect that applied acceleration has on the left and right sternocleidomastoid muscles for the variables of peak EMG (V), normalized EMG (percentage of isometric maximal voluntary contraction), and force equivalent of EMG (N). From the experimental design conditions here, and where specifically we have not included the possibility of head impact, the risk of cervical muscle injury is likely less than when the head and trunk in the neutral position. This is contrary to previous research findings. 4 6 The previous research, however, focused on dummy responses, which may explain the difference in our findings, and also some of he dummy experiments were of much higher velocity impacts. In low velocity collisions, at least, we would argue that when the trunk is flexed, less contact between the trunk and seat leads to less differential in the inertial moments between the trunk and the head. The only seat contact is at the lower hip/buttock region, and not the entire trunk. The torso-head lag is what promotes the pattern of differential head and torso movements leading to the S-shaped curve. This differential should be less in this scenario given that there is nothing initially to stop the rearward torso movement relative to the head movement if the torso is initially held away from the seat back. With less of a head-torso lag there is less stretch of cervical muscles in response to impact and less expected muscle activation overall. Furthermore, when flexing the trunk and looking down, there may be a volitional activity achieved by exerting the muscles, thereby putting them in a state of readiness to resist any perturbation. Thus, assuming no other bodily impact, having the trunk flexed at the time of impact should be protective, just as expecting an impact may be protective against injury. Obviously, with higher velocity impacts these protective factors may be overcome, resulting in injury, the nature of which we are unable to predict without experimental results. Key Points With head rotation at the time of rear impact, the sternocleidomastoid muscle opposite the direction of head rotation is activated more than the trapezii, splenii capitis, or ipsilateral sternocleidomastoid muscles.the sternocleidomastoid contralateral to the direction of head rotation reaches near its maximal voluntary contraction with an acceleration of 13.1 m/s 2. If the head is rotated out of neutral posture at the time of rear impact, the injury risk tends to be greater for the sternocleidomastoid muscle contralateral to the side of rotation. References 1. Spitzer WO, Skovron ML, Salmi LR, et al. Scientific monograph of the Quebec Task Force on whiplash-associated disorders. Spine 1995;120(Suppl 8):1S 73S. 2. Severy DM, Mathewson JH, Bechtol CO. Controlled automobile rear-end collisions, an investigation of related engineering and medical phenomena. Can Serv Med J 1955;11: Ferrari R. The Whiplash Encyclopedia. The Facts and Myths of Whiplash. Gaithersburg, MD: Aspen Publishers Inc., 1999; Warner CY, Stother CE, James MB, et al. Occupant protection in rear-end collisions: II. The role of seat back deformation in injury reduction. Proceedings of the 25th Stapp Car Crash Conference. Paper , Warrendale, PA: Society of Automotive Engineers;1991: Foret-Bruno JY, Tarriere C, Le Coz JY, et al. Risk of cervical lesions in real-world and simulated collisions. Proceedings of the 34th Conference of the Association for the Advancement of Automotive Medicine, 1990 October 1 3; Scottsdale, AZ. Barrington, IL: Association for the Advancement of Automotive Medicine; p Hu AS, Bean SP, Zimmerman RM. Response of belted dummy and cadaver to rear impact. Proceedings of the 21st Stapp Car Crash Conference. Paper Warrendale, PA: Society of Automotive Engineers; p West DH, Gough JP, Harper GTK. Low speed rear-end collision testing using human subjects. Acc Reconstr J 1993;5:22 6.

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