Leg Extension Power and Walking Speed in Very Old People Living Independently. Taina Rantanen 1 and Janne Avela 2

Transcription

1 Journal of Gerontology: MEDICAL SCIENCES 1997, Vol. 52A, No. 4." M225-M23I Copyright 1997 by The Gemntological Society of America Leg Extension Power and Walking Speed in Very Old People Living Independently Taina Rantanen 1 and Janne Avela 2 'Finnish Center for Interdisciplinary Gerontology and Department of Health Sciences, and ^Department of Biology of Physical Activity, University of Jyvaskyla, Jyvaskyla, Finland. Background. Leg extension power can be determined as the product of the force and velocity of movement. Its association with maximal walking speed was studied in and 85-year-old men and women. Methods. Leg extension power was measured with the help of a sledge ergometer in a sitting position using a facilitated "jump test." The participant was attached by belts to a sliding chair on rails inclined at 12.6 to the floor. The feet were placed on the force plate attached perpendicularly to the rails, and the knee angle was 90 at the starting position. The participant was advised to extend his or her legs powerfully. The highest value of five to eight attempts was accepted as the result. The results were adjusted for body mass and expressed as watts-kilogram" 1. Maximal walking speed was measured in the laboratory corridor over a distance of 10 m. Results. Men and 80-year-old subjects exhibited greater leg extension power and were faster walkers than women and 85-year-old persons. Leg extension power correlated positively with maximal walking speed in all groups: the correlation coefficients were.412 in the 80-year-old men (n = 41, p =.007),.619 in the 80-year-old women (n = 56, p <.001),.939 in the 85-year-old men (n = 8, p =.001), and.685 in the 85-year-old women (n = 23,p<.001). The regression lines for leg extension power and walking speed were coincident, indicating that the power requirements to attain a given walking speed were similar for both sexes. The minimum power threshold for those with a maximal walking speed of m-s"' was on the order of 4 Wkg" 1 ; a maximal walking speed of m-s" 1 required 7 Wkg"'; and for a speed over 2.00 m-s" 1 the power threshold was 9.5 Wkg" 1. Conclusions. Their lower average leg extension power may be one of the factors explaining the greater prevalence of mobility problems among women than men. IN higher age groups mobility problems become more common as the maximal walking speed decreases. Attempts to measure and study leg extension power in elderly people have been rare, although the inability to produce joint torque rapidly may be an essential factor in causing mobility problems among the elderly population (1). Leg extension power is determined as the product of force and velocity of movement. Force production in relation to speed of movement follows the well-known force-velocity curve in a shape of transposed S (2). Consequently, maximum leg extension power is gained when concentric fcrce production is performed at a level of approximately 40% of maximal isometric strength. Maximal isometric muscle strength (3) and maximal movement velocity (4) decrease with increasing age, while the time needed to reach a predetermined relative level of strength increases (5). In a cross sectional study among healthy 65- to 85-yearold men and women Skelton et al. (6) found maximal leg extension power to deteriorate at a rate of 3.0% per year in men and 1.1% per year in women. They also found that leg extension power was significantly correlated with maximal walking speed and maximum stair-mounting height in healthy elderly people. Bassey et al. (7) reported that among frail female nursing home residents, 86% of the variation in walking speed was explained by leg extension power. The association between physiological function (such as leg extension power) and functional performance (such as walking speed) has been suggested to be curved in shape and include a threshold (8,9) below which the amount of power is insufficient to perform a given functional task. As maximum leg extension power rises well above threshold level, a reserve capapity evolves; although this may not improve the functional performance as such, it can be seen as a safety margin. The correlation between physiological function and functional performance is assumed to be strongest near the threshold level. The purpose of the current study was to analyze the association of maximal leg extension power with maximal walking speed in independently living 80- and 85-year-old men and women and to detect potential power threshold levels for different walking speeds. PARTICIPANTS This study was carried out as a part of a larger longitudinal gerontological research program on functional aging known as the Evergreen Project. Tests on maximal leg extension power were added to the project for the second wave of data collection. M225

2 M226 RANTANENAND AVELA At the baseline (in 1989 and 1990) the target group was the total population of the city of Jyvaskyla born in the years 1914 and 1910 (10). During the second wave, those who had participated in the baseline survey 5 years earlier and were still alive constituted the target population. Eligible at the beginning of the laboratory examinations were 87 men and 181 women aged 80, and 40 men and 127 women aged 85. Of these people, 63 men and 155 women aged 80, and 27 men and 69 women aged 85, entered the laboratory. The test of leg extension power was carried out on those subjects who were willing to try the test and had no evident contraindications such as severe cardiovascular disease, severe dementia, endoprothesis in knee or hip joint, or severe arthritis in lower limbs. The assessment of contraindications was discretionary in the absence of exact guidelines or any prior experience on administering this test in people of these ages. The physician who decided on the subject's suitability to undergo power tests was an experienced practitioner of geriatric medicine. The physician also collected information about walking problems and chronic diseases by observing and interviewing each subject. Eventually, 41 men and 56 women aged 80, and 8 men and 24 women aged 85 (total, n = 131), were tested for leg extension power. METHODS Leg extension power was measured with the help of a sledge ergometer (11) in a sitting position using a facilitated "jump test." The participant was fastened by a seat belt to a sliding chair on rails inclined 12.6 to the floor. The acceleration of the sledge was 9.81 ms" 2 sin(12.6 ) = 2.14 ms" 2. The weight directed at the force plate was 0.218(body weight + chair weight). The mass of the chair was 27.7 kg. For example, for a person weighing 75 kg, the total weight directed toward the force plate was 22.4 kg. In the starting position the feet were placed on the force plate attached perpendicularly to the rails, and the knee angle was adjusted to 90. The participant was advised to extend his or her legs powerfully, "as if trying to jump," so that the seat started to slide upwards on the rails. The distance the seat moved was measured using an optical encoder. During the descent the measurer held the chair in order to slow it down and thus avoid impact and accidents while coming to rest. The highest result of five to eight attempts was accepted as the result. In a single "jump" the duration of the leg extension effort was about 1 sec. The between-trial rest period was at least 1 min. Leg extension power (P) was calculated as follows: P = W-t~\ where W is the external work. W was calculated by the integral of the function F(x) between the position limits of the sledge p, and p 2 as follows: W=]F(x)dx, P2 where F is the reaction force, x is the displacement of the sledge, pi is the beginning of the push-off, and p 2 is the end of the push-off from the force plate. Thereafter leg extension power was calculated by dividing W by the push-off time (12). The coefficient of variation of maximal leg extension power between two tests held at intervals of 1-2 weeks was 7.5% (SD 7.4, n = 13). For the analysis, maximal force during push-off and leg extension power were adjusted for body mass and expressed as Newtons-kilograirr 1 (Nkg" 1 ) and watts-kilogram" 1 (W-kg- 1 ). Maximal walking speed was measured in the laboratory corridor over a distance of 10 m. The subjects were allowed an extra 3-4 m to increase their speed and were advised not to stop until well past the finish line. Time was measured using a regular stop watch. Speed was calculated from the single best trial. Self-reports of mobility problems were studied by asking the participants whether they experienced difficulty in walking outdoors. The answers were classified in two categories: (7) those reporting no problems and (2) those reporting difficulties (such as tiredness, reduced speed, need of help, or totally unable). In addition, the physician observed the participant walking indoors. The categories were (a) normal gait and (b) need of cane or frame (staggering) or assistance of another person. Everyday physical activity was recorded by interviewing the subject using a question validated by Grimby (13). The categories were as follows: (7) mostly sitting; (2) mostly sitting, sometimes walking, or light tasks like straightening up, heating food, and dusting; (3) light physical exercise 2-4 h a week such as walking, fishing, gardening; main responsibility for domestic work such as cooking, dusting, straightening up; or taking part in weekly cleaning; (4) moderate exercise 1-2 h a week such as jogging, gymnastic exercise, heavier gardening, home repairs, or easier activities more than 4 h a week, or responsible for all domestic activities light and heavy; (5) moderate exercise at least 3 h a week; and (6) strenuous exercise several times a week. Categories 7 and 2 were combined to form the low activity category, category 3 was rated as moderate physical activity category, and categories 4-6 were combined to form a high activity category. Statistical methods. Cross-tabulation with x 2 was used to assess the difference in the prevalence of mobility problems between the power-tested subjects and the rest of the participants of the larger study. The number of chronic diseases of participants and nonparticipants was compared by Mests. Two-way analysis of variance (ANOVA) was used to analyze the age and sex effects on maximal leg extension force and power as well as on maximal walking speed. Correlation coefficients were calculated to study the associations of leg extension ground force and leg extension power and between leg extension power and walking speed. The LISREL 8 program was used to estimate the regression models for force and power and for power and walking speed in the four subgroups according to age and sex (14). The coincidence of the regression lines for the subgroups was tested using nested models. Cumulative percentage distributions of leg extension power in groups based on walking speed were produced by cross-tabulation.

3 LEG EXTENSION POWER AND WALKING SPEED M227 RESULTS The power-tested subjects were more physically active and had less gait abnormalities (such as staggering or need of cane or other assistance to walk) than the rest of the participants of the larger study (Table 1). In addition, the self-reported problems in walking outdoors were less common in the power-tested 80-year-olds than the other 80-year-olds. Also, in men aged 80 and women aged 85, the number of chronic diseases was lower in the powertested subjects than in the other subjects. The maximal ground reaction force recorded from the force plate as well as the maximal leg extension power were significantly greater in the 80-year-olds and in the men (Table 2). The coincidence of regression lines illustrated in Figure 1 was tested using nested LISREL models. The regression models for force and power were not coincident (\ 2 = 25.45, df 6, p <.001), indicating a divergence between the groups in the power produced in relation to force. Women were able to produce a smaller increase in power in relation to the same increase in maximal force than the men (\ 2 = 17.54, df 4, p =.002). In addition, 85-year-old women were able to produce a smaller increase in power in relation to the same increase in maximal force than the 80-year-old women (x 2 = 12.56, df 2, p =.002). In men the models did not differ significantly between age groups (x 2 = 4.98, df 2, p =.083). Maximal walking speed was significantly faster in the men than in the women and in the younger subjects than in the older subjects (Table 2). The correlations between maximal walking speed and leg extension power were significant (p <.001) in all groups: in 80-year-old men r =.412; in 85-year-old men r -.939; in 80-year-old women r =.619; and in 85-year-old women r =.685. The regression models for leg extension power and walking speed shown in Figure 2 did not vary according to age (x 2 = 1.319, df 4, p =.858) or sex (x 2 = 6.312, df 4, p =.177). Thus, it may be concluded that power requirements for a given walking speed were similar for men and women, regardless of age. The curved cumulative percentage distributions for the maximal leg extension power of the groups according to maximal walking speed provide evidence for the existence Table 1. Number of Diseases, Prevalence of Mobility Problems, and Physical Activity Compared Between Power-Tested Volunteers and the Other Participants of the Evergreen Project 80-Year-Old Men 85-Year-Old Men 80-Year-Old Women 85-Year-Old Women Power-tested M=41 2.2(1.4) n = 8 1.4(1.6) n = (1.3) n = (2.0) Others n = (1.7) «= (2.0) n= (1.9) n = (1.6) Mest (p) {p) Notes: Shown are /-test, means (SD), x 2 test, and percentages.

4 M228 RANTANENAND AVELA Table 2. Body Height and Mass, Maximal Leg Extension Force, and Power and Maximal Gait Speed in 80- and 85-Year-Old Men and Women Men Women Main Effects Body height (cm) Body mass (kg) Maximum leg extension force (N-kg- 1 ) Maximum leg extension power (Wkg- 1 ) Maximum gait speed (ms~') Mean years (n = = 41) 1.78 (SD) (5.9) (11.1) (2.0) (2.9) (0.40) Note: Shown are ANOVA and mean (SD) year-old Mean years (n = 8) (SD) (7.3) (13.9) (2.9) (0.32) 80 years («= 56) Mean (SD) women D y^ (5.2) (8.2) (2.6) (0.28) Mean years (us= 24) 1.22 (SD) (5.6) (10.5) (0.29) F Age P F Sex P Age by Sex Interactions F P jg n= Figure 1. Association of the maximal leg extension force and power in 80- and 85-year-old men and women. The equations for the regression lines (standard error) are as follows: 80-year-old men: y = -5.43(1.21)+1.34(0.10);t; 85-year-old men: y = -4.67(1.87)+1.19(0.16)^; 80-year-old women: y = (0.52)+1.14(0.05)x; and 85-year-old women: >> = -2.41(0.60)+0.92(0.08)-JC.

5 LEG EXTENSION POWER AND WALKING SPEED M year-old men Q O year-old men o n= n= year-old a D a women 0.5 a n=24 o Figure 2. Association of the maximal leg extension power and maximal walking speed in 80- and 85-year-old men and women. The equations for the regression lines (standard error) are as follows: 80-year-old men: y = 1.13(0.24)+0.058(0.020)-*; 85-year-old men: y = 0.74(0.l3)+0.l03(0.0l4)vc; 80-yearold women: y = 0.98(0.09)+0.067(0.01 1)-JC; and 85-year-old women: y = 0.80(0.10)+0.087(0.020)x I I I I I I W/kg Figure 3. Cumulative percentage distributions of maximal leg extension power in groups based on maximal walking speed.

6 M230 RANTANENAND AVELA of power thresholds for fast walking (Figure 3). In the three fastest groups a plateau followed by a clear upward turn was observed in the curves. In the group with a maximal walking speed of m-s~' it was of the order of 4 Wkg" 1 ; in the group with a maximal walking speed of ms" 1 it was 7 Wkg' 1 ; and in the fastest group with walking speed over 2.00 ms" 1 it was 9.5 Wkg" 1. DISCUSSION The threshold theory (8,9) was supported by the cumulative percentage distributions for leg extension power in groups according to maximal walking speed. However, instead of a single threshold level, a critical range of power can be detected. Within that range the probability of attaining a predetermined walking speed increases. The lower level of the range can be seen as the minimum level, below which it is virtually impossible to perform the task. The upper level of the range can be seen as the "safe" level, above which a further increase in power does not increase the probability of attaining that particular walking speed but acts as a safety margin. For example, the majority (95%) of those with a maximal walking speed in the range of ms" 1 exhibited leg extension power below 9.5 Wkg- 1, which would seem to be the upper level of the critical range of leg extension power for that walking speed. On the other hand, only 12% of those with a walking speed of more than 2.00 m«s~' had maximal leg extension power below that level. Therefore, 9.5 Wkg" 1 can be seen as the lower or minimum level of the critical power range for a walking speed of more than 2.00 ms" 1. For that speed the curve in the upper region of the threshold zone did not flatten out in the current study population, indicating that safe level was not reached. The coincidence of the estimated regression lines for leg extension power and walking speed in the four groups also provides evidence for the existence of threshold phenomena. The lines suggest that roughly the same amount of leg extension power is required for a specific walking speed in both sexes. In the current study, 80-year-old men exhibited greater power-test results than the other groups, and the variability of walking speed not explained by power appeared to be greater. In this group, leg extension power was probably not a limiting factor for walking speed, which indicated a safety margin. In women and older men the association between power and gait speed was clear: an increase in power improved gait speed. These groups had poorer power levels and were thus situated within the critical power range for walking. In a prospective study among an initially disability-free population aged 71 years or older at baseline, a slow customary walking speed predicted subsequent mobilityrelated disability (15). It is possible that in that study, the slower walkers were already near the minimum power levels for walking and thereafter deteriorated to below the minimum threshold level, probably due in part to reduced activity. Slow walkers often experience difficulties in moving about outdoors, as a walking speed of 1.5 m-s' 1 is considered the norm for pedestrians at traffic lights. Slow walkers often have been found to be homebound (16), which is generally associated with a reduced quality of life. The slowing of movement with age (4) results in less power (force X velocity) produced at a set level of force in the older age groups, which was observed in the present study among the women. Consequently, power deterioration with age is greater than strength deterioration. In addition, the sex difference is marked; the 80-year-old women exhibited 66% of the power of the men, and in the 85- year-olds the ratio was 54%. In measuring leg extension power in elderly people the already weakened performance capacity of the subjects has to be considered. The ability to produce power in vertical jumps has been found to be very poor in elderly persons. Bosco and Komi (17) found that 72-year-old healthy meft could produce 25% of the power measured in young adults. In women the corresponding figure was 20%. Conse L quently, most 85-year-old women are probably unable t6 jump vertically at all. Moreover, jumping tests performed in standing position involve risk of accidents in the elderly, such as falling or muscle and tendon injuries. The present method of measuring leg extension power was a modified form of a method developed by Kaneko et al. (11). The inclination of the rails of the sledge ergometer was set at 12.6 to avoid excessive strain in the lower extremities. At this inclination the force against the force platform at the starting position (body and seat mass) was some one-third of the force that would have been directed toward a horizontal force plate by the body mass of the participant had he or she been in a vertical position. Thus, the participants were able to perform the test. Also, a sitting position during testing is a safety advantage. After familiarization with the test, the participants found it easy to perform and acceptable. No accidents occurred during the testing. Disability in old age is much more common in women than in men (18). The lower average leg extension power in women than in men is probably one of the contributory factors, as the power requirements for walking are similar for both sexes. It is thus highly appropriate to consider women as a primary target group for exercise interventions aiming to maintain and improve lower extremity functional capacity in old age. ACKNOWLEDGMENTS The Evergreen Project has received funding from the Academy of Finland, Ministry of Education, Ministry of Social Affairs and Health, City of Jyvaskyla, and the University of Jyvaskyla. We thank Markku Kauppinen, MSc, and Mr. Esko LevSlahti for statistical analyses. We wish to expres our gratitude to Professor Paavo V. Komi for loan of the sledge ergometer, and to Dr. Heikki Kyrolainen for comments on the manuscript. We also wish to acknowledge Professor Eino Heikkinen for his valuable contribution in the realization of the Evergreen Project. The elderly volunteers of the study deserve acknowledgment for their kind participation. Address correspondence to Dr. Taina Rantanen, Epidemiology, Demography and Biometry Program, N1A, NIH. 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