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Journal of Motor Behavior

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Effect of Age and Gender in the Control of Elbow Flexion Movements

Aron S. Buchmana; Sue Leurgansb; Gerald L. Gottliebc; Chi-Hung Chend; Gil L. Almeidae; Daniel M. Corcosf a Department of Neurological Sciences, Rush-Presbyterian-St. Lukes Medical Center, b Department of Preventative Medicine, Biostatistics, Rush-Presbyterian-St. Lukes Medical Center, c Neuromuscular Research Center, Boston University, d College of Kinesiology Department of Psychology, University of Illinois, e Instituto de Reabilitacao de Campinas Universidade Estadual de Campinas, f Department of Neurological Sciences Rush-Presbyterian-St. Lukes Medical Center College of Kinesiology Department of Psychology, University of Illinois, Online publication date: 02 April 2010 To cite this Article Buchman, Aron S. , Leurgans, Sue , Gottlieb, Gerald L. , Chen, Chi-Hung , Almeida, Gil L. and Corcos,

Daniel M.(2000) 'Effect of Age and Gender in the Control of Elbow Flexion Movements', Journal of Motor Behavior, 32: 4, 391 — 399 To link to this Article: DOI: 10.1080/00222890009601388 URL: http://dx.doi.org/10.1080/00222890009601388

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Journal of Motor Behavior, 2000,Vol. 32,No. 4,391-399

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Effect of Age and Gender in the Control of Elbow Flexion Movements Aron S. Buchman

Gerald L. Gottlieb

GiI L. Almeida

Department of Neurological Sciences, Rush-PresbyterianSt Lukes Medical Center

Neuromuscular Research Center Boston University

lnstituto de Reabilitacao de Campinas Universidade Estadual de Campinas

Sue Leurgans

Chi-Hung Chen

Department of Preventative Medicine, Biostatistics Rush-PresbyterianSt Lukes Medical Center

College of Kinesiology Department of Psychology University of Illinois

Daniel M. Corcos Department of Neurological Sciences Rush-Presbyterian-St.Lukes Medical Center College of Kinesiology Department of Psychology University of Illinois

cos, 8z Agarwal, 1989b). It has been suggested that when task parameters such as movement speed or distance are varied, participants use different control strategies to modulate biphasic EMG activity (Gottlieb, Corcos, & Agarwal, 1989b). Those control strategies can be described in terms of characteristic changes in the initial rate of rise of the agonist EMG bursts and in the areas of the agonist and antagonists EMG bursts. With increasing age, there is loss of muscle function as manifested by declines in strength, power, and speed, which may be associated with decreased muscle bulk. Those changes in body composition and motor function that accompany aging have been termed surcopeniu (Dutta & Hadley, 1995). The changes in body composition and muscle function raise the question of whether there are associated age-related changes in EMG patterns. In addition, the differences in strength between male and female participants may alter EMG patterns (Hoffman & Strick, 1993; Pfann, Hoffman, Gottlieb, Strick, 8z Corcos, 1998). Our purpose in this study was to test the hypothesis that similar patterns of biphasic musclc activity can be observed in both young and old men and women. Elbow flexion movements of different distances and speeds were investigated in young men because those movements have been associated with predictable patterns of musclc activity (Corcos, Gottlieb, & Agarwal, 1989; Gottlieb. 1996; Gottlieb, Corcos, 8z Agarwal, 1989a, Gottlieb, Corcos, & Agarwal, 1990, Gottlieb, Latash, Corcos, Liubinskas, & Agarwal, 1992).

ABSTRACT. In previous studies of rapid elbow movements in young healthy men, characteristic task-dependent changes in the pattei‘ns of muscle activation when movement speed or distance was haried have been reported. In the present study, the authors invcsligated whether age or gender is associated with changes in thc piillterns of muscle activity previously reported in young men. Arm tnovements of 10 healthy older and 10 healthy younger participmts ( 5 men and 5 women in each group) were studied. Surface electromyograms (EMGs) from agonist (biceps) and antagonisl (triceps) muscles, kinematic and kinetic parameters, as well as anlhropometric and strength measures were recorded. All 4 groulrs of participants showed similar task- (distance or speed) depwidcnt changes in biphasic EMG activity. Similar modulation of thc initial rate of rise of the EMG, integrated agonist and aqtaglonist EMG activity, as well as their relative timing were observed in all 4 groups. Those results suggest that older individuitls of both genders retain the control strategies for elbow mtivaments used by young individuals. Despite the qualitative similprities in the patterns of muscle activation, the men moved mrire quickly than the women, and younger participants moved mrirc quickly than older participants. Those performance differenced could not be explained in terms of differences in body size and xtrength alone. K6.y words: aging. arm movements, EMG, gender, motor control

W

hen healthy young adults make rapid elbow flexion movements, electromyographic (EMG) recordings denirmstrate sequential activation of the agonist and antagonist muscles. Biphasic muscle activity accelerates and decelerates the limb, and the biphasic pattern is sometimes follolwed by a second agonist burst that may function to “clarnp” the limb into position (Hallett & Khoshbin, 1980). The focus in most subsequent studies, which have been basal almost exclusively on young male participants, has been on characterizing the biphasic EMG bursts and how they change with different task parameters (Gottlieb, Cor-

Correspondence address: Aron S. Buchman, Even Shniuel37/5, Ramot 02, Jerusalem 97230, Israel. E-mail address: con&@ wisdom.weizmann.ac.il

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A. S. Buchman, S. Leurgans, G. L. Gottlieb, C.-H. Chen, G. L. Almeida, & D. M. Corcos

Materials and Method Participant Selection Informed consent was obtained in compliance with RushPresbyterian-St. Lukes Medical Center's Human Investigations Committee. The 10 young participants (n = 5 men and 5 women) were 20-35 years old, and the 10 older participants (n = 5 men and 5 women) were more than 65 years old. All participants were right-handed; their detailed medical histories and neurologic examinations were normal.

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Experimental Set-Up and Data Recording Participants were seated, and they abducted their right shoulder 90" and rested their forearm on a horizontal manipulandum that allowed free rotation about the right elbow. They viewed a computer monitor that displayed a cursor, positioned horizontally by the angle of the elbow joint. Zero degrees was defined with the forearm and upper arm forming a right angle. Extension was toward -90" and flexion approached +90". A small elliptical marker on the screen corresponded to the starting position of the limb. A second, broader marker was a target (6" wide) centered at thc desired angular position. Flexion movements displaced Ihc cursor to the desired target. A small elliptical marker on the screen corresponded to the starting position of the limb. A second, broader marker was a target (6" wide) centered at the desired angular position. Flexion movements displaced the cursor to the desired target. An audio tone (lasting 2 s) signaled the participants to initiate the requisite elbow flexion task. The participants were not instructed to initiate movements as quickly as possible, as in reaction time studies. The participants were instructed to make a smooth movement that terminated within the target and not to undershoot or overshoot. They were instructed that once thcy reached the target they were to maintain the cursor in the target for the duration of the audio signal. Once the participants returned to the starting position, there was an 8-s lime delay until the next trial. All of their movements were made with a manipulandum (moment of inertia = 0.1818 N m s'had). Joint angle and acceleration were transduced and low-pass filtered at 30 Hz. Joint velocity was computed from angle. EMG surface electrodes were placed over the bellies of the biceps brachii muscle and the lateral head of the triceps muscles. EMGs were amplified (X 1,600) and band-pass filtered (60-500 Hz). All measured signals were digitized with 12-bit resolution at a rate of l,OoO/s.

Protocol Anthropometric Measurements Height, weight, and arm circumference were measured. We measured upper extremity segments in order to calculate the moment of inertia of the arm (Miller & Nelson, 1973). Muximal Voluntary Isometric Contraction

The manipulandum was locked, the participant was instructed to generate maximal elbow flexor torque and 392

maximal elbow extensor torque as forcefully and as quickly as possible, and three trials were obtained. Movement Tasks Distances. Participants were instructed to move "as fast as possible" to the target. The target was placed at three different distances (72", 54", and 36"). Speeds. Participants were instructed to move to the 36" target in three different movement times (350,450, and 550 ms). Participants received feedback after each trial. The sequence of testing was the same for all participants.

Data Analysis The data were processed, and we used custom software to determine kinematic and EMG parameters. The onset of agonist and antagonist EMG, as well as the onset of acceleration, were marked on the basis of visual identification. Individual trials were included for further analysis if (a) there were no technical artifacts that interfered with marking and processing, (b) the distance moved was within +3"of the target distance, and (c) the movement time was within *lo% of the target time in the speed experiments. We normalized EMG recordings by using results of maximal voluntary contractions (Gottlieb, Corcos, & Agarwal, 1989a). Measures Used to Characterize Biphasic EMG Initial agonist activation (Qso).This measure is the integral of the agonist EMG over the first 30 ms. Agonist burst (QAG). Elbow flexion movements were initiated with a burst of agonist EMG activity, which was integrated from its onset through the time of peak velocity. Antagonist burst ( Q A W ) . The antagonist EMG burst began after the agonist burst and was active for a longer period of time. We calculated the EMG activity of the antagonist burst by integrating from the marked onset of agonist EMG through the time at which velocity had decreased to 5% of its peak value. Latency of the antagonist burst (CANT). The onset of antagonist EMG can be difficult to identify visually, especially for slower movements. Because the movements of the elderly and the young women were slower than those of the young men, visual identification of the onset of muscle activity was sometimes ambiguous. Instead, we calculated the time at which the centroid of the antagonist EMG activity occurred (Gottlieb, 1996). Constructed Slope Sensitivity Parameter

Testing one type of movement would have been sufficient to enable us to compare the effects of age and gender on the patterns of EMG activation. In this study, however, we tested two different tasks (distance and speed) with three levels of variation for each task (three movements per task) to determine the effects of age and gender on the task-dependent changes in EMG patterns. Journal of Motor Behavior

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Effect of Age and Gender on Arm Movements

Predic,firdChanges in EMG Parameters .for Different Moior 'Thsk.sks ure Captured by Slope Analysis

of the 5 participants in each of the four groups arc prcsented in Table 1.

According to the hypothesized task-dependent changes in EM('; measures, different changes in EMG activation are predictcd for the speed and distance movements. To test for the pnisence of those differences, we grouped each of the three blocks of movements for each motor task together (dislance and speed). We used all the accepted trials for each ofthe three blocks of movements within a task to calculate ii separate regression line. For each EMG parameter (Q3,,. (JAG, QAN.,. and CANT), we derived slope measures (one li)r each task) for each participant. For instance, for the EMG parameter Q3[)we calculated the slope of the two following regression lines: (a) 4 3 0 versus distance and (b) Q3" versus speed. Each of those slopes measures summarized the task- (distance or speed) dependent changes in Q,o across three blocks of movements and were compared among the four groups. The use of slope in this context of the analyses should be distinguished from the use of the word dope in regard to the initial rising phases of EMG. To avoid confusion we use here the term sensitivity measure (S) to refer to the slope of the regression line for each task. In Figure 1. we show Q3o data from 1 participant that were used lo depict how the sensitivity measure (SQlo) was derivccl for the three blocks of movements performed at different speeds. The data in part A show how the area of the first 21) ms,and hence the slope, increased with movement speed The data in part B show that the regression line for the t h w blocks of movements had a positive slope (Su30).

SimilaritiesAmong Groups Averaged EMGs for 1 participant from each group (young male, elderly male, young female, and elderly female) are shown in Figure 2. In all four groups similar

0

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Mornrnt ojlnertia and Strength We used the calculated moment of inertia of the forearm and hand as a composite measure of anthropometry. Force or strengrh was based on the measured maximal flexor torque dividcd by the moment arm.

Statis K i C d Analyses All data are presented as means and standard errors, unless otherwise noted. We used logistic regression analysis to determine whether there were an equal number of trials in the six wperimental conditions (three distances and three speeds).We used analysis of variance and covariance on logtransfirmed data to determine whether age and gender mfluenced the scaling of muscle activiation for different movement tasks and whether the scaling was influenced by force or moment of inertia. Analyses were performed both for movements of different distance and different speeds. We applied a Bonferroni correction to the significance level and used an alpha level of .025 (.05/2). With paired t tests, we tested the hypothesis that the sensitivity measures would be equal to zcrt) for movements over different distances and speeds.

Results

Demographics Rventy participants completed the experimental protocol. The ages, heights, weights, and arm moments of inertia December 2000,Vol. 32,No. 4

2

ag

1

+ o -1 -2

B,

B2 Block

B3

FIGURE 1. Calculating Q3"and constructed electroniyographic (EMG) parameters: The derivation of the EMG parameters Qso and SQxlare depicted in this figure,. A. Q3() is the integrated EMG for the first 30 ms of muscle activation. The insert in Figure 1A shows the averaged time series for agonist EMG for three speeds from a single participant. The small area in this figure, which displays the differences in area that are captured by 430,is enlarged in Figure IA. The area of the slowest speed is identified by X. The increased EMG area observed with increasing speed is shown by the additional area observed in areas Y and Z as compared with X. B. The Q30s for the three speeds wcre log-transformed (4301. Q~o2,and Q303) and are plotted in Figure 1B. We determined a regression line in order to fit those data points; and the slope measure SuJ0summarizes the task-dependent changes in Q3(, for the thrce different speeds tested in the speed task.

393

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A. S. Buchman, S. Leurgans, G. L. Gottlieb, C.-H. Chen, G. L. Almeida, & D. M. Corcos

changes in biphasic EMG responses were observed as dislance or speed was varied. The EMG data displayed in Figure 2 were processed; the mean values and the standard errors for the four EMG parameters used to characterize biphasic EMG responses are displayed in Figure 3. It should be noted that to simplify Figure 2, we display EMGs from only two distances and speeds; whereas in Figure 3, three speeds and distances are shown. In all 4 participants, the agonist EMG showed similar rates of rise for movements to different distances (Figure 2, distances), and therefore Qlo did not change with distance (Figure 3A, distances). In contrast, in all 4 participants, initial agonist EMG activity increased with increasing speed (Figure 2, speeds) and that increase was reflected in increases in Q ~ (Figure o 3B, speeds). For all movements, agonist (biceps) EMG area (QAG) increased with distance or with speed (Figure 3C, distances; Figure 3D, speeds). Antagonist (triceps) EMG area (QANT) was relatively constant in the distance experiments (Figure 3E, distances) but increased with speed for all 4 participants (Figure 3F, speeds). Antagonist latency (CANT)was proportional to movement time and increased with distance but decreased with speed (Figure 3G, distances; Figure 3H, speeds).

Similar analyses for SQAG,SQANT, and SCANT showed no significant differences in sensitivity measures between the four groups of participants. SQAG: distance, F(3, 16) = 3.16, p = .053;speed, F(3, 16) = 3.96, p = .027. SQANT: distancc, F(3, 16) = 2.93, p = .07; speed, F(3, 16) = 0.84, p = .49. SCANT: distance, F(3, 16) = 1.64, p = .22; speed, F(3, 16) = 0.28,p = 34. Those analyses suggested that task-dependent changes in biphasic EMGs were similar in all four groups. Are There Consistent Task-Dependent Changes for Movements of Diflerent Distances and Speeds?

Were Sensitivity Measures Similar Among the Four Groups?

We performed analyses to determine whether there were differences in the sensitivity measures among the groups and whether those findings were associated with differences in moment of inertia or force. The estimated values of S Q ~ O did not differ significantly among the four groups for the two tasks at a = 325 for distance, F(3, 16) = 0.33, p = .8, or speed, F(3, 16) = 3.37, p = .045, even after we conducted analyses to isolate the contributions of moment of inertia and force. Those results showed that in all four groups similar changes in SQlowere observed when movement distance and speed were varied, and those changes were not influenced by age or gender.

We also performed statistical analyses to determine whether the slope of the four different sensitivity measures differed from zero for the different motor tasks. If the mean slope is not different from zero, then it means that the sensitivity measure did not change for the given movement task. In contrast, a non-zero slope would mean that the sensitivity measure was changing for the given movement (ask. The four participant groups showed task-dependen t changes in the four EMG parameters similar to thosc observed in the 4 representative participants. As predicted by the dual strategy hypothesis, S Q ~ increased O with faster speeds in the speed task and showed no change for the distance task. The statistical analysis for S Q ~ is O presented in Table 2. One can also see that the other three sensitivity measures showed consistent increases for both tasks, with the exception of SQANT, which did not change in the distance tasks (Table 2).

Differences Among Groups For movements over different distances, the estimated magnitude of maximal velocity (Figure 4) differed significantly among groups, F(3, 16) = 12.00, p = ,0002. Thc women were 126.45”hslower than the men, t(18) = -5.7 I , p = .001, and the older participants were 51.84”/s slower than the younger participants, t(18) = -2.34, p = .031X. There was no significant Age x Gender interaction, t( 18) 2: .25, p = .8. Moment of inertia made a statistically significant contribu-

TABLE 1

Group Characteristics. Demographic Information for the Participants in the Study

Age (yeas) Group Young men Young women Elderly men Elderly women

Height (cm)

Weight (kg)

Moment of inertia (10” Nm . m2)

M

SD

M

SD

M

SD

M

SD

25.8 26.0 69.0 69.8

4.3 3.5 5.6 5.4

178 166 177 162

4.8 8.4 9.6 5.2

77 59 80 70

6.8 5.9 13.6 14.5

15.7 11.8 17.6 13.7

0.21 0.27 0.41 0.38

Note. There were 5 participants in each of the four groups.

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Journal of Motor Behavior

Effect of Age and Gender on Arm Movements

tion 10 the model, but even after isolating the effects of moment (it' inertia the speed differences were still significant. After isolating the effects of moment of inertia on velocity, the women were 86.9"/s slower than the men, t(18) = -3.83. p = .0015, and the older participants were 69.4"/s slowei than the young participants, t( 18) = -3.6, p = .0024.

-__

As expected by design, there were no significant differences between the four groups for speed movements.

Discussion Recent attention has been focused on the changes in body composition and decreased muscle function that occur with

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FIGURE 2. Biphasic electromyographic (EMG) responses in the young and the older healthy participants. Averaged EMGs for 1 participant from each of the four groups are shijwn for both motor tasks (young man, elderly man, y t m g woman, and elderly woman).The EMGs from all 4 participants show similar rates of rise of agonist EMG (Q30) for the different distances that are displayed. In contrast, agonist EMGs diverged early in the come of the movement in the speed movements. Analyses of EMG parameters suggcsted that all four groups showed similar biphasic EMG responses for the tasks that were tested.

I I

FIGURE 3. Quantifying electromyographic (EMG) responses of the young and the older healthy participants. The EMG data from the 4 participants displayed in Figure 2 were processed, and the mean and standard error of the four parameters used to characterize biphasic EMG responses are shown. All 4 participants showed similar initial rates of agonist EMG rise (QJo)despite the different target distances (A). In contrast, Q3() increased with increasing speeds in the speed movements (B).In both tasks, one can see that agonist EMG ( Q A ~increased ) with increasing distance (C) or faster speeds (D). In the speed experiments, antagonist EMG (QANT) increased with increasing speeds for all 4 participants (F). Antagonist latency (CAWr) increased with increasing distance (G) and slower speeds

(HI.

December 2000,Vol. 32,No. 4

395

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A. S. Buchman, S.Leurgans, G. L. Gottlieb, C.-H. Chen, G.L. Almeida. & D. M. Corcos

increasing age. The term sarcopenia refers to the loss of inuscle function that is manifested as a loss of normal muscle properties and may often be associated with decreased inuscle bulk (Buchman, Mendes Deleon, & Bennett, 1998). Although changes in body composition are often considcred benign concomitants of aging, the results of recent studies have suggested that decreased muscle mass and slrength are associated with functional impairments that lead to morbidity and disability (Dutta & Hadley, 1995; Evans & Campbell, 1993; Guralnik, Ferrucci, Sinonsick, Salive, &Wallace, 1995; Harris, 1997). In humans, direct information about central neuronal processing is difficult to obtain. Therefore, investigators have relied on kinematic and kinetic descriptions of movement, in conjunction with surface EMG recordings of muscle activation, in an effort to detcrmine the strategies that the central nervous system employs for muscle activation. In a serics of earlicr investigations of healthy young men, Gottlieb and his colleagues (Corcos et al., 1989; Gottlieb et al., 1989a; Cottlieb et al., 1990; Gottlieb et al., 1992) have parameterized EMG bursts and characterized task-dependent changes in EMG activity. They proposed the dual strategy hypothesis to explain the task-dependent changes in EMC activity that they observed in young men. In the present study, both young and older participants exhibited scquential agonist and antagonist muscle activation when performing rapid elbow movements. Those findings are consistent with earlier observations by Hallett and Khoshbin (1980). In a recent study, Seidler-Dobrin, He, and Stelmach (1998) showed that elderly and young participants scale EMGs to increasing loads in a similar fashion. In contrast, Darling, Cooke, and Brown (1989) reported that although older participants demonstrated qualitatively normal agonist muscle bursts, their phasic antagonist bursts showed several abnormalities. In the latter study, when older participants performed rapid elbow movements there was increased tonic activity. The antagonist muscle activity was often absent, and even when antagonist bursts were

identified, the timing of the antagonist burst was abnormal. It could precede or occur simultaneously with the agonist. Seidler-Dobrin and colleagues reported that elderly participants coactivate agonist and antagonist muscles more than younger participants do. One possible explanation for the disparity between the results of the studies is the differences between the characteristics of the participants that were studied. For instance, the group studied by Darling et al. (1989) were older; their ages ranged from 68 to 95 years. The age range of our participants was from 64 to 78 years, and they were in excellent health as determined by neurological examinations. Bennett et al. (1996) have shown that parkinsonism increases from 14.9% in individuals 65-74 years of age to more than 50% for individuals older than 85 years of age. Because one of the signs of parkinsonism is bradykinesia, it is possible that the antagonist abnormalities suggested by Darling may reflect parkinsonian signs rather than just increasing age. Additional studies and considerable care in characterizing the participants that are studied will be needed to determine the clinical-neuroanatomic substrate for changes in muscle activation patterns in aging.

Task-DependentChanges in EMG Patterns All four groups exhibited similar task-dependent changes in EMG as movement distance or speed was varied. The similarities between the four groups can be seen in the averaged EMGs from 4 participants depicted in Figures 2 and 3 When distance alone was increased, the initial rate of EMG rise (Q3d did not change (Figures 2 and 3A, distances) and SQ3"was zero (Table 2). That finding suggests that participants employed a speed-insensitive strategy to control those movements. In contrast, agonist EMGs diverged early in the course of the movements when speed was varied (Figure 2 , speeds) and therefore had different values for Qln (Figure 3B, speeds), and S Q was ~ ~not zero (Table 2). That result suggests that participants used a speed-sensitive strategy to control those movements. The present findings suggest that

TABLE 2 Task-Dependent Changes in EMG Parameters Experiment Distance Speed Distance Speed Distance Speed Distance Speed

Parameter

Mean slope -0.01 1 -0.497 24.97 -15.21 4.03 -14.22 21.31 25.47

Confidence interval 0.076 0.096 7.61 4.34 6.21 5.79 3.87 7.03

f

statistic

Significance

-0.29 8.889 6.87 -7.34 1.36 -5.14 11.53 7.59

.77 ,0001 < .0001 < ,0001 .1905 .0001

< .0001 < ,0001

Note. The mean slope and confidence intervals for all participants are shown for all four parameters for

both motor tasks. The t statistic and significance are also included. Similar results are seen in Figure 3 for the representative participants. See text for an explanation of abbreviations.

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Journal of Motor Behavior

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Effect of Age and Gender on Arm Movements

older participants retain the ability to modulate the input to the mc )toneuron pool depending on whether movement distance or movement speed is varied. All four groups showed similar changes in the EMG parameters Q Aand ~ CANTas movement distance and speed were varied. Agonist EMG area ( Q A ~ was ) larger with increacing distance or with faster speeds (Figure 3C, distance!,; Figure 3D, speeds). Antagonist latency (CANT) increased with increasing distance and slower speeds (Figure 3 ; ,distances; Figure 3H, speeds). Those changes are consibtent with the rules for muscle activation described by the dual strategy hypothesis. Antagonist EMG (QANT) was relatively constant in the distance experiments (Figure 3E, distances) and increased with \peed for all four groups (Figure 3E speeds). That finding is consistent with the dual strategy hypothesis, and increiises in antagonist EMG with speed have also been showti by Mustard and Lee (1987). Changes in antagonist EM m

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FIGURE 4. Peak velocity for threc distances. The mean peak velocities in the distance experiments for each 01 the four groups tested are shown. Three distances (36". 54". and 72') were tested, and a uniform speed command of move as fast as possible was employed. The men were significantly faster, and the older participants were significantly slower than the younger participants.

of this study suggest that gender- and age-related differences in kinematics cannot be explained on the basis of anthropometric differences alone. There have been conflicting reports regarding possible gender-related differences in maximal velocity. In some previous studies, women's maximal velocity for arm tlexion movements (Ives, Kroll, & Bultman, 1993; de Koning, Binkhorst, Vos, & van't Hof, 1985) and maximal rates of isometric force production (Bell & Jacobs, 1986; Bemben, Clasey, & Massey, 1990) have been found to be decreased as compared with those of men. There have been cxceptions to the just-mentioned findings in studies in which no gender-related differences in maximal velocities for knee extension (Housten, Norman, & Froese, 1988) and elbow flexion (Nygaard, Houston, Suzuki, Jorgensen, & Saltin, 1983) have been reported. The results of histopathologic studies have suggested that there are similar proportions of different muscle fiber types in men and women (Nygaard et at., 1983). In light of the force-velocity relationship with muscle fiber composition, in most studies investigators have assumed that there are no intrinsic differences in how men and women activate the motoneuron pool (Thorstensson, Grimby, & Karlsson, 1976). Therelore, in most studies in which gender-related kinematic differences have been reported it has been assumed that those differences derive from anthropometric disparities (men are bigger and stronger and therefore move faster). The results of the present study suggest that anthropometric disparities alone are insufficient to account for gender differences. Additional mechanisms such as longer twitch duration and half-relaxation times (Vandervoort & McComas, I986), longer time to tetany (Lennmarken, Bergman. Larsson, & 397

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A. S. Buchman, S. Leurgans, G. L. Gottlieb, C.-H. Chen, G. L. Almeida, & D. M. Corcos

Larsson, 1985), or differences in agonist-antagonist activation (Ives et al., 1993) may contribute to gender-related kinematic differences. In the present study, maximal arm flexion velocity was slower in older than in young participants. Similar findings have been noted in earlier studies of the arms (Darling et al., 1989) and the lower extremities as well (Larsson et al., 1979). and have been noted in many subsequent studies (Pratt, Abrams, & Chasteen, 1997). The decreased speed in older participants is only one manifestation of declining motor function. Increasing attention has been focused on changes in body composition and muscle function that occur with increasing age (Guo et al., 1997). Although those changes are often considered benign concomitants of aging, the results of recent studies have suggested that decreased muscle mass and strength are associated with functional impairments that lead to morbidity and disability (Evans, 1995; Harris, 1997). Those concerns have led to increased focus on determining the underlying pathophysiology of the changes. In addition to changes in body composition, in both neurophysiologic and histologic studies changes that suggest an ongoing chronic neurogenic process with increasing age have been documented. Muscle biopsies in humans have shown that with increasing age there is evidence of fiberLype grouping consistent with neurogenic atrophy and denervation-reinnervation (Lexell, Henriksson-Larsen, Winblad, & Sjostrom, 1983). Those findings are supported by histologic changes in other components of the motor unit that might contribute to the denervation-reinnervation process observed in muscles. Age-related decreases in the number of myelinated fibers as well as degenerative neuronal changes have been noted in peripheral nerves and ventral root (Kawamura, Okazaki, O’brien, & Dyck, 1977). In postmortem studies in humans, a reduction in the number of anterior horn cells with increasing age has been found (Tomlinson & Irving, 1977). Those histologic changes are complemented by neurophysiologic studies in which an ongoing neurogenic process has been suggested. The chronic neurogenic changes that develop with increasing age may cause older participants to activate their motoneuron pool differently than young participants and may contribute to movement slowing and motor impairment with increasing age (Kamen, Sison, Kuke Du, & Patten, 1995; SeidlerDobrin et al., 1998). A number of lines of research suggest that neural control of movements is different in the elderly and may contribute to movement slowing and decreased muscle function (Grabiner & Enoka, 1995). Force control modulation is different in elderly participants (Galganski, Fuglevand, & Enoka, 1993). With increasing age, power declines to a greater extent than strength, and the decline in power cannot be explained on the basis of decreased muscle mass (Metter, Conwit, Tobin, & Fozard, 1997). It has been suggested that in addition to the changes in the neuromuscular apparatus, changes in its innervation as well as changes in central 398

motor and sensory function may affect the rate of torque development and may contribute to the decreased speed and functional impairment observed in the elderly (Pohl, Winstein, & Fisher, 1996). We conclude, however, that despite changes in motor function that can occur with increasing age, older healthy participants can successfully perform different elbow flexion tasks by using control strategies similar to those that are observed in younger participants. ACKNOWLEDGMENTS This work was supported in part by National Institutes of Heallh and National Science Foundation Grants KO8-NS01575, K04NS01508, R01-NS28127, R01-AR33189, and NSF DMS9023906. We would like to acknowledge the support of the RushPresbyterian-St. Lukes Medical Center Outpatient Neurology Clinic and the Rush Aging Institute for the recruitment of healthy older participants, and Steve Creech for preparing data for analysis.

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