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Experimental Aging Research: An International Journal Devoted to the Scientific Study of the Aging Process Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ uear20
Motor Skill Learning of Concentric and Eccentric Isokinetic Movements in Older Adults Denise M. Connelly, Heather Carnahan, Anthony A. Vandervoort Published online: 11 Nov 2010.
To cite this article: Denise M. Connelly, Heather Carnahan, Anthony A. Vandervoort (2000) Motor Skill Learning of Concentric and Eccentric Isokinetic Movements in Older Adults, Experimental Aging Research: An International Journal Devoted to the Scientific Study of the Aging Process, 26:3, 209-228, DOI: 10.1080/036107300404868
To link to this article: http://dx.doi.org/10.1080/036107300404868
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Motor Skill Learning of Concentric and Eccentric Isokinetic Movements in Older Adults Denise M. Connelly
School of Rehabilitation Therapy, Physical Therapy Programme, Queen’s University, Kingston, Ontario, Canada
Heather Carnahan
Department of Kinesiology, The University of Waterloo, Waterloo, Ontario, Canada
Anthony A. Vandervoort
School of Physical Therapy, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada Neuromuscular adaptation at the onset of resisted exercise can be observed as increases in torque and surface electromyography. T he e†ect of learning the motor task has been hypothesized to contribute to these early increases, especially for older people. T hus, the purpose of this study was to investigate the facilitatory e†ects of practice on motor performance in older adults during short± term isokinetic training of the ankle dorsiÑexors (DF). T wenty± eight men and women (M 5 76.3 6 4.6 years) volunteered for a 2± week, 3± days/week strength training program. T hey were tested in a sitting position on a KinCom isokinetic dynamom± eter at 30, 90, and 180° s ] 1 through 40° of DF movement (concentric and eccentric contractions). Criterion curves of lever arm angle patterns Received 10 August 1998 ; accepted 12 September 1999. This study was supported by the Natural Sciences and Engineering Research Council of Canada and the Royal Canadian Legion and Physiotherapy Foundation of Canada. Address correspondence to Dr. A. A. Vandervoort, Rm. 1400, Elborn College, School of Physical Therapy, The University of Western Ontario, London ON N6G 1H1, Canada. E-mail :
[email protected]
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were cross± correlated with subject± generated angle patterns, showing signiÐcantly better correlations on posttest versus baseline. Smoothness and proÐciency of muscle contraction improved with practice by fewer hesitations in movement and increased ability to change between concen± tric and eccentric muscle contractions. Increased agonist electro± myography and torque were hypothesize d to be secondary to greater neural drive and/or synchronizatio n of motor unit Ðring rate and/or recruitment during maximal voluntary contraction, improved coordi± nation, and adapted neural control of concentric and eccentric muscle contraction.
Research on aging and the learning of motor skills has not received much attention (Anshel, 1989 ; Swanson & Lee, 1992 ; Williams, 1989). Further, research about the learning involved in maximizing torque pro± duction at various velocities of movement during a motor skill has not been addressed to date. Many components of the motor system that a Œect movement abilities are reduced with aging, including total muscle mass, amount of fast twitch muscle, number of motoneurons, nerve con± duction velocity, muscle metabolic rate, overall strength, speed of move± ment, perceptual skills, memory/retention, vision, proprioception, and anticipation (Williams, 1989). With these age± related losses, an under± standing of the role of learning for increased force production is needed to maximize and maintain muscle strength and functional ability in older individuals. Strength training programs in older adults have been shown to dra± matically increase torque with hypertrophic changes in the muscle occurring at about 8 weeks and beyond (for review, see Porter, Vander± voort, & Lexell, 1995). Because improvements in strength are not sec± ondary to muscle ber hypertrophy in less than 8 weeks of resisted training, the hypothesis has been that early strength improvements must be due to neural factors (see reviews by Enoka, 1997 ; Sale, 1992). Strength is not solely a property of muscle, but rather it is a property of the motor system (Enoka, 1988), that is, the interaction between the nervous and muscle systems. These neurophysiological inuences have been referred to in the literature primarily as ‘‘neural factors.’’ If only muscle ber cross± sectional area, as an example of a muscle property, were the primary factor for increased muscle strength, then muscle length, type of contraction, and velocity of movement during training would not aŒect muscle torque production (Enoka, 1997). However, strength gains are specic to the muscle properties challenged during training. For example, the same increased muscle torque production would be found by all methods of force measurement regardless of the
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strength training regimen. Instead, this specicity suggests that the neural factors are of major importance in strength gain (Enoka, 1997). The skill in weight lifting has been described by Kelso (1982) as assigning maximal force to the muscles in a coordinated manner. If force production is considered to be a motor skill, like any skill, force pro± duction can be improved with practice; it has even been suggested that practice is the single most important factor for learning a skill (Schmidt, 1988). Practice refers to the attainment of skill through repetition of per± formance (Southard & Higgins, 1987). Sometimes millions of repetitions are required before a novice attains the level of a skilled expert in a task such as cigar making, violin playing, or baseball throwing (Kottke, Halpern, Easton, Ozel, & Burrill, 1978). Acquisition of a motor skill through the use of practice in the per± formance of everyday life tasks, such as kicking a soccer ball (Anderson & Sidaway, 1994) or lifting free weights in a bench press (AlmaÊsbakk & HoŒ, 1996), has been investigated in young adults having no previous experience in the sport for which they were study subjects. The improve± ment in the performance of a soccer kick resulted from a change in coor± dination, between hip and knee joint angle movements, rather than simply an increase in speed of the entire movement pattern (Anderson & Sidaway, 1994). In addition, in the early phase of the bench press strength training program, improved coordination or learning the bench press movement increased torque production (AlmaÊsbakk & HoŒ, 1996). However, improved coordination alone, at the onset of exercise, did not maximize strength gains, and weighted movement was required for optimal strength gain (AlmaÊsbakk & HoŒ, 1996). No previous studies of older adults have investigated changes in movement performance and torque production with training in the lower limb. Studies investigating movement performance in young adults included measuring the ability to negotiate level changes during walking (McFadyen & Carnahan, 1997) and coordinated upper extremity move± ments during walking (Carnahan, McFadyen, Cockell, & Halverson, 1996). Previous studies using only motor performance as an outcome measure indicated that old adults could make improvements with repeti± tions of a task in the lower limb (Meeuwsen, Sawicki, & Stelmach, 1993) and in the upper limb (Wishart & Lee, 1997). Repetitive practice of foot position matching, for example, resulted in improved performance con± sistency, decreased performance errors, and increased speed of decision making (Meeuwsen et al., 1993). Most previous studies investigating torque both as the measured quantity and the criterion have been related to isometric or static torque contractions (Newell & Carlton, 1985 ; Schmidt, Zelaznik, Hawkins, Franks, & Quinn, 1979 ; Sherwood & Schmidt, 1980 ; Whitley & Elliot, 1968). A study employing dynamic contractions investigated variations in the accuracy of the force produced by elbow exor muscles during
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feedback± assisted acquisition of a motor skill (Yizhar & Dvir, 1995). An isokinetic dynamometer was used to practice concentric elbow exion± extension continuous movement cycles over a 3± day training period in young adults. Strength changes were not highlighted in this study. The authors had purposefully limited practice of the movement to a 3± day study period to investigate acquisition of the skill and the involvement of the nervous system, rather than hypertrophic factors, in the muscular system. Measures of strength can be inuenced by a variety of neurophysio± logical processes (Enoka, 1997). Improved coordination (Keen, Yue, & Enoka, 1994 ; Rutherford & Jones, 1986), increased attention to proprio± ceptive feedback (Cordo, Bevan, Gurnkel, Carlton, Carlton, & Kerr, 1995 ; Meeuwsen et al. 1993), and increased neural activity (electromyography) (Moritani & deVries, 1979, 1980) are cited neural factors involved in strength gain. Smith’s review (1974) noted an interest in the role of the nervous system in strength as far back as 1955 when studies suggested that learning may be associated with increases in strength. In the review, the author compared previous plotted curves of learning variables (i.e., time, number of errors made) which had been plotted from the performance of subjects who practiced laboratory tasks for motor learning under massed± and distributed± practice and fatigue conditions. The results showed that there were close similarities between strength± learning curves and laboratory motor± learning curves (Smith, 1974). Thus, borrowing from motor learning paradigms, it was possible to quantify coordination and performance of a task. The purpose of this study was to investigate the facilitatory eŒects of practice on motor per± formance in older adults with short± term training of the ankle dorsiexor muscle group during isokinetic concentric and eccentric muscle actions.
METHODS Subjects Subjects were recruited from a community health club and a uni± versity alumni social group. None had previously taken part in isokinetic resisted exercise training programs, nor were currently strength training the dorsiexor (DF) or plantar exor (PF) muscle groups. All subjects maintained their habitual level of activity during the training program. Participation in recreational physical activity was quantied for each subject using the Yale Physical Activity Survey for Older Adults (Dipietro, Casperson, Ostfeld, & Nadel, 1993). Exclusion criteria for subject participation in the study included neurologic or orthopedic pathologies of the lower limb, recent bedrest for more than 3 days,
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uncontrolled hypertension or angina, requirement for supplemental oxygen, or recent (, 3 months) myocardial infarction. It was determined that all subjects were right leg dominant after answering the question ‘‘with which leg would you kick a ball ?’’ (Carnahan & Elliott, 1987). The subjects gave their informed consent and all procedures were approved by the University Review Board for Health Sciences Research Involving Human Subjects. Subject characteristics for age, height, mass, and recreational activity level are compiled in Table 1.
Isokinetic Exercise Training Subjects participated in a short± term isokinetic exercise program of the ankle joint. Six training sessions were completed with the right ankle in 2 weeks ; 3 sessions per week on a KinCom isokinetic dynamometer (KinCom 500H, Chattecx Corporation, Chattanooga Group Inc., TN). A pre± and postexercise session was completed using the subjects’ left leg as the control. Subjects were seated in the dynamometer with the hip exed about 30° , the knee at an angle of 90° , and the ankle in the neutral position. The rotational axis of the dynamometer was positioned to be coaxial with the ankle joint axis during movements of plantar and dorsi± exion (bular malleolus) (Norkin & Levangie, 1983). The resistance pad was positioned directly under the head of the metatarsals. Extraneous leg and trunk movement was eliminated by a seat belt xed rmly across the hips, around the thighs and chair seat, and the knee was supported and strapped in the exed position. The nonexercising leg was supported by a stool for subject comfort during ankle exercise. Subjects were instructed to keep their arms folded across their chest during the muscle contractions. Subjects exercised through their pain± free active ankle range of motion which was determined with the foot xed in the dynamometer and the leg in the training position. Pain± free active range of motion was mea± sured from a neutral ankle position to a plantar exed position using a large 360° universal goniometer. Subject ankle range of movement was
TABLE 1 Summary of Subject Characteristics Age (years)
Height (cm)
Mass (kg)
Recreational activity (hours/week)
74.7 6 3.8 (70È83) 78.2 6 4.9 (71È85)
158.3 6 7.5 (142.1È175.0) 169.8 6 7.0 (156.8È178.9)
67.3 6 11.8 (47.1È87.0) 75.6 6 11.1 (61.1È99.5)
4.1 6 3.5 (0È10.0) 5.0 6 3.7 (1.0È14.0)
Subjects Women (n 5 Men (n 5
13)
15)
Note. Values are mean 6
standard deviation (range) for each subject characteristic.
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established during the initial training session, entered into the KinCom program, and standardized for the duration of training. The extremes of joint angle were subject specic (pain± free range of motion) and deter± mined the range required for muscle contraction during training. These extremes of joint angle range were set as the stop and start angles for the range of movement required for ankle exercise. The subject had to com± plete the full range of the contraction before the KinCom lever arm would permit a change in direction, or turn, to begin the next contrac± tion in the sequence. At the beginning of each training session, a warm± up of six repetitions at a minimal level of exertion of reciprocal ankle concentric DF and PF was completed at each of the three velocities. On the rst training day, subjects were given a verbal description of the movements required and a practice trial for concentric and eccentric DF muscle contractions, at each velocity to be tested, just prior to recording the actual training eŒort. The instruction to the subject for both concentric and eccentric DF movement was to ‘‘pull up.’’ As a safety feature of the isokinetic dynamometer, the lever arm would only move if pressure was exerted on the load cell. The subject must produce force against the lever arm to produce a smooth, continuous joint angle trace. The two types of muscle contraction, concentric and eccentric DF, and the joint angle trace produced on the computer screen were explained during these trial contractions. The instructions to the subject during training were ‘‘pull as hard and as fast as you can.’’ The subject’s foot was then positioned for exercise at the start angle and the command ‘‘ready, go’’ was given. No verbal encouragement was given by the investigator during the exercise con± traction sequences. The investigator did, however, coach subjects for eŒective breathing during the maximal eŒorts to avoid increased intra± thoracic pressures. Assurance was sought from the subjects periodically during rest periods to reinforce that they were performing maximal vol± untary contractions (MVC) at each velocity throughout their full range of motion. Visual feedback to the subject was the simultaneous joint angle movement trace on the KinCom computer screen during the ankle contraction sequences. The movement trace was correct for directional feedback in that concentric or upward movement of the foot correspond± ed with the upward drawing of the joint angle line trace and eccentric or downward foot movement was a downward line drawing. Four sets of six maximum voluntary contractions were performed during each training session for both concentric and eccentric DF muscle contraction at each of the three velocities : 30, 90, and 180° s ] 1. Subjects performed the six repetitions per set as a continuous series of DF muscle actions, as concentric then eccentric contraction sequences at all three velocities. Sets of concentric± eccentric DF were completed in ascending order of velocity, 30, 90, and 180° s ] 1, throughout the training program.
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Sufficient rest was given between exercise sets to minimize fatigue during the training session.
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Measurement of Muscle Torque All KinCom les were converted to ASCII les and imported into the data analysis program SigmaStat (SPSS, San Rafael CA). Ankle DF torque data from the KinCom were mathematically ltered for velocity (30 or 90 or 180 6 1° s ] 1) to eliminate impact artefact secondary to accel± eration or deceleration and to ensure a velocity± specic torque value for the contraction. Concentric and eccentric DF peak torque values were compiled for 30, 90, and 180° s ] 1 of exercise.
Measurements of Electromyography (EMG) Agonist DF (tibialis anterior) and antagonist PF (soleus) EMG was collected using 10 mm diameter Beckman silverÈsilver chloride elec± trodes (Sensormedics Corporation, Anaheim, CA) with a 16± channel, ber optic, biological amplier (total gain 2000 3 , low± pass lter 1000 Hz, high± pass lter 10 Hz, CMRR . 70 db at 100 Hz, sampling fre± quency 2500 Hz) custom± made system. A custom± designed software program sampled the digital EMG signal at 200 Hz/channel before EMG integration was completed. Skin preparation, electrode placement, and recording procedures were standardized over the study period. Alcohol pads were used to clean the skin, followed by abrasion with a terry cloth towel and conducting gel until an erythema was produced. Two surface electrodes were positioned and xed, straddling the center of the muscle bulk of the tibialis anterior at end range of movement during a maximal voluntary toe raise contrac± tion. PF recording electrodes were positioned during a heel raise over the soleus muscle, just lateral to the midline of the leg, inferior to the bulk of the gastrocnemius muscle. The ground electrode for the DF channel was xed over the head of the bula. A ground electrode xed over the lateral malleolus completed the PF channel circuit. The elec± trode positions were measured to minimize variability in electrode place± ment when retesting.
Criterion Curve Correlation with SubjectsÏ Curves Criterion curves of six contraction repetitions were generated by the KinCom at each of 30, 90, and 180° s ] 1. The range of motion to be completed and the velocity of the movement were programmed into the KinCom computer; with the lever arm moved to the start position and running the exercise protocol generated the criterion curves at the three velocities. Subject± generated joint angle curves at 30, 90, and 180° s ] 1
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were cross± correlated with the corresponding criterion curve. A corre± lation value (r) of best t was generated for subjects pre± and post± training at each velocity. Examples of a subject’s curves at 30, 90, and 180° s ] 1 are plotted against the computer criterion curves for 30, 90, and 180° s ] 1 in Figure 1.
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Plateau and Turn Durations Stops during concentric and eccentric contractions, between the start and stop preset subject± specic joint angles, were labeled as plateaus. Recording continued during an interruption in the contraction and was seen on the joint angle trace as a straight line until the subject resumed the movement. Various durations were identied and measured on the joint angle trace (Figure 1). The change± over, or turns, between the concentric± eccentric and eccentric± concentric phases of DF muscle contraction were also quanti± ed. Any interruption between muscle contraction types was seen as an extended line at the extremes or turns of the joint angle trace (Figure 1). The durations of turns were quantied similarly as the plateaus described previously.
Plantar Flexor Cocontraction Index Antagonist muscle (soleus) activity during concentric and eccentric DF was quantied to determine the eŒect of practice on cocontraction.
FIGURE 1 Subject± generated joint angle curves of concentric± eccentric dorsi±
exor muscle contractions recorded at (A) 30°s ] 1 , (B) 90° s ] 1 and (C) 180°s ] 1 are shown for a 76± year± old subject. The subject’s curves for pretest and posttest are plotted against the computer criterion curves for concentric± eccentric muscle contractions at each movement velocity (note: time scales vary). Several charac± teristics used for quantication of change in performance over time are evident in the subject traces. (1) A stop between the turning points of the contractions, seen as a straight line as time continued, was labeled as a plateau and was measured in milliseconds (ms). A plateau was evident in the eccentric portion of the pretest contraction series at 180° s ] 1 in the fourth movement. (2) Prolonged turns between contraction phases, where direction of the lever arm changed, were mea± sured in ms. The turn, in the same subject trace, at the fth movement was between the concentric and eccentric phases of muscle contraction. At this point in the contraction series, the dorsiexor muscle group has just completed a full range of concentric muscle work and is at its most shortened length before start± ing the eccentric muscle contraction phase. (3) The whole subject curve and the computer curve, for example at 30°s ] 1 , were cross± correlated to determine a correlation value of best t. In this example, the cross± correlation value was r 5 .54.
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The same integral of time to determine the EMG for agonist CONC and ECC DF was used to calculate antagonist PF EMG. The same time frame was used to calculate agonist PF EMG activity. Cocontraction index for CONC and ECC DF at each of the three velocities of move± ment was calculated by the equation : cocontraction index 5 PF agonist EMG/PF antagonist EMG.
Statistical Analysis Separate repeated± measures analysis of variance tests were run for the control leg and for the experimental leg data. The dependent measures for the right leg were peak torque, electromyography, plateau duration, turn duration, and antagonist cocontraction index. The dependent mea± sures assessed for the left leg were peak torque, electromyography, and criterion curve correlation. For each of these dependent measures, the three± factor design included Time of Test (pretest, posttest) by Type of Contraction (concentric, eccentric) by Movement Velocity (30, 90, 180° s ] 1). The remaining dependent measure, criterion curve correlation, was analyzed by a repeated± measures analysis of variance with two factors, Time of Test (pretest, posttest) by Movement Velocity (30, 90 180° s ] 1). Based on an a priori hypothesis that if peak torque values increased with practice, the EMG value would also increase, a one± tailed test of signicance was used to assess EMG change with time of test. Tukey’s posthoc tests were used for posthoc comparison of eŒects signi± cant at p , .05. Values for all group data are indicated as the mean and standard deviation (M 6 SD).
RESULTS Isokinetic Torque With time, comparing pretest to posttest, the mean concentric torque (across movement speeds) increased by 27% and mean eccentric torque increased by 20%. For isokinetic torque there were signicant main eŒects for time (F(1,27) 5 49.28, p , .01), velocity (F(2,54) 5 57.79, p , .01), and type of contraction (F(1,27) 5 328.18, p , .01). There was a signicant interaction with velocity by type of contraction (F(2,54) 5 85.78, p , .01). Posthoc Tukey’s tests revealed signicant diŒerences between eccentric and concentric torque production (eccentric . concentric) at each of the movement speeds : 30° s ] 1 (24.3 6 7.4, 19.9 6 6.8 N É m), 90° s ] 1 (25.6 6 7.7, 16.8 6 5.5 N É m), and 180° s ] 1 (24.7 6 7.5, 8.1 6 3.6 N É m). For concentric contractions torque production decreased as movement speed increased, whereas eccentric torque remained stable with increasing movement velocity. For concen±
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tric torque production (N É m), signicant diŒerences were found between 30° s ] 1 (19.9 6 6.8) vs. 90° s ] 1 (16.8 6 5.5), 30° s ] 1 vs. 180° s ] 1 (8.1 6 3.6), and 90 vs. 180° s ] 1 movement speeds. However, for eccentric contrac± tions there were no diŒerences in torque found across movement speeds.
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Plateau Durations There were signicant main eŒects for both time (F(1,27) 5 10.67, p , .01) and velocity (F(2,54) 5 37.29, p , .01), in addition to a time± by± velocity interaction (F(2,54) 5 7.96, p , .01) when the plateaus were analyzed. At the 30° s ] 1 speed, the subjects had shorter plateau durations on posttest after practice, in comparison to pretest (Figure 2). For pretest and posttest, a similar pattern in plateau duration was maintained across movement speeds. Signicantly longer plateaus were found at the 30° s ] 1
FIGURE 2 Group plateau durations, mean 6
standard deviation, for the dorsi± exors were plotted against movement velocity. Submovements or stop± start pauses were signicantly decreased with practice at 30° s ] 1 .
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FIGURE 3 Turn duration between dorsiexor concentric± eccentric (Conc± Ecc) and eccentric± concentric (Ecc± Conc) muscle contractions were combined across movement velocities to generate group mean 6 standard deviation. At pretest, subjects’ performances of the turn transition in a shortened muscle length was signicantly worse than when completing the turn from a lengthened or eccentric muscle contraction (p , .05). Training signicantly improved subjects’ ability to complete Conc± Ecc movements in a shortened muscle length because the diŒer± ence in performance between Conc± Ecc and Ecc± Conc was not found on posttest measurement.
speed in comparison to the plateau durations at 90° s ] 1 and at 180° s ] 1 . For both the pretest and the posttest, plateau duration scores decreased with increases in movement speed.
Turn Durations There were signicant main eŒects for time (F(1,27) 5 6.83, p , .05), velocity (F(2,54) 5 13.50, p , .01), and direction of turning (F(1,27)
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FIGURE 4 Turn duration, quantifying the transition time between concentric (Conc) and eccentric (Ecc) contraction phases, was combined across test occasion to generate group mean 6 standard deviation. Conc± Ecc and Ecc± Conc turn durations were plotted against movement velocity. Signicantly longer turns (p , .05) were found for the Conc± Ecc muscle contraction sequence, compared to Ecc± Conc sequence, at all tested velocities (30, 90, and 180°s ] 1 ). 5 32.49, p , .01) in the turn duration analysis. In addition, signicant, interactions existed for time± by± turn direction (F(1,27) 5 4.53, p , .05) and velocity± by± turn direction (F(2,54) 5 6.25, p , .01). The time± by± turn direction interaction showed pretest turn scores were signicantly diŒer± ent between turn directions, that is, the time between DF concentric± eccentric muscle actions was longer than the time between DF eccentric± concentric muscle contraction sequences. This time diŒerence, or ability to change from concentric to eccentric DF muscle contraction
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type (concentric± eccentric vs. eccentric± concentric), was eliminated after practice on the posttest measurement (Figure 3). In addition, for the velocity± by± turn direction interaction, posthoc testing demonstrated that turn duration was signicantly longer when moving from a concentric to an eccentric muscle contraction than an eccentric to a concentric contraction combination at 30° s ] 1 and 90° s ] 1 . No diŒerence in turn duration was found for contraction order at 180° s ] 1. Also, turn durations for the concentric± eccentric contraction order were signicantly diŒerent between velocities 30° s ] 1 vs. 90° s ] 1 , and 30° s ] 1 vs. 180° s ] 1 (Figure 4). No di Œerences were found across velocity for eccentric± concentric contractions.
Criterion Curve Correlation There was a signicant main eŒect for time when the correlation values were analyzed (F(1,26) 5 5.56, p , .05). The shape of the subject± generated curves improved to better match the computer criterion curves after repeated practice. Higher correlations were found in the posttest (M 5 .72, SD 5 .20) compared to the pretest (M 5 .64, SD 5 .23). This eŒect did not interact with movement velocity.
Electromyography The integrated EMG recordings of the tibialis anterior (TA) showed signicant main eŒects for time (F(1,25) 5 4.04, p , .05), velocity (F(2,50) 5 55.73, p , .01), and type of contraction (F(1,25) 5 10.06, p , .01), in addition to a time± by± velocity± contraction interaction (F(2,50) 5 3.73, p , .05). The source of the interaction was that the concentric and eccen± tric contractions di Œered for the post± 30° s ] 1 condition only. All other concentric/eccentric comparisons across velocity were the same. Posthoc tests indicated that EMG during concentric and eccentric contractions at 30, 90, and 180° s ] 1 was signicantly increased after 2 weeks of practicing the isokinetic contractions (Table 2).
Plantar Flexor Cocontraction Integrated EMG of the PF showed a main eŒect for the type of con± traction (F(1,27) 5 6.06, p , .05). PF activity was signicantly higher during eccentric vs. concentric DF contraction regardless of movement velocity (4.4 6 3.7 . 3.6 6 2.0). No eŒects for time of test were found in the analysis (p 5 .93).
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TABLE 2 Mean Concentric and Eccentric Dorsiexor Surface
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Electromyography Values for Pretest and Posttest at 30, 90, and 180°s ] 1 Dorsiexor muscle contraction
Velocity of contraction (° s ] 1)
Concentric
30 90 180 30 90 180
Eccentric
Note. Values are mean 6
Pretest (mV É s) 707.3 6 258.0 6 148.3 6 642.2 6 237.4 6 139.3 6
713.3 258.7 150.9 647.1 250.8 150.7
Posttest (mV É s) 1128.8 6 435.6 6 243.8 6 897.0 6 390.2 6 225.7 6
standard deviation. * p ,
930.1 * 406.8 * 210.0 * 665.2 * 338.6 * 184.4 *
.05.
Left Control Leg No signicant di Œerences were found pretest vs. posttest in left leg control torque values (p 5 .38), agonist electromyography (p 5 .70), or criterion curve correlation (p 5 .89) with subject± generated curves.
DISCUSSION The main results of this study suggest signicant improvement in the movement pattern of the subject± generated curves and the improved ‘smoothness’ of movement with practice. Subject± generated movement patterns were matched signicantly better to the criterion curve with repetitions of the motor task. Stops during movements, as measured by plateau and turn durations, were signicantly reduced with practice. The plateaus and turns that comprised the resistance exercise can be concep± tualized as being similar to the movement patterns produced by a gener± alized motor program (Schmidt, 1988). Thus, the more practiced the skill, the more invariant the movement patterns become. Similar improvement in movement performance after training was found in a small hand muscle (Keen et al., 1994). Kinematic techniques were used to quantify the uency and pro± ciency of each of the types of muscle movement, concentric and eccen± tric. Stopping or hesitation, resulting in plateaus during concentric and eccentric muscle action, was signicantly reduced with practice. The purpose of stops or submovements during a motor task is thought to be to home in on the target (Meyer, Smith, Kornblum, Abrams, & Wright, 1991). Hesitancy has also been used as an indicator of increased reliance on guidance during movement execution (Morgan, Phillips, Bradshaw,
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Mattingley, Iansek, & Bradshaw, 1994). Hesitancy in this study may rep± resent a combination of the need for guidance during the task, although no target was given, and the subjects’ abilities to perform and monitor their production of an eccentric or concentric muscle contraction in a novel situation. Plateau duration or hesitancy in this study was greater at slower movements than at faster speeds as previously found by Morgan et al. (1994). Movements by the elderly have been described as more variable and more jerky than those of the young (Connelly, Carna± han, Vandervoort, & Inglis, 1996 ; Cooke, Brown, & Cunningham, 1989), a pattern which may reect an underlying loss of certainty of voluntary movement (Morgan et al., 1994). The slowness of movement, with several pauses or plateaus during the movement, may reect carefulness and uncertainty. The limits of move± ment in this study were mechanically imposed by the KinCom lever arm, therefore, a trade± oŒ for speed of movement or accuracy may only have been a factor at the initial practice sessions, that is as the subject did not have to ‘‘target’’ the movement endpoints to change direction of the lever arm. Eliminating the subject’s responsibility for the movement range focused the subject on the task of making concentric and eccentric smooth, quick muscle contractions. However, the subject had to become aware, by repetition, of the fact that simply maintaining a high level of torque would facilitate the change in direction of the lever arm and ease the performance of the turn. The training program reected the acquisi± tion of skill and that training improved muscle activation at the trained velocities. The link between the measure of strength (peak torque) and skill (kinematics) is that for maximal voluntary contraction, the active state must be prolonged, allowing time for the maximal number of muscle bers to contract, the uptake of the passive elastic component, and the synchronization of synergist muscles, all of which contribute to torque development. Short, interrupted, jerky contractions require the muscle bers to stop, slacken, and then to contract again before contin± uing the motion. When the movement is interrupted, the cumulative eŒect of the number and simultaneous contribution of contracting bers and uptake of the passive tissues is lost, resulting in lower peak torque values. The attainment of maximal concentric or eccentric peak torque therefore is during smooth, continuous movements. Learning must have been a large factor in the early strength gains across velocity, as reected by the improved correlation between the subject curve and the criterion curve after practice. Practice had a signicant eŒect in reducing the mean duration of stop± start incidents during muscle contraction. Thus, the kinematic prole (joint angle trajectory) of the movements became more stereotyped as a result of practice. Essentially, the subjects were practicing how to do a maximal voluntary concentric and eccentric DF muscle contraction at diŒerent velocities. Practice± related decreases in movement variability
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support the view that motor performance can be enhanced in the elderly through repetition (Brown, 1996). Improved control over concentric and eccentric muscle actions of exor digitorum interosseus at 80% maximum voluntary contraction (MVC) during self± paced contraction speed occurred by 4 weeks in the training study of Keen et al. (1994), as measured by a reduced coefficient of variation for the force uctuations. This reduction occurred before the time of signicant hypertrophy and was thought to be secondary to increased coordination (Keen et al., 1994). In addition to reducing variability of the movement trajectory, extended practice modied the quantity of movement± related muscle activity in the agonist, but not the antagonist. Increased agonist EMG indicated increased neural activity in the prime mover muscle with practice, but no change was found in the coac± tivation activity of the PF group. Increased neural activity represents factors associated with motor unit activity : recruitment and modulation of discharge frequency. Short± term training, less than 5 weeks with typi± cally two to three training sessions per week, is generally regarded as too short to induce gross morphological changes in muscle (Hakkinen, Alen, & Komi, 1985 ; Moritani & deVries, 1979 ; Rutherford & Jones, 1986). Neural factors were then hypothesized to be primarily responsible for increased strength in this short time frame. The methods of the present study were unable to lter out motor unit (MU) recruitment and dis± charge rate changes with practice. However, Milner± Brown, Stein, and Lee (1975) reported signicantly greater incidence of MU synchro± nization in weight lifters compared to control subjects. The implications were that torque production was further enhanced by MU synchro± nization. Also, Milner± Brown et al. (1975) demonstrated synchronization of MU ring after daily exercise with maximal contractions. They con± cluded that synchronization may be the result of regularly using muscles at or near maximal voluntary forces. Schmied, Ivarsson, and Fetz (1993) also found with short± term voluntary training of the extensor digitorum muscle that increased synchronization of motor units occurred with vol± untary isometric contractions. The increased agonist EMG in this study may reect, therefore, increased motor unit recruitment and/or increased rate of motor unit ring and/or increased synchronization of motor unit ring. Turning delays were eliminated with practice. Delays between move± ments were hypothesized to represent intervals for movement planning in the control of motion (Goggin & Stelmach, 1990). The delays in turning were larger at 30° s ] 1 and 90° s ] 1 when moving from a concen± tric to an eccentric muscle action. No delays were found between an eccentric± concentric sequence, and the hypothesis was that the muscle length at this point was optimal for torque production (the DF muscle on a stretch in the plantarexed position). Also, the neural strategies may diŒer to control an eccentric contraction vs. a concentric contraction
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(Enoka, 1996), and perhaps the aging adult muscle contraction control system adapts more slowly to training. In addition, Laidlaw, Bilodeau, and Enoka (1996) found that elderly subjects had the greatest difficulty at the transition from the shortening to the lengthening contraction, as seen by a sudden decrease in force at the beginning of the lengthening phase of rst dorsal interosseous (FDI) muscle eccentric abduction. These FDI muscle contractions were submaximal in an approximate 10° range of motion in a slow velocity position± tracking task. However, our study showed that with repetition, older adults must adapt eventually because the turns became smoother and faster. The neural mechanisms that might contribute to the training± induced increase in torque during isokinetic ankle concentric and eccentric contractions achieved by older adults in this study include: (i) greater muscle activation as indicated by increased agonist EMG, perhaps due to greater neural drive and/or syn± chronization of MU ring rate and/or recruitment during maximal vol± untary contraction ; (ii) improved coordination as demonstrated by reduced submovements and improved movement smoothness ; and (iii) adaptation to neural control strategies of concentric and eccentric muscle actions as found by improved transitions during turns. In summary, this study used kinematic analysis and surface EMG to investigate the mechanisms for the early strength changes observed in resistance exercise. Thus, learning of the motor skill is one of the neural factors hypothesized to be primarily responsible for muscle strength improvement during dynamic movement at the onset of resisted exercise programs for older adults.
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