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Older human subjects experience less of a change in EMG frequency content during a fatiguing contraction
674
Abstracts
OLDER HUMAN SUBJECTS EXPERIENCE DURING A FATIGUING CONTRACTION
LESS OF A CHANGE
IN EMG FREQUENCY
CONTENT
Roger M. Enoka, Andrew J. Fuglevand, and Patricia M. Barreto Departments of Exercise & Sport Sciences and Physiology, University of Arizona, Tucson, AZ 85721. Although human muscle does not become more fatigable with age, them is a substantial age-related reorganizauon of motor units that may influence the neuromuscular adaptations that occur with fatigue. The purpose of this study was to compare the age-related changes in the frequency content of the EMG during a fahgumg contraction. The performance of the first dorsal interosseus muscle of older subjects (64-79 yrs, n = 15) was compared to that of younger adults (18-49 yrs, n = 22) for an isometric abduction task in which the index finger sustained a force of 35% MVC to exhaustion. There was no difference between the two groups (old vs young) in terms of endurance time (194&84 vs 204f85 s). Although tbe final average EMG increased from 35% MVC to a similar final value for the two groups during the fatiguing contraction (final values of 57.4zh14.2 vs 55.1f16.2 %MVC), tbe mean power frequency declined less for the older subjects (55.*12-l vs 43.ti6.1 % initial, p < 0.05). Furthermore, these changes in surface EMG were accompanied by parallel changes in the direct motor response to stimulation (M wave) that included a similar decline in amplitude (72.3f19.0 vs 77.1f19.7 % initial) but less of an increase in duration (117.6k33.7 vs 138.5k23.5 8 initial; p < 0.05). These data indicate that although the old and young subjects had a similar endurance time, the older subjects experienced less of a change in the mechanisms underlying the reduction in propagation velocity of muscle action potentials. Supported by USPHS grants AG 0900, NS 20544, NS 07039, and NS 08634.
SINGLE
MUSCLE
CONTRACTIONS CAN PRODUCE MUSCLE-TENDON INJURY
, James G. Hay*, Warren G. Darling” IDepartment of Biomedical Engineering and Applied Therapeutics, The Cleveland Clinic Foundation, Cleveland, OH 44195, USA ZDepartment of Exercise Science, The University of Iowa, Iowa City, IA 52242, USA JonW. Feuerbachl
The purpose of this study was to examine the effect of a single eccentric contraction on the incidence and severity of muscle-tendon injury in a rat model. Other investigators have modeled muscle injury by: (1) lengthening a muscle-tendon unit beyond its physiological length limit until the muscle ruptured; or (2) subjecting the muscle to repeated slow eccentric contractions at physiological lengths. The eccentric contractions of the present study were maintained within physiological limits of length and velocity. Injury was defined functionally as a statistically significant reduction in isometric force production. Each of 30 rats was stabilized in a testing jig mounted on a Materials Test System and the distal tendon of the TA was released from its insertion and clamped to a load cell. Pre- and post-test isometric contractions were performed by stimulating the sciatic nerve and fixing the initial length of the TA fibers at 90% L,__. A test eccentric contraction was performed by stimulating the sciatic nerve and lengthening the TA from the initial length to /s) did not suffer a significantly L . TAs that were subjected to a slow eccentric contraction (ILL a&Ftz loss in isometric force production than that of the correspondini?ontrols. TAs that were subjected to /s) experienced a significant loss in isometric force production (7.9%) a fast eccentric contraction (16 L As p
POSTER SESSION-ANATOMICAL
JOINTS
FACTORS AFFECTING ELASTIC ENERGY RECOVERY IN STRETCH-SHORTENING MOVEMENTS and Jack M. Winters2 Michael Mungiolel 2 Exercise L Sport Research Inst. 2Chemica1, Bio, and Materials Engrg., Arizona State finiversity, Tempe, AZ 85287, USA Past exnerimental research which have investigated factors influencing series elasti; component (SEC) energy recovered during the concentric phase of stretch-shortening cycle (SSC) movements have resulted in equivocal conclusions when attempting to extrapolate external measures to internal phenomena. In this study, a validated muscle model was used to investigate the influence of fiber type proportion, eccentric phase movement amplitude, and SEC compliance on an ankle plantarflexion movement task and the SEC enercrv recovered. When compared to the nominal case, each of these factors influenced the relative proportions of the contractile component (CC) and SEC to overall muscle power, although performance (joint excursion in a particular time period) was.affected tb a lesser extent. The interaction of the CC and SEC influenced the joint power developed and the amount of elastic energy recovered during the concentric phase. Also, an increase in the SEC energy recovered does not necessarily imply improved performance.
Abstracts
OLDER HUMAN SUBJECTS EXPERIENCE DURING A FATIGUING CONTRACTION
LESS OF A CHANGE
IN EMG FREQUENCY
CONTENT
Roger M. Enoka, Andrew J. Fuglevand, and Patricia M. Barreto Departments of Exercise & Sport Sciences and Physiology, University of Arizona, Tucson, AZ 85721. Although human muscle does not become more fatigable with age, them is a substantial age-related reorganizauon of motor units that may influence the neuromuscular adaptations that occur with fatigue. The purpose of this study was to compare the age-related changes in the frequency content of the EMG during a fahgumg contraction. The performance of the first dorsal interosseus muscle of older subjects (64-79 yrs, n = 15) was compared to that of younger adults (18-49 yrs, n = 22) for an isometric abduction task in which the index finger sustained a force of 35% MVC to exhaustion. There was no difference between the two groups (old vs young) in terms of endurance time (194&84 vs 204f85 s). Although tbe final average EMG increased from 35% MVC to a similar final value for the two groups during the fatiguing contraction (final values of 57.4zh14.2 vs 55.1f16.2 %MVC), tbe mean power frequency declined less for the older subjects (55.*12-l vs 43.ti6.1 % initial, p < 0.05). Furthermore, these changes in surface EMG were accompanied by parallel changes in the direct motor response to stimulation (M wave) that included a similar decline in amplitude (72.3f19.0 vs 77.1f19.7 % initial) but less of an increase in duration (117.6k33.7 vs 138.5k23.5 8 initial; p < 0.05). These data indicate that although the old and young subjects had a similar endurance time, the older subjects experienced less of a change in the mechanisms underlying the reduction in propagation velocity of muscle action potentials. Supported by USPHS grants AG 0900, NS 20544, NS 07039, and NS 08634.
SINGLE
MUSCLE
CONTRACTIONS CAN PRODUCE MUSCLE-TENDON INJURY
, James G. Hay*, Warren G. Darling” IDepartment of Biomedical Engineering and Applied Therapeutics, The Cleveland Clinic Foundation, Cleveland, OH 44195, USA ZDepartment of Exercise Science, The University of Iowa, Iowa City, IA 52242, USA JonW. Feuerbachl
The purpose of this study was to examine the effect of a single eccentric contraction on the incidence and severity of muscle-tendon injury in a rat model. Other investigators have modeled muscle injury by: (1) lengthening a muscle-tendon unit beyond its physiological length limit until the muscle ruptured; or (2) subjecting the muscle to repeated slow eccentric contractions at physiological lengths. The eccentric contractions of the present study were maintained within physiological limits of length and velocity. Injury was defined functionally as a statistically significant reduction in isometric force production. Each of 30 rats was stabilized in a testing jig mounted on a Materials Test System and the distal tendon of the TA was released from its insertion and clamped to a load cell. Pre- and post-test isometric contractions were performed by stimulating the sciatic nerve and fixing the initial length of the TA fibers at 90% L,__. A test eccentric contraction was performed by stimulating the sciatic nerve and lengthening the TA from the initial length to /s) did not suffer a significantly L . TAs that were subjected to a slow eccentric contraction (ILL a&Ftz loss in isometric force production than that of the correspondini?ontrols. TAs that were subjected to /s) experienced a significant loss in isometric force production (7.9%) a fast eccentric contraction (16 L As p
POSTER SESSION-ANATOMICAL
JOINTS
FACTORS AFFECTING ELASTIC ENERGY RECOVERY IN STRETCH-SHORTENING MOVEMENTS and Jack M. Winters2 Michael Mungiolel 2 Exercise L Sport Research Inst. 2Chemica1, Bio, and Materials Engrg., Arizona State finiversity, Tempe, AZ 85287, USA Past exnerimental research which have investigated factors influencing series elasti; component (SEC) energy recovered during the concentric phase of stretch-shortening cycle (SSC) movements have resulted in equivocal conclusions when attempting to extrapolate external measures to internal phenomena. In this study, a validated muscle model was used to investigate the influence of fiber type proportion, eccentric phase movement amplitude, and SEC compliance on an ankle plantarflexion movement task and the SEC enercrv recovered. When compared to the nominal case, each of these factors influenced the relative proportions of the contractile component (CC) and SEC to overall muscle power, although performance (joint excursion in a particular time period) was.affected tb a lesser extent. The interaction of the CC and SEC influenced the joint power developed and the amount of elastic energy recovered during the concentric phase. Also, an increase in the SEC energy recovered does not necessarily imply improved performance.