Academic journal article Research Quarterly for Exercise and Sport

Practicing a Maximal Performance Task: A Cooperative Strategy for Muscle Activity

Academic journal article Research Quarterly for Exercise and Sport

Practicing a Maximal Performance Task: A Cooperative Strategy for Muscle Activity

Article excerpt

The effect of practice on predicting elbow flexion movement time was studied. Participants (N = 18) performed 400 elbow flexion trials to a target in the horizontal plane. The trials were distributed equally over four sessions. The goal was to decrease the movement time (MT) for the same degree of accuracy. The electromyographic (EMG) activity of the biceps and triceps brachii was monitored with standard Beckman Ag/AgCl surface electrodes. The EMG measures formed two variable sets within one prediction equation. One variable set was composed of the onset of muscle activity relative to the start of movement (motor time) and the duration of muscle activity. The other variable set consisted of the mean amplitude value of the entire burst and of the first 30 ms ([Q.sub.30]) of activity. As the maximal speed of limb movement increased, the duration of muscle activity (motor time and EMG duration) decreased, and the magnitude of muscle activity (MAV and [Q.sub.30]) increased. Most of the change in the duration of m uscle activity occurred in Session 1, while the magnitude of muscle activity continued to increase until Session 3. Multiple regression analysis revealed a cooperative strategy between the magnitude and duration of muscle activity. Early in learning, participants adjusted the magnitude of muscle activity to increase limb movement speed. As practice continued, alterations in the duration of muscle activity became more important, while the magnitude changes were less involved. Late in learning, both dimensions of muscle activity were used to decrease MT. We suggest that the interplay between the magnitude and duration of muscle activity may be due to: (a) cognitive factors related to the division of attention in a motor skill, (b) an increase in the frequency of motor unit firing that affects both dimensions of muscle activity, or (c) some combination of (a) and (b).

Key words: elbow flexion, EMG, learning, practice

This paper focuses on changes in electromyographic (EMG) activity that occur as a maximal performance task is practiced. The specific question posed was: How do the magnitude and duration of muscle activity combine to increase the maximum speed of limb movement? Rapid limb movements are associated with a triphasic EMG pattern. The agonist muscle is recruited first to move the limb in a desired direction. This activity terminates near the midpoint in the range of motion, resulting in a silent period. The antagonist muscle then initiates deceleration during the silent period. Finally, there is a second burst of agonist muscle activity that occurs near target achievement (Cooke & Brown, 1990; Wierz-bicka, Wiegner, & Shahani, 1986).

The speed-sensitive strategy predicts how muscle activity changes when participants decrease movement time (MT) for the same distance under submaximal conditions (Corcos, Gottlieb, & Agarwal, 1989). This strategy is part of the dual strategy (Gottlieb, Corcos, & Agarwal, 1989a) hypothesis, wherein the alpha motoneuron pool is modeled as a first-order, low-pass filter, and descending commands take the form of an excitation pulse. The nervous system modifies either pulse height or pulse width depending on the movement strategy. To increase the speed of limb movement for the same distance, the nervous system uses a speed-sensitive strategy and increases only the height of the excitation pulse. The result is an increase in the slope and magnitude of muscle activity, while the duration remains the same. The onset of antagonist muscle activity relative to the start of movement (motor time) also decreases.

When participants practice a maximal performance task, the changes in EMG are consistent with the speed-sensitive hypothesis with one exception: antagonist motor time is participant-dependent and can increase (Corcos, Jaric, Agarwal, & Gottlieb, 1993). What is not known, however, is how the magnitude and duration (motor time and EMG duration) of muscle activity combine to increase the speed of limb movement under maximal effort conditions. …

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