Frequent alternate muscle activity of plantar flexor synergists and muscle endurance during low-level static contractions as a function of ankle position

Abstract

We have investigated the appropriate joint angle for detecting frequent alternating activity in synergistic muscles and the relationship between muscle activation patterns and endurance during static low-level contractions. Eleven healthy men performed prolonged static plantar flexion of the ankle at 10% of the maximal voluntary contraction, with the ankle flexed at 100°, 110°, or 120°, while seated with the right leg in full extension. The onset of muscle activation and/or inactivation was detected using quantitative analysis, and alternate activity among muscles was detected using a threshold criterion of ×e or ×1/e multiplied by the levels of mean electromyograms (EMG) calculated at 1-min intervals. Surface EMG revealed frequent alternations of activity among the lateral and medial gastrocnemius and soleus muscles at an ankle flexion of 110°. The first alternation occurred after approximately 15 min of exercise. The number of alternations per hour was four- to sevenfold higher at 110° than at 100° or 120°. Endurance was longest and shortest at 110° and 120°, respectively. These findings suggest that synergistic motor pools activated at a specific joint angle (110°) affect muscle endurance during static low-level fatiguing tasks.

Fig. 1

Test windows for event detection of alternate activity among muscles. Two consecutive windows (Window_k−1, Window_k) are displayed for each muscle (LG lateral gastrocnemius, MG medial gastrocnemius, Sol soleus muscle). m Mean rsEMG [raw electromyogram (EMG) signals that had been amplified, full-wave rectified, and smoothed (time constant 300 ms)] level in the previous Window_(k−1) with a period of 60 s. The threshold for the offset was set at 1/e while that of the onset was set at e time m (Window_k). Binary events detecting the active (ON) and inactive (OFF) periods are shown in the three muscles. The typical EMG overlap period is also shown (below left) See Statistical analysis for a detailed definition of parameters

Fig. 2

Simultaneous EMG recordings from the lateral gastrocnemius (LG), medial gastrocnemius (MG), and soleus (Sol) muscle during static contractions at 10% maximal voluntary contraction (MVC). The bottom line (angle) indicates ankle joint angles. Continuous EMG recordings during static contractions at an ankle joint angle of 100° (a) and 120° (c), respectively. Note the difference in time to first recruitment of LG between 100° and 120°. Few events of alternating muscle activity were shown, although each muscle showed repeated fluctuations in EMG activity with increasing time. b An example of clear alternation of EMG activity during static contractions at 110°. Note that EMG activities are switched from MG to LG + Sol with increasing duration of contractions. All traces (a–c) were obtained from the same subject

Fig. 3

a The influence of ankle joint angle on number of events of alternate activity among synergistic muscles per hour. b The influence of ankle joint angle on endurance time and maximal force level normalized to initial value after fatiguing exercise. A higher force level for maximal plantar flexion is maintained at an angle of 110° compared to 100° or 120°. Values are given as the mean ± standard deviation (SD). ^#P < 0.05, *P < 0.05, Significantly different from the value at 110°

Fig. 4

a A comparison of time to first alternation of activity among muscles during static contractions at each joint angle. The first alternation of muscle activity is observed early in all exercise times during static contractions at an angle of 110°. *P < 0.05, Significantly different from the value at 110°,^#P < 0.05, significantly different from the value at 100°. b A comparison of time to first recruitment of the lateral gastrocnemius (LG) during static contraction at each joint angle. Note that LG is hardly recruited at an angle of 100°, i.e., far from the recruitment threshold of LG (approximately 130°). There is no significant difference between values at the ankle angles of 110° and 120°. *P < 0.05, Significantly different from the value at 100°

Fig. 5

Percentage of total activation (%Ta) and inactivation (%Ti) time during the whole endurance exercise in the lateral gastrocnemius (LG), medial gastrocnemius (MG), soleus muscle (Sol)