Alternating Muscle Activation Patterns Across The Deltoid Muscles During Sustained Fatiguing Contractions

Alternating muscle activation (AMA) across synergists has been identified in lower-extremity muscles that may function to prolong force output during sustained contractions; however, this has not yet been identified in upper extremity muscles. AMA consists of two periods: co-activation (Co-A), in which a pair of synergists are simultaneously increasing or decreasing EMG amplitude, and trade-off (TO), in which the EMG amplitude of one muscle is increasing while the EMG amplitude of a synergist is decreasing. We hypothesized that TO duration would increase and Co-A duration would decrease during fatigue. PURPOSE: To investigate AMA patterns during sustained submaximal isometric fatiguing scaption contractions in healthy adults. METHODS: Surface electromyographic (EMG) data was collected from 10 healthy individuals (5 males and 5 females, 21.5 ± 2.0 yrs) from the anterior deltoid (AD), middle deltoid (MD), and posterior deltoid (PD) during an isometric fatiguing contraction of 25% MVC in the scapular plane. Synergistic muscle pairs (AD-PD, AD-MD, PD-MD) were analyzed for Co-A duration (time difference between the zero-crossing points occurring at the onset and offset of a positive correlation phase), TO duration (time difference between the zero-crossing points occurring at the onset and offset of a negative correlation phase), and the frequency of AMA cycles (number of zero-crossing events/15 s). We selected 15 s from the beginning of the stable force level period as the pre-fatigue phase, and 15 s at the end of the stable force level period as the fatigue phase. A two-way repeated measures ANOVA (factors: muscle pair and time) with Tukey’s post-hoc analysis was used to compare Co-A duration, TO duration, and the frequency of AMA. RESULTS: There was a significant increase in TO duration (pre-fatigue: 3.2 ± 0.5 s; post-fatigue: 5.3 ± 0.6 s, p = 0.006) and a decrease in Co-A duration (pre-fatigue: 11.8 ± 0.5 s, post-fatigue: 9.7 ± 0.6 s, p = 0.006). The frequency of AMA cycles did not change over time (pre-fatigue: 4.7 ± 0.5 cycles/s, post-fatigue: 4.9 ± 0.6 cycles/s, p = 0.6). There were no significant correlations between endurance time and the AMA patterns. CONCLUSION: This finding demonstrates that there is rotation of muscle activation levels across the deltoid muscles during sustained isometric fatiguing contractions. Supported by the College of Education, University of Texas at Austin