Sex differences in quadriceps alternating muscle activation patterns during fatigue

Synergistic alternating muscle activation (AMA) consists of a period of co-activation (Co-A) and a period of trade-off (TO). Together they form a load-sharing cycle which is a neuromuscular control strategy that attenuates fatigue. However, the structure of AMA interactions of synergistic muscles has only been investigated during low-level contractions of 2.5-10% maximal voluntary contraction (MVC) and has yet to be investigated at moderate force levels (≥ 20% of MVC) and differentiated between the sexes. The purpose of this study was to quantify the activation relationship between pairs of synergistic quadriceps muscles to further understand the patterns (durations of Co-A and TO and frequency of AMA cycles). Surface electromyographic (EMG) data was collected from 16 individuals (8 male, 8 female) from the rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), and vastus medialis oblique (VMO) during a fatiguing contraction at 20% MVC. Synergistic muscle pairs (VL-RF, VL-VM, VL-VMO, RF-VM, RF-VMO, VM-VMO) were analyzed for Co-A, TO, and AMA frequency during 3 phases of the fatiguing contraction. The synergistic pairs were in Co-A significantly longer than TO during all fatigue phases. Some muscle pairs differed significantly from each other in time spent in each state (Co-A or TO) during the final contraction phase. There was no significant difference in AMA patterns within individual muscle pairs between fatigue phases. There were strong positive and negative correlations between endurance time and Co-A and TO durations respectively for every muscle pair in males during the final two fatigue phases. For the same measures in females, only the RF-VL, RF-VMO, and VL-VM muscle pairs demonstrated a significant negative and positive correlation in the middle fatigue phase for Co-A and TO respectively. AMA was present in both male and female EMG data, but contrary to expectations that AMA cycle frequency would produce significant differences throughout the contraction, the endurance time correlations were where significant differences were present.