Author(s): FUNK, J., KO, R., CHAYA, T., PYNN, J., LANOVAZ, J., CHILIBECK, P., FARTHING, J., Institution: UNIVERSITY OF SASKATCHEWAN, Country: CANADA, Abstract-ID: 1711

INTRODUCTION:
Associated Activity (AA) refers to unintended activity in non-target contralateral muscles during strong unilateral contractions. AA has been identified as a candidate mechanism for cross-education effects and tends to increase as task demand (e.g. fatigue) increases. There is limited research on AA in the lower limbs and during sustained maximal contractions. While females are generally less fatigable than males, there is limited research on sex differences in AA. The purpose of this study was to examine sex differences in fatigue and AA during a sustained unilateral knee extension maximum voluntary contraction (MVC) until time to task failure (TTF).
METHODS:
Thirty healthy participants (15 Females (F), 15 Males (M); Age:25±3yrs) with at least one year training experience completed brief 3-5s unilateral isometric knee extension (KE) and flexion (KF) MVCs on both limbs with simultaneous electromyographic (EMG) recordings (vastus lateralis and biceps femoris) to determine peak torque and maximal EMG signal. Participants then completed a sustained unilateral isometric KE MVC until failure, defined as torque below 40% MVC for five continuous seconds. Torque and EMG were analyzed as %MVC for the active (ACT) and EMG was analyzed as %MVC in the opposite contralateral (CONT) limb. TTF data were normalized to percentage of contraction duration (DUR) and analyzed across six time points: start-of-contraction (2-4s at start) followed by five 20% contraction-duration bins (0–20%, 20-40%, 40-60%, 60-80%, 80–100% of TTF duration).
RESULTS:
Seconds to failure was greater in females (F:98.4±26.7 vs M:81.4±12.3s; p=.038). For the ACT limb, there were significant decreases from start-of-contraction to 80-100% DUR for torque (F:90.6±5.9 vs 39.4±3.1; M:88.3±5.5 vs 40.6±3.5%MVC), KE EMG (F: 100.5±25.2 vs 67.6±30.0; M: 80.8±11.3 vs 58.2±20.7%MVC) and KF EMG (F:13.5±8.4 vs 8.4±5.0; M:11.0±8.5 vs 7.5±5.2%MVC; all p<.001), with no differences between sexes. For CONT limb, KE EMG was significantly greater in females (F:8.6±3.2 vs M:4.4 ±3.1%MVC; p=.002), pooled across all time points. CONT KF EMG increased over time (p=.028) pooled for both sexes from start-of-contraction to peak at 60-80% DUR (17.0±28.2 vs 25.2±32.6%MVC).
CONCLUSION:
When normalized based on percent contraction duration, torque and ACT EMG decreased as fatigue progressed for both sexes, with a similar fatigue profile. CONT limb muscle activity was evident in both the KF and KE, with higher activity in extensor muscles in females, despite greater resistance to fatigue. CONT antagonist (KF) muscle activity was relatively high, but quite variable. Higher CONT knee extensor activation in females may indicate sex-based differences in lower body AA.