Katie G. Kennedy, Ryan J. Colquhoun, Sydnie R. Fleming, Kaitlyn F. Overstreet, Keelan I. Stricklin, Christian T. Macarilla, Abby E. Turnbow. University of South Alabama, Mobile, AL.

BACKGROUND: It is well established that maximal force declines with the accumulated fatigue from intermittent isometric contractions. However, the changes in neuromuscular properties following high- and low-intensity exercise remain unclear. Therefore, the purpose of this investigation was to determine changes in maximal force and muscle activation of the knee extensors following high- and low-intensity intermittent isometric contractions. METHODS: Ten recreationally active, healthy females (Mean ± SD; Age: 21±2 y) completed two experimental visits, consisting of repeated isometric contractions of the right knee extensors to failure at either 30 or 70% maximum voluntary isometric contraction (MVIC). Each contraction lasted 20 s., with 6 s. rest between repetitions. The order of each visit was randomized and counterbalanced, and each visit was separated by 5±2 days, and the same time of day (±1 h). Prior to each bout (PRE), participants completed two MVICs to determine their target torque. A single MVIC attempt was recorded immediately following (POST) each bout, which ended when participants could no longer maintain the target torque. Electromyographic signals from the vastus lateralis (VL) and vastus medialis (VM) were recorded during each MVIC. MVIC strength and root mean square (RMS) of the VL and VM were calculated offline. RESULTS: Condition × time interactions were observed for MVIC (p = 0.006), VL RMS (p = 0.011), and VM RMS (p = 0.018). Post-hoc analyses indicated that MVIC significantly decreased from PRE to POST in both the 30% (p<0.001; PRE30: 183.8 ± 60.9 N‧m; POST30: 111.1 ± 42.5 N‧m) and 70% (p=0.001; PRE70: 177.4 ± 73.3 N‧m; POST70: 125.2 ± 50.5 N‧m) conditions but was significantly less at POST30 when compared to POST70 (p=0.012). Both VL RMS (p = 0.001; PRE30: 0.08 ± 0.04 mV; POST30: 0.05 ± 0.03 mV) and VM RMS (p=0.040; PRE30: 0.10 ± 0.06 mV; POST30: 0.07 ± 0.02 mV) significantly decreased in the 30% condition, while no changes were observed in either the VL (p=0.946) or the VM (p=0.856) in the 70% condition. CONCLUSIONS: While both high- and low-intensity conditions produced significant declines in MVIC, 30% MVIC produced significantly greater reductions. Further, muscle activation was only altered following low intensity exercise. These data suggest that the mechanisms of fatigue differ between high- and low-intensity intermittent isometric contractions in college-aged females.

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