Lacey Harper1, Benjamin Dalton2, Michaela Alesi1, Michael Cooper1, Jacob McDougle1, Trisha VanDusseldorp3, Robert Buresh, FACSM1, Yuri Feito, FACSM4, Garrett Hester1. 1Kennesaw State University, Kennesaw, GA. 2University of Guelph, Guelph, ON. 3Bonafide Health, Harrison, NY. 4American College of Sports Medicine, Indianapolis, IN.

BACKGROUND: Neuromuscular fatigue following maximal voluntary isometric contractions (MVIC) dampens both rate of torque development (RTD) and rate of muscle activation (RMA), but the influence of contraction speed (i.e., rapid vs. slow) is less clear. The purpose of this study was to determine responses in explosive neuromuscular function and peripheral fatigue after fatiguing, rapid and ramp maximal isometric contractions. METHODS: Following a familiarization visit, twelve recreationally active, young males (22.8 ± 2.5 yrs) performed either rapid (FAST; as rapid as possible) or ramp (SLOW; 2 sec ramp-up) maximal voluntary isometric knee extensions. Protocol order was randomized and both protocols were stopped upon a 50% reduction in peak torque. Before and after each protocol, a rapid MVIC was performed and early (0-50 ms and 0-100 ms) and late (0-200 ms) RTD were calculated from the torque-time curve. RMA was assessed for the vastus lateralis using electromyography and was calculated as the root mean square from 0-50 ms relative to maximum root mean square. Following the MVICs, a series of electrically evoked singlets and doublets (10 Hz and 100 Hz) were delivered to assess indices of peripheral fatigue such as twitch amplitude and half relaxation time. Two-way (condition × time) repeated measures ANOVAs were used to identify differences between conditions across time. RESULTS: RTD from 0-50 ms (-56% vs -27%; p=0.020) and RMA (-64% vs +6%; p=0.026) were reduced more after SLOW compared to FAST, whereas similar reductions were found for other RTD time phases. Indices of peripheral fatigue were decreased similarly between conditions, but the larger effect size and relative increase for half relaxation time after SLOW (+43% vs +31%; p = 0.062) was noteworthy. CONCLUSIONS: These results suggest that early rapid torque production and RMA is reduced more after slow fatiguing contractions than fast. Though the physiological underpinnings cannot be completely elucidated, the impairment in RMA after slow contractions may be due to greater inhibitory feedback resulting from peripheral fatigue. Despite a similar total time under tension between protocols, the longer ramp-up during SLOW contractions likely allowed for greater peripheral disturbance.

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