A comparison of maximal rate of force development between burst and maximum voluntary contractions: reliability and neuromuscular determinants.

PURPOSE: Explosive strength, measured as the maximal rate of torque development (RTDmax), can be assessed during rapid isometric contractions (bursts) or high-force maximum voluntary isometric contractions (MVIC). However, the ideal approach for assessing knee extensor RTDmax in trained individuals with respect to the reliability, RTDmax output, and neuromuscular activation is unclear. Thus, the purpose of this study was to compare inter-session test-retest reliability, RTDmax output, and neuromuscular activation between burst and MVIC methods. METHODS: Trained (n = 30; females: n = 13) participants completed knee extension MVIC and burst contractions. Neuromuscular activation was assessed by the rate of rise of the electromyography (EMG) root mean square (RMS) signal normalized to the intra-repetition peak EMG rms amplitude (RERmax) for the vastus lateralis (VL) and vastus medialis (VM). RTDmax was computed from the derivative of the torque-time curve as the average RTD around the instant of the peak RTD using a ± 25 ms window. RESULTS: RTDmax demonstrated excellent test-retest reliability for MVIC (right limb: ICC = 0.87; left limb: ICC = 0.93) and burst (right limb: ICC = 0.86; left limb: ICC = 0.88) contractions. Across limbs and sessions, RTDmax and RERmax were significantly greater (p < 0.01) with the burst method (RTDmax = 1751 Nm·s- 1; VL RERmax = 382%RMSmax·s- 1; VM RERmax = 403%RMSmax·s- 1) compared to the MVIC method (RTDmax = 1493 Nm·s- 1; VL RERmax = 309%RMSmax·s- 1; VM RERmax = 343%RMSmax·s- 1). CONCLUSION: Burst contractions provide an advantage over traditional MVICs for assessing the true knee extensor explosive strength.