Influence of stimulation frequency on early and late phase rate of torque and velocity development

The early (≤50 ms) rate of torque development (RTD) is dependent upon the rate of neuromuscular activation; however, few studies have evaluated the determinants of rate of velocity development (RVD), which may be load-dependent. The purpose here was to explore the relationship between stimulation frequency with the early and late (≥100 ms) phase isometric RTD and isotonic RVD. The knee extensors of 16 (5 female) young recreationally active participants were stimulated using 14 frequencies from 1-100 Hz during isometric and isotonic ('unloaded' and 7.5% of the isometric maximal voluntary contraction [MVC]) contractions. Isometric RTD and isotonic RVD were evaluated for the early (0-50 ms) and late (0-100 ms) phases from torque and velocity onset, respectively. Sigmoid functions were fit and bi-linear regressions were used to examine the slopes of the steep portion and the plateau frequency. RTD- and RVD-frequency relationships were well described by a sigmoid function (all r >0.96). Compared with the late phase, early isometric RTD and unloaded RVD displayed lower slopes (all p≤0.001) and higher plateau frequencies (all p