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 speed 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 (five female) young recreationally active participants were stimulated using 14 frequencies from 1 to 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 bilinear regressions were used to examine the slopes of the steep portion of the curve and the plateau frequency. RTD- and RVD-frequency relationships were well described by a sigmoid function (all r 2  > 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 < 0.001). In contrast, early and late RVD of a moderately loaded isotonic contraction did not display different slopes ( P = 0.055) or plateau frequencies ( P = 0.690). Early isometric RTD and unloaded isotonic RVD are more dependent on changes in stimulation frequency compared with late phases. However, RVD for a moderately loaded isotonic contraction displayed similar responses for the early and late phases. Therefore, a high frequency of activation is critical for early torque and velocity generation but dependent upon the load for isotonic contractions. NEW & NOTEWORTHY We show that during an "unloaded" isotonic contraction, the early phase rate of velocity development is more dependent upon a high electrical activation frequency compared with the late phase, similar to isometric torque. However, early and late phase rates of velocity development of moderately loaded isotonic contractions display similar responses. These results indicate that the determinants of isotonic shortening function are dependent on the externally applied load, highlighting the importance of task-specificity of contraction.