Differential effects of stimulation frequency on isometric and concentric isotonic contractile function in human quadriceps.

Electrically evoked contractions are used to assess the relationship between frequency input and contractile output to characterize inherent muscle function, and these have been done mostly with isometric contractions (i.e., no joint rotation). The purpose was to compare the electrically stimulated frequency and contractile function relationship during isometric (i.e., torque) with isotonic (i.e., concentric torque, angular velocity, and mechanical power) contractions. The knee extensors of 16 (5 female) young recreationally active participants were stimulated (~1-2.5s) at 14 frequencies from 1-100 Hz. This was done during four conditions, repeated on separate days, which were: isometric; and isotonic at loads of 0, 7.5 and 15% isometric maximal voluntary contraction (MVC). Comparisons across contractile parameters were made as a % of 100 Hz. Independent of the load, the mechanical power-frequency relationship was rightward shifted compared with isometric torque-, concentric torque- and velocity-frequency relationships (all p≤0.04). With increasing load (0-15% MVC) the isotonic concentric torque-frequency relationship was shifted leftward systematically from 15-30 Hz (all p≤0.04). Conversely, the same changes in load caused a rightward shift in the velocity-frequency relationship from 1-40 Hz (all p≤0.03). Velocity was leftward shifted of concentric torque in the unloaded isotonic condition from 10-25 Hz (all p≤0.03), but concentric torque was leftward shifted of velocity at 15% MVC isotonic condition from 10-50 Hz (all p≤0.03). Therefore, isometric torque is not a surrogate to evaluate dynamic contractile function. Interpretations of evoked contractile function differ depending on contraction type, load, and frequency which should be considered relative to the specific task.