Load- and frequency-dependent impairments to isotonic power are greater than isometric torque following concentric-eccentric fatiguing contractions in humans.

Prolonged low-frequency force depression (PLFFD) is the disproportionate loss of force in response to low frequencies (e.g., <20 Hz) of activation compared to high frequencies (e.g., >50 Hz) following fatiguing contractions. While traditionally assessed using isometric contractions, the effect of PLFFD during dynamic contractions is much less explored. The purpose was to assess PLFFD at two loads (unloaded and 12.5% of maximal voluntary contraction) during dynamic isotonic and isometric contractions following a fatiguing task. Eighteen participants (23.3 ± 1.6 yr, 6 females) performed continuous, maximal isokinetic cycles of concentric and eccentric knee extension contractions until a 75% loss of concentric peak power. Percutaneous electrical stimulation of the quadriceps was used to assess PLFFD beginning at 30 min of recovery by comparing pre- and postfatigue 20:50-Hz ratios. The 20:50-Hz ratio for isometric torque declined 15.6% (P < 0.001), indicating PLFFD. The loaded dynamic isotonic 20:50-Hz power ratio was significantly more depressed than isometric torque (36.4%, P < 0.001), but there was no difference in the relative decline between the unloaded and isometric conditions (22.9%, P = 0.57). Furthermore, the 20:50-Hz isotonic velocity ratio was depressed with both loads (unloaded, 12.9% and loaded, 34.4% decrease, and both P < 0.001), and this decline was greater than concentric torque in the loaded condition (34.4% and 7.4% decrease, respectively, P < 0.001). Therefore, the larger decline in the power ratios was due to the combined deficits in both torque and velocity, indicating that during PLFFD the impairment of isotonic power is greater than isometric torque, which is further impaired at higher loads (i.e., 12.5%).NEW & NOTEWORTHY The assessment of prolonged low-frequency force depression (PLFFD) following a dynamic fatiguing task of the knee extensors is underestimated when using traditional measures of isometric torque, compared to isotonic power, which is a more functionally relevant measurement of this state. Furthermore, when isotonic assessments were performed against a moderate load, the combined deficit in torque and angular velocity led to a greater decline in evoked power compared to isometric torque in response to low-frequency stimulation.