Comparison of prolonged low-frequency force depression assessed using isometric torque and isotonic power following a dynamic fatiguing task

Purpose Prolonged low-frequency force depression (PLFFD) occurs following both dynamic and static fatiguing tasks, but it has been assessed predominately using measures of isometric torque. However, it is unknown whether PLFFD induced during dynamic tasks is adequately characterized by isometric torque, which excludes velocity and power. The purpose of this study was to compare PLFFD assessed using isometric torque and isotonic power following a concentric fatiguing task. Methods Young (18-31 years) males (n = 9) and females (n = 4) performed isotonic plantar flexion contractions until a similar to 75% reduction in peak power. Isotonic and isometric contractions were electrically evoked at 10 Hz and 50 Hz via tibial nerve stimulation. Isotonic and isometric PLFFD was assessed as the ratio of 10 to 50 Hz for power and torque, respectively. Recovery was assessed immediately, and at 2.5, 5, 10, 20, and 30 min after task termination. Results Relative to baseline, 10:50 Hz ratio assessed using isotonic power was reduced more than isometric torque (30 min 41 +/- 17 vs. 25 +/- 12% reduction, p = 0.001); however, both contraction modes displayed similar trajectories throughout recovery (p = 0.906). The larger reduction in isotonic 10:50 Hz ratio was due to greater impairments in 10 Hz power compared to 10 Hz isometric torque (30 min 38 +/- 20 vs. 21 +/- 11% reduction, p < 0.001). Conclusion The similar trajectories of 10:50 Hz ratios throughout recovery indicate that PLFFD can be adequately characterized using either isometric torque or isotonic power.