Isometric torque and shortening velocity following fatigue and recovery of different voluntary tasks in the dorsiflexors.

The present study was designed to compare the relative influence of various fatigue-related factors involved in isometric and dynamic task failure following an equivalent decrease in isometric maximum voluntary contraction (MVC) torque. Using a similar duty cycle (~1-s contraction per 2 s) and contraction load (50% of MVC), 9 young males performed in the dorsiflexors a dynamic task, and on a separate occasion an intermittent isometric task, to an equal decrease in isometric MVC torque. Dynamic contractions had greater motor unit activation and maximum rate of torque development, and required fewer contractions to task failure than the isometric task, indicating a faster development of fatigue during the velocity-dependent dynamic contractions. Peripheral factors, rather than impairments in voluntary drive, were responsible for the equivalent decrease in MVC torque at task failure and its subsequent incomplete recovery. These included, for both tasks, similar changes during fatigue and recovery in voluntary isometric MVC torque, shortening velocity, stimulated twitch and 50 Hz torque, and 50 Hz maximum rate of relaxation. Irrespective of the task, however, the greater reduction in shortening velocity at task failure and its subsequent faster recovery than MVC torque suggest that changes in metabolites affect velocity to a greater extent than isometric torque.