Effect of Decreases in Joint Excursion on the Torque-Length Relationship and Velocity after Shortening Contractions.

Abstract

PURPOSE: To compare changes in joint excursion (ROM) with maximum isometric voluntary torque (MVC) and contractile properties at various joint angles, as well as peak shortening velocity following an isotonic dorsiflexion fatiguing task. METHODS: Using a Biodex System 3 dynamometer, seven young men performed fatiguing isotonic (30% MVC through 40° of ROM) dorsiflexion contractions at maximum effort, until ROM was reduced to 60% of baseline (PRE). Fatigue and recovery of ROM, MVC and evoked isometric contractile properties of doublet and 50-Hz tetanus [at long, optimal and short muscle lengths: 30° and 10° plantar flexion (PF), and 10° dorsiflexion (DF), respectively], and velocity were measured at task failure (POST), 0.5, 2, 5, and 10 minutes of recovery (R). RESULTS: ROM at POST decreased to ~55% of the value at PRE (40°), but recovered by R5. At POST, velocity decreased to ~35% of the value at PRE (165 ± 23 deg/s) and recovered at R5, whereas, the decrease in MVC (~70%) relative to PRE (49 ± 9 Nm at 30° PF) was similarly reduced at all joint angles and remained depressed up to R10. Voluntary activation at all angles at PRE (~ 99%) remained high (>92%) at POST and throughout recovery. Doublet torque was depressed at POST for all angles, however the decrease at 10DF (~20% relative to PRE (6 ± 3 Nm)) was greater than the decrease (~60% relative to PRE) for 30PF and 10PF. At POST, 50Hz torque declined similarly for all angles to ~75% of PRE (23 ± 5 Nm) but showed different rates of recovery. That is, at R10, 50-Hz torque for 10DF recovered to ~78% of PRE, whereas 10PF and 30PF recovered to ~88% of PRE. Normalized maximum rates of rise and relaxation of the 50-Hz response declined similarly at POST for all angles (~70% relative to PRE) and recovered by R5. CONCLUSION: The decrease in ROM and shortening velocity following dynamic fatigue recovered quickly, but this was not related to the decrease and incomplete recovery of MVC assessed at short, optimal, and long muscle lengths. Furthermore, 50-Hz tetanus maximum rates of rise and relaxation appeared closely related to the recovery in ROM. Results indicate that peripheral mechanisms impairing dynamic crossbridge function were related to failures in ROM and velocity with dynamic fatigue. Supported by NSERC and OGS.