The effects of exercise-induced muscle damage on critical torque

Exercise-induced muscle damage (EIMD) has been shown to reduce endurance exercise performance. This study examined the effects of EIMD on critical torque (CT) and the sum of the torque integral above CT during (∫ _{T total}) during a 5-min all-out, intermittent isometric knee extension exercise.

CT was determined in eight participants prior to and 48-h following EIMD. EIMD was induced using electrically stimulated eccentric knee extensions until maximal voluntary strength (MVC) was reduced by 40%. EIMD was assessed by changes in MVC and ratings of muscle soreness using a VAS scale. EMG and near-infrared spectroscopy (NIRS) were collected from the vastus lateralis and vastus medialis, respectively, during the CT test to assess neuromuscular electrical activity and microvascular circulation.

MVC decreased 22% (p = 0.006) and soreness increased from 2.1 ± 1.9 to 50.4 ± 31.5 mm (p = 0.002) 48-h following eccentric exercise. CT declined from 61.6 ± 17.8 to 52.0 ± 14.1 Nm (-14%; p = 0.005) post-EIMD. ∫ _{T total} declined 33% (p = 0.0006) post-EIMD. No changes were observed in neuromuscular electrical activity (p = 0.95 for EMG RMS and p = 0.57 for EMG median frequency) or any parameter of microvascular circulation (p = 0.60 for tissue saturation index, p = 0.27 for total hemoglobin and myoglobin, p = 0.51 for oxyhemoglobin, and p = 0.26 for deoxyhemoglobin) between conditions.

Our finding that EIMD-reduced CT may explain the decrements in endurance performance following EIMD observed in the previous studies. The disproportionate reduction in ∫ _{T total} compared to MVC and CT is suggestive of a more rapid depletion of anaerobic energy stores and/or accumulation of metabolic by-products leading to fatigue following EIMD.