(2) EFFECT OF ECCENTRIC CONTRACTION-INDUCED INJURY ON INDIVIDUAL QUADRICEPS MUSCLES: IMPACT ON MUSCLE ACTIVATION, JOINT TORQUE AND MOTOR CONTROL

PUPROSE When exercise is unaccustomed, the repetitive loading can lead to a grade I injury of skeletal muscle often referred to as exercise-induced muscle injury. Research has shown that exercise-induced muscle injury can cause 25-50% reductions in maximal joint torque. Strength deficits associated with exercise-induced muscle injury are normally assessed by measuring changes in joint torque which reflect the functional integrity of a given set of synergistic muscles. However, it is unknown whether individual muscles of a synergist group are injured to the same extent following injurious exercise. We hypothesized that the extent of injury among synergistic muscles is not uniform, and the primary cause of the weakness stems from the failure of muscle and not the ability of the nervous system to activate the muscle. We also presumed that muscle injury would alter balance (postural sway) and quadricep muscle activation patterns (electromyography [EMG]) during locomotion. METHODS 15 healthy sedentary or recreationally active male subjects between 18 and 35 years old completed the study. Subjects performed either downhill running (DHR) on a treadmill for 60 min to induce injury (n=8) or level treadmill walking for 30 min as control (n=7). Before and after (immediately and 2-days) exercise, we measured 1) maximal voluntary contraction (MVC) torque of quadricep muscles (QMs), 2) torque produced by vastus medialis (VM) and vastus lateralis (VL) via electrical stimulation (20 and 80 Hz), 3) soreness rating of VM, VL, and rectus femoris (RF), 4) QMs EMG root mean square (RMS) during running and MVCs, and 5) standing postural sway. RESULTS MVC torque was significantly reduced immediately (25.3%) and 2-days (14.0%) after DHR, whereas torque was unchanged after level walking. Immediately following DHR, 20 Hz stimulated torque relative to baseline trended lower (p=0.054) in the VL (67.1±0.4%) than the VM (77.5±0.6%). 20 Hz stimulated torque deficit remained lower (p=0.051) in the VL (87.5 ± 0.4%) than the VM (94.5±0.4%) 48 hours after DHR. There were no statistical differences (p≥0.810) in the 80 Hz torque deficit (89.7±0.4% to 91.5±0.2%) for the injured vasti muscles. Immediately after DHR, the 20/80 Hz torque ratio decreased (p=0.001) by 20.6% across both muscles. No muscle interaction (p≥0.109; Observed Power =0.442) was present for 20/80 Hz torque ratio. At 48 hours, the VL experienced greater (p≤0.004) soreness than RF and VM in the DHR group. After DHR (immediately and 2-days), MVC RMS across the QMs was significantly reduced by 16.8% immediately following injury. Running RMS of the QMs increased immediately after DHR. Postural sway increased immediately after DHR and decreased in the control group at 2-days. CONCLUSION DHR caused differences in muscle soreness across the individual QMs, and tends to cause differential submaximal (20 Hz) torque loss between the VM and VL muscles. Reduced activation (i.e., RMS) of the quadriceps and intrinsic force depression (i.e., 20 & 80 Hz torque) account for the decreases in MVC torque after DHR. Muscle injury from DHR disrupted standing balance and normal muscle activation patterns during running. PRACTICAL APPLICATIONS The results show that there are differences in markers of injury between synergists muscles following eccentrically bias exercise. Further research is waranted to see how differential muscle injury may impact joint stability during locomotion and balance after eccentrically bias exercise.

Acknowledgements: None