Author ORCID Identifier

Date of Award

Summer 8-2022

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Kinesiology and Health

First Advisor

Christopher Ingalls

Second Advisor

Feng Yang

Third Advisor

Jeff Otis

Fourth Advisor

Kyle Bradenberger


Research has shown that exercise-induced muscle injury can cause 25-50% reductions in maximal joint torque. However, it is unknown whether individual muscles of a synergist group are injured to the same extent following injury. 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. 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), rectus femoris (RF) and vastus lateralis (VL) via electrical stimulation (20 and 80 Hz), 3) soreness of individual QMs, 4) QMs EMG root mean square (RMS) during running and MVCs, and 5) standing postural sway. MVC 90° torque was significantly reduced immediately (25.3%) and 2-days (14.0%) after DHR, whereas torque was unchanged after level walking. After DHR (immediately and 2-days), MVC RMS across all three quadriceps muscles was significantly reduced by 16.8% immediately following injury. There was a 13.2% decrease in stimulated torque (20 & 80 Hz) collapsed across all muscles for the injury group immediately post and a decrease of 9.1% at 48H following. At 48 hours, the VL experienced greater soreness than RF and VM. Running RMS of the knee extensor muscles increased immediately after DHR. Postural sway increased immediately after DHR and decreased in the control group at 2-days. In conclusion, DHR caused the differential injury of the QMs, and reduced activation (i.e., RMS) of the quadriceps and force depression (i.e., 20 & 80 Hz torque) account for the decreases in MVC torque after DHR. In addition, muscle injury from DHR disrupted standing balance and normal muscle activation patterns during running.


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