Fatigue-induced changes in VMO-VL EMG correlation in individuals with patellofemoral pain

Patellofemoral pain (PFP) is commonly aggravated during prolonged activity, suggesting that neuromuscular responses to fatigue may influence quadriceps activation. We examined how fatigue affects the similarity of activation between the vastus medialis oblique (VMO) and vastus lateralis (VL) in individuals with and without PFP, and whether responses differ by sex.

Twenty-three patients with PFP (11 females, 12 males) and 23 non-injured controls (13 females, 10 males) performed sustained isometric knee extensions at 20% maximal voluntary contraction until task failure. VMO-VL activation similarity was quantified using cross-correlation of surface and intramuscular EMG amplitude envelopes across four equal phases of fatigue. Peak correlation reflected maximal temporal similarity of activation, and integrated correlation strength (ICS) represented overall correlation magnitude.

Peak correlation and ICS increased progressively with fatigue in all groups (p < 0.001). However, fatigue-related increases were significantly smaller in individuals with PFP and in females compared with non-injured males. Pre-fatigue values did not differ between groups. Surface and intramuscular EMG showed comparable fatigue-related trends.

Fatigue increased VMO-VL EMG cross-correlation in healthy participants, whereas this fatigue-related increase was attenuated in individuals with PFP. These findings suggest reduced capacity to adapt quadriceps activation patterns during sustained load in PFP, supporting fatigue as a useful physiological stressor for evaluating quadriceps function.