Reduction in Quadriceps Neuromuscular Function during and after Exercise-induced Fatigue in Patients with Patellofemoral Pain

Abstract

Context Exercise-induced fatigue reduces muscle force production and motoneuron pool excitability. However, it is unclear if patients with patellofemoral pain (PFP) experience further loss in quadriceps neuromuscular function due to fatigue during and after exercise. Objective To observe how quadriceps maximal strength, activation, and force-generating capacity change during and after repetitive bouts of isokinetic knee-extension exercise in patients with PFP. Design Cross-sectional study. Setting Laboratory. Patients or Other Participants Twenty-two patients with PFP (mean = pain severity: 4.2 out of 10 cm in Visual Analog Scale; symptom duration: 38.6 months) and 19 healthy controls matched on age and body mass index. Main Outcome Measure(s) Quadriceps peak torque (PT), central activation ratio (CAR), and rate of torque development (RTD) were assessed at baseline and immediately after every five sets of knee-extension exercise (times 1 to 5). Participants continued knee-extension exercises until the baseline quadriceps PT dropped below 50% for three consecutive contractions. Results No group-by-time interaction was observed for quadriceps PT (F5,195=1.03, P=.40). However, group-by-time interactions were detected for quadriceps CAR (F5,195=2.63, P=.03) and RTD (F5,195=3.85, P=.002). Significant percentage reductions in quadriceps CAR (− 3.6%, P=.04, Cohen d=0.53) and RTD (−18.9%, P=.0008, Cohen d=1.02) were observed between baseline and time 1 in patients with PFP, but not in their healthy counterparts (CAR: −1.9%, P=.86; RTD: −9.8%, 25 P=.22). A significant percentage reduction in quadriceps RTD was also observed between times 4 and 5 in patients with PFP (−24.9%, P=.002, Cohen d=0.89), but not in their healthy counterparts (−0.9%, P=.99). Conclusions Patients with PFP appear to experience an additional reduction in quadriceps activation and/or force-generating capacity both at the early and late stages of exercise compared to healthy individuals. Clinicians should be aware of such potential acute changes during and after exercise and utilize fatigue-resistant rehabilitation programs for PFP.