Pain response on neuromuscular function after 30 minutes of physical activity in individuals with knee osteoarthritis

Abstract

Purpose: Physical activity, such as walking, is currently recommended as a non-pharmacological intervention for individuals with knee osteoarthritis. Despite consistent recommendations across clinical guidelines, the individual symptomatic response immediately following a prescribed bout of physical activity is less consistent and how these responses affect joint function is unclear. Insight into this relationship may be provided by examining whether varied symptomatic responses to a bout of physical activity are associated with altered neuromuscular activation patterns. The purpose of this study was to investigate the effect of pain responses on neuromuscular activation patterns during gait before and immediately after 30 minutes of continuous treadmill walking between asymptomatic individuals, and individuals diagnosed with moderate knee osteoarthritis. Methods: Twenty-one asymptomatic individuals and 23 individuals diagnosed with symptomatic, moderate knee osteoarthritis underwent standard gait analysis during over ground walking immediately before and after 30 minutes of continuous treadmill walking at a self-selected pace. Surface electromyography (EMG) data was collected during over ground walking from five knee muscles [rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL), and medial/lateral hamstrings (MH/LH)] and normalized to percent maximum voluntary isometric contraction (MVIC). Key shape and magnitude features of each waveform were extracted using principal component analysis (PCA). Self-reported pain was collected using a numeric pain rating scale (NPRS) at baseline, 10, 20 and 30 minutes of walking. Participants with moderate knee osteoarthritis were further categorized into two groups: (1) individuals who self-reported an increase in knee pain (>0) from baseline (before treadmill walking) to 30 minutes of walking, and (2) individuals who self-reported no change (=0), or a decrease (<0), in knee pain from baseline to 30 minutes of walking. Mean differences and 95% confidence intervals for neuromuscular PC scores were calculated between over ground walking sessions. Ensemble averages represent group neuromuscular patterns before and after treadmill walking. Results: Eleven participants with moderate knee osteoarthritis reported an increase in knee pain immediately after treadmill walking, and 12 participants reported no change or a decrease in pain (Table 1). Compared to asymptomatic participants, participants with knee osteoarthritis had more radiographic severity (p<0.001). These individuals also reported more clinical severity using pain (p<0.003) and total (p<0.005) WOMAC scores, with significantly worse scores for individuals reporting increased pain (p<0.004). No other significant differences were observed at baseline between groups. Neuromuscular EMG magnitudes (PC1) decreased after treadmill walking. Participants with knee osteoarthritis plus increased pain had higher initial EMG magnitudes, which remained elevated after treadmill walking. These participants also had the largest decrease in quadriceps activation [VM: -29.7 (95%CI: -50.7, -8.8), VL: -33.7 (95%CI: -60.1, -7.3), and RF: -20.1 (95%CI: -40.8, 0.5)] compared to participants with no change in pain (Figure 1). Conversely, asymptomatic participants had the largest decrease in hamstring activation [MH: -27.0 (95%CI: -45.3, -8.8) and LH: -23.5 (95%CI: -45.8, 1.1), respectively]. Although higher PC2 scores (i.e., difference between early and late stance amplitude) were observed in the lateral compartment for participants with increased pain, no changes were observed. Similarly, no changes were observed for PC3 scores (i.e., difference between mid-stance and early swing amplitude) except for VM in asymptomatic participants [5.7 (95%CI: 0.7, 10.6)] and participants with knee osteoarthritis plus increased pain [6.2 (95%CI: 0.2, 12.3)], and VL in asymptomatic participants only [6.2 (95%CI: 2.2, 10.2)]. Conclusions: Despite varied pain responses, all participants had reduced quadriceps and hamstring activation after a bout of physical activity. However, individuals with knee osteoarthritis who experienced increased pain had higher neuromuscular activation for all muscle groups before and after walking. Muscle strength was similar between groups, which may indicate higher co-activity of the quadriceps and hamstrings as a potential guarding mechanism to increase joint stiffness, reduce pain and possibly counterbalance joint instability. These preliminary results suggest that a bout of physical activity can reduce neuromuscular activation in quadriceps and hamstrings, however, pain levels as well as the pain response to physical activity must be considered during prescription.