The influence of chronic knee pain and age on conditioned pain modulation and motor unit control

BACKGROUND: Over 25% of U.S. older adults experience chronic knee pain, which worsens with inactivity, creating a cycle of pain, disability, and sedentary behavior. However, a limited understanding of the underlying mechanisms hinders the development of effective treatments. PURPOSE: This study integrated assessments of Conditioned Pain Modulation (CPM) and motor unit control to elucidate the role of pain sensitization and neuromuscular impairments specific to chronic knee pain. METHODS: Seventy-five participants were divided into three groups: young adults (n = 25), older adults without pain (n = 30), and older adults with chronic pain (n = 20). CPM efficiency was evaluated using heat and pressure test stimuli alongside a cold-water bath conditioning stimulus. Motor unit assessments involved isometric contractions of the dominant/painful quadriceps at 50 % of maximal torque, with surface electromyographic signals recorded from the vastus lateralis. RESULTS: Independent of chronic knee pain, older adults demonstrated more efficient heat-CPM than young adults (p = 0.014, ηp² = 0.080). The slope of the mean firing rate versus recruitment threshold relationship indicated that older adults showed greater firing rates for high threshold motor units, independent chronic knee pain (p = 0.010, ηp² = 0.136). The y-intercept of this relationship was greater in younger versus chronic-pain older adults (p = 0.024, ηp² = 0.111). CONCLUSION: Contrary to our hypothesis, older adults displayed more efficient heat-CPM, independent of chronic pain. Similarly, motor unit control was mostly influenced by age but not chronic knee pain. These findings suggest that age-related changes in pain modulation and motor unit behavior may play a greater role in neuromuscular function than the presence of chronic pain itself.