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

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.

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.

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.

Independent of chronic knee pain, older adults demonstrated more efficient heat-CPM than young adults (p = 0.014, η_p² = 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).

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.