Treadmill running and static stretching improve long-lasting hyperalgesia, joint limitation, and muscle atrophy induced by cast immobilization in rats.

The effects of exercise on chronic pain induced by immobilization are incompletely understood. The purpose of this study was to investigate whether 30min of treadmill running (TR; active exercise) and 10min of static stretching (SS; passive exercise) of the immobilized hindlimb reduce widespread chronic pain, joint limitation, and hindlimb muscle atrophy induced by cast immobilization in rats. One hindlimb of Sprague Dawley (SD) rats was immobilized for 2 weeks with a cast, and remobilization was conducted for 7 weeks. MRI study showed that cast immobilization had induced inflammatory changes in the immobilized hindlimb, beginning as early as 2h after cast removal; these changes continued for 2-3 days. Mechanical hyperalgesia in the calf and hindpaw developed as early as 2h after cast removal and continued for 7 weeks. TR and SS were initiated 3 days after cast removal and were continued 3 times per week for 2 weeks. Both forms of exercise significantly inhibited mechanical hyperalgesia in the calf and hindpaw in immobilized rats. Range-of-motion limitations in the knee and ankle joints and calf muscle atrophy after cast removal were also decreased by both TR and SS. This study is the first to demonstrate the beneficial effect of TR and SS on widespread chronic pain, joint limitation, and muscle atrophy in a cast-immobilized rat model.