Skeletal muscle adaptations following spinal cord contusion injury in rat and the relationship to locomotor function: A time course study

Experimental spinal cord injury (SCI) via contusion of moderate severity re sults in residual locomotor deficits, including a lack of coordination and trunk stability. Given that muscle contractile properties and fiber composi tion adapt to reduced neural input and/or weight bearing, contusion-induced locomotor deficits may reflect changes in hindlimb skeletal muscle. Theref ore, we examined muscle adaptations during early (1 week), intermediate (3 week), and late (10 week) stages of motor recovery after moderate SCI. Fort y-two Sprague Dawley rats underwent SCI via 1.1mm cord displacement with th e OSU impact device or served as age and weight-matched or laminectomy cont rols. Subsets of rats had soleus (SOL) in vitro physiological testing or SO L and extensor digitorum longus (EDL) myosin heavy chain (MHC) fiber type a nalysis. At 1 week post-SCI during paralysis/paresis, a significant decreas e in wet weight occurred in the plantaris, medial/lateral gastrocnemius (MG /LG), tibialis anterior, and SOL. Changes in contractile properties of the SOL did not accompany muscle wet weight changes. By 3 weeks, the loss of we ight-bearing activity early after SCI induced significant decreases in SOL peak twitch and peak tetanic tension as well as significantly greater IIx M HC expression in the EDL. By 10 weeks post-SCI, after several weeks of weig ht supported stepping, muscle wet weight, contractile properties and MHC co mposition returned to baseline levels except for MG/LG atrophy. Thus, muscl e plasticity appears to be extremely sensitive to locomotor deficits and th eir resolution after moderate spinal cord contusion.