Changes in muscle T2 relaxation properties following spinal cord injury and locomotor training.

Magnetic resonance (MR) is frequently used to study structural and biochemical properties of skeletal muscle. Changes in proton transverse relaxation (T2) properties have been used to study muscle cellular damage, as well as muscle activation during exercise protocols. In this study, we implemented MR imaging to characterize the T2 relaxation properties of rat hindlimb muscles following spinal cord injury (SCI) and locomotor training. After moderate midthoracic contusion SCI, Sprague-Dawley rats were assigned to either treadmill training, cycle training or an untrained group. T2 weighted images were obtained and mean muscle T2 times were calculated in the tibialis anterior, soleus, and gastrocnemius (GAS) muscles at pre-injury as well as at 1, 2, 4, 8, and 12 weeks post-injury. Following SCI, hindlimb muscles in untrained animals showed a significant increase in muscle T2, with the most dramatic shift (+5.46 ms) observed in soleus muscle at 1 week post-SCI. Subsequently, all muscle groups showed a spontaneous recovery in muscle T2 with normalized T2 values in the GAS and tibilias anterior muscles at 4 weeks and the soleus at 12 weeks post-SCI. Both training paradigms, treadmill and cycling training, accelerated the recovery of soleus muscle T2. As a result, soleus muscle T2 recovered back to pre-injury values within 3 weeks of training in both training groups. Finally, in vitro histological assessments of rat skeletal muscles demonstrated that there was no apparent muscle injury in any of the muscles studied at 1 week post-SCI.