Minimal Dose Of Electrical Stimulation Training Adapts Skeletal Muscle After Spinal Cord Injury

Abstract

PURPOSE After complete spinal cord injury (SCI), the skeletal muscles undergo significant adaptations, leading to fast and highly fatigable properties. Loss of chronic activity is one likely mechanism underlying these adaptations. The skeletal system also loses bone mineral density (BMD) at the rate of 2–3% per month, leading to osteoporosis and other secondary complications. No previous studies have investigated the long-term effects (>1 year) of electrical stimulation after acute SCI. We investigated the long term adaptations induced by a unilateral electrical stimulation training program on plantar flexion muscle fatigue, torque, and speed properties in individuals with SCI. We also analyzed the end point distal tibia BMD. METHODS Nine individuals (ages 21 to 57) with acute and complete spinal cord injury (< 3 months) received supra-maximal electrical activation of one calf muscle (isometric) for 5 days each week for 3 years. The opposite leg served as a control. The subjects were assessed weekly in the laboratory, but also trained at home. The training protocol used a custom designed stimulator programmed to deliver 4 bouts of 120 contractions every 2 s for 667 ms. The stimulator had an impedance based compliance meter. Plantar flexion (soleus) muscle fatigue (FI), peak torque (PT), contractile speed (1/2RT), and a twitch difference ratio (TTD) were calculated for both groups. Tibia BMD (4% site) was analyzed with peripheral quantitative computed tomography (pQCT). RESULTS After 3 years, the FI and PT were significantly greater (P <0.05) for the trained group than for the untrained group (150% and 50% greater, respectively). The 1/2RT and TTD changed minimally (∼25%, P = 0.12) during repetitive stimulation in the trained group, while the untrained group showed a large increase (∼200%, P <0.05). Interestingly, the absolute contractile speeds were not different between the trained and untrained groups after 3 years (P =.231). The distal tibia BMD was over 20% greater in the trained limbs than in the untrained limbs (P <0.05). CONCLUSIONS These findings provide evidence to support that less than 16 minutes of activity per day was adequate to prevent muscular adaptations after SCI. This study included a within subject control so that the impact of daily exercise could be ascertained within the same subjects. We believe a larger study using a method to load the entire lower extremities to maintain the musculoskeletal properties after SCI is now warranted. SUPPORT NIH R01 HD39445