RELATIONSHIP BETWEEN CHANGES IN MUSCLE SIZE FOLLOWING 12 WEEKS OF NMES INDUCED RESISTANCE TRAINING USING MRI and FIBER SPECIFIC ANALYSES IN PERSONS WITH COMPLETE SPINAL CORD INJURY.

Abstract

Purpose/Hypothesis: Chronic spinal cord injury (SCI) results in extreme atrophy of skeletal muscle below the level of lesion. We previously reported substantial muscle hypertrophy measured via magnetic resonance imaging (MRI) resulting from a neuromuscular electrical stimulation (NMES) induced resistance training program (RT). The purpose of this follow-up study was to determine the relationship between changes in muscle cross-sectional area (CSA) via magnetic resonance imaging (MRI) and fiber CSA measured using in-vitro histochemical techniques after 12 weeks of NMES-induced RT in subjects with chronic complete SCI. Number of Subjects: Three men (35.6 ± 4.9 yrs, 76.6 ± 21.5kg) with chronic (13.± 6.5yrs post-injury) complete SCI (ASIA A, C5-T10) participated in this study. Materials/Methods: Subjects performed four sets of ten NMES-induced knee extensions, two days per week for 12 weeks with resistance progressively increased over the training period. Percutaneous skeletal muscle biopsies from the m. vastus lateralis and MR images of the thigh were obtained from each subject both pre- and post-resistance training. Histological staining of individual fibers in muscle cross-sections was performed and fiber specific CSA was determined using computerized planimetry (NIH Scion Image Program (Version 4.0.2). Transaxial MR Images of the thighs were collected with a 1.5-T magnet (TR = 500 ms; TE = 14 ms; FOV = 20cm; encoding matrix=256x256; 1cm slice thickness separated by 0.5cm) from the hip joint to the knee joint using the whole body coil. Linear regression analysis was used to determine the relationship between changes in whole muscle CSA and changes in individual fiber CSA. Dependent samples t-tests were used to assess for differences in fiber area pre- and post-NMES training. Significance was set at p < 0.05. Results: Changes in muscle CSA explained 99.5% of the variance in fiber hypertrophy following NMES induced RT (y = 2.215x − 0.0128; R2 = 0.9956). In addition, following RT, average muscle fiber CSA increased from (2807 ± 327cm2) at baseline to (4581 ± 510cm2) after training (p < 0.05). Conclusions: The main finding of this study was that 12 weeks of NMES-induced RT elicited substantial hypertrophy in individual muscle fibers in subjects with complete SCI. In addition, the relationship between these changes and improvements in whole muscle CSA via MRI were extremely strong. Clinical Relevance: These data provide further evidence of the responsiveness of skeletal muscle to RT in the SCI population as well as support the use of NMES as a training modality capable of eliciting substantial hypertrophy in human skeletal muscle.