Neuromuscular electrical stimulation resistance training enhances oxygen uptake and ventilatory efficiency independent of mitochondrial complexes after spinal cord injury: a randomized clinical trial

The purpose of the study was to determine whether neuromuscular electrical stimulation resistance training (NMES-RT)-evoked muscle hypertrophy is accompanied by increased V̇o 2 peak, ventilatory efficiency, and mitochondrial respiration in individuals with chronic spinal cord injury (SCI). Thirty-three men and women with chronic, predominantly traumatic SCI were randomized to either NMES-RT ( n = 20) or passive movement training (PMT; n = 13). Functional electrical stimulation-lower extremity cycling (FES-LEC) was used to test the leg V̇o 2 peak, V̇E/V̇co 2 ratio, and substrate utilization pre- and postintervention. Magnetic resonance imaging was used to measure muscle cross-sectional area (CSA). Finally, muscle biopsy was performed to measure mitochondrial complexes and respiration. The NMES-RT group showed a significant increase in postintervention V̇o 2 peak compared with baseline (ΔV̇o 2 = 14%, P < 0.01) with no changes in the PMT group (ΔV̇o 2 = 1.6%, P = 0.47). Similarly, thigh (ΔCSA thigh = 19%) and knee extensor (ΔCSA knee = 30.4%, P < 0.01) CSAs increased following NMES-RT but not after PMT. The changes in thigh and knee extensor muscle CSAs were positively related with the change in V̇o 2 peak. Neither NMES-RT nor PMT changed mitochondrial complex tissue levels; however, changes in peak V̇o 2 were related to complex I. In conclusion, in persons with SCI, NMES-RT-induced skeletal muscle hypertrophy was accompanied by increased peak V̇o 2 consumption which may partially be explained by enhanced activity of mitochondrial complex I. NEW & NOTEWORTHY Leg oxygen uptake (V̇o 2 ) and ventilatory efficiency (V̇E/V̇co 2 ratio) were measured during functional electrical stimulation cycling testing following 12-16 wk of either electrically evoked resistance training or passive movement training, and the respiration of mitochondrial complexes. Resistance training increased thigh muscle area and leg V̇o 2 peak but decreased V̇E/V̇co 2 ratio without changes in mitochondrial complex levels. Leg V̇o 2 peak was associated with muscle hypertrophy and mitochondrial respiration of complex I following training.