H-reflex, muscle voluntary activation level, and fatigue index of flexor carpi radialis in individuals with incomplete cervical cord injury

Background. Individuals with incomplete spinal cord injury (SCI) are predisposed to muscle fatigue during voluntary exercise. However, the origin of fatigue is unclear. Objective. The authors examined the motoneuron excitability, muscle activation level, and fatigue properties of the flexor carpi radialis muscle, just below the level of injury. Methods. Nine individuals with chronic, incomplete cervical cord injury and 9 age-matched healthy individuals were recruited. The authors tested maximum voluntary contraction (MVC), motoneuron excitability by the maximum amplitude of the H-reflex (Hmax at C-7), and muscle voluntary activation level measured by the interpolated twitch technique. Subjects were fatigued by repetitive maximal voluntary isometric wrist flexion. General fatigue index (GFI), central fatigue index (CFI), and peripheral fatigue index (PFI) of flexor carpi radialis were examined before, during, and immediately after exercise. Results. The Hmax in the SCI group was significantly higher (P =.0028) than in controls. The MVC (P <.001) and voluntary activation level (P =.016) in the SCI group were significantly lower. The GFI and CFI decreased in both the SCI and the non-SCI groups. The PFI in the SCI group was significantly higher (ie, less fatigue) than that in controls at 30 repetitive contractions. Conclusions. In individuals with incomplete SCI, the deficit in central drive is an important source of muscle weakness and fatigue in the muscle below the level of injury.