Doublet Electrical Stimulation Enhances Torque Production in People With Spinal Cord Injury

Background. Muscle fatigue prevents repetitive use of paralyzed muscle after spinal cord injury (SCI). Objective. This study compared the effects of hybrid patterns of muscle stimulation in individuals with acute and chronic SCI. Methods. Individuals with chronic (n = 11) or acute paralysis (n = 3) underwent soleus muscle activation with a constant (CT) or doublet (DT) stimulation train before and at various times after a fatigue protocol. Results. The chronically paralyzed soleus was highly fatigable with a fatigue index (FI) of 19% +/- 6%, whereas the acutely paralyzed soleus was fatigue resistant (FI = 89% +/- 8%). For the chronically paralyzed group, the DT protocol caused less than 5% improvement in peak and mean force relative to the CT protocol before fatigue; however, after fatigue the DT protocol caused an increase in peak and mean force (>10%), compared with the CT protocol (P < .05). As the chronically paralyzed muscle developed low-frequency fatigue, the DT protocol became more effective than the CT protocol (P < .05). The DT protocol increased the rate of torque development before fatigue (42% +/- 78%), after fatigue (62% +/- 52%), and during recovery (87% +/- 54% to 101% +/- 56%; P < .05). The acutely paralyzed group showed minimal change in peak and mean torque with the DT protocol. Conclusions. Chronic reduced activity is associated with muscle adaptations (slow to fast) that render the muscle more amenable to force enhancement through doublet train activation after fatigue. These findings are applicable to patients using neuromuscular stimulation.