Enhancing Muscle Force and Femur Compressive Loads Via Feedback-Controlled Stimulation of Paralyzed Quadriceps in Humans

Objective: To compare paralyzed quadriceps force properties and femur compressive loads in an upright functional task during conventional constant-frequency stimulation and force feedback-modulated stimulation. Design: Crossover trial. Setting: Research laboratory. Participants: Subjects (N=13; 12 men, I woman) with motor-complete spinal cord injury. Interventions: Subjects performed 2 bouts of 60 isometric quadriceps contractions while supported in a standing frame. On separate days, subjects received constant-frequency stimulation at 20Hz (CONST) or frequency-modulated stimulation triggered by a change in force (FDBCK). During FDBCK, a computer algorithm responded to each 10% reduction in force with a 20% increase in stimulation frequency. Main Outcome Measures: A biomechanical model was used to derive compressive loads on the femur, with a target starting dose of load equal to 1.5 times body weight. Results: Peak quadriceps force and fatigue index were higher for FDBCK than CONST (P<.05). Within-train force decline was greater during FDBCK bouts, but mean force remained above CONST values (P<.05). As fatigue developed during repetitive stimulation, FDBCK was superior to CONST for maintenance of femur compressive loads (P<.05). Conclusions: Feedback-modulated stimulation in electrically activated stance is a viable method to maximize the physiologic performance of paralyzed quadriceps muscle. Compared with CONST, FDBCK yielded compressive loads that were closer to a targeted dose of stress with known osteogenic potential. Optimization of muscle force with FDBCK may be a useful tactic for future training-based antiosteoporosis protocols.