Enhancing muscle force and femur compressive loads via feedback-controlled stimulation of paralyzed quadriceps in humans

Abstract

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, 1 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