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<.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.

Figure 1

Schematic representation of the active-resisted stance experimental paradigm. The force transducer measures the posterior force during the isometric activation of the quadriceps. The biomechanical calculation of compressive (Fc), shear (Fs) and quadriceps force (Fq) is detailed by Frey Law and Shields . The model uses the subject’s height, weight, and hip/knee position to resolve the compression and shear forces at the distal femur, expressed as a percent of the subject’s body weight.

Figure 2

Representative example of CONST (left) and FDBCK (right) bouts. Representatives of all frequencies are plotted (20, 24, 28, 32, 36, 40, 44, and 48 Hz) in the FDBCK bout. The corresponding contractions for the CONST bout are also depicted.

Figure 3

Representative example of a 48 Hz FDBCK contraction, showing the instantaneous force values associated with each of the 100 stimulus pulses. The dotted line illustrates the within-train force decline slope for pulses 30–100.

Figure 4

Mean (SE) normalized quadriceps force (A), normalized mean force (B), and fatigue index (C) during repetitive stimulation. Percentages to the right of the plots in (A) and (B) correspond to the value of contraction 60. * = greater than corresponding CONST bout (p < 0.05).

Figure 5

Modeled femur compressive load, expressed as a percent of body weight (%BW). * = significant difference between FDBCK and CONST for bout 1 (p < 0.05). ** = significant difference between FDBCK and CONST for bout 2 (p < 0.05).