Hybrid stimulation enhances torque as a function of muscle fusion in human paralyzed and non-paralyzed skeletal muscle

Abstract

Objective: After spinal cord injury (SCI), hybrid stimulation patterns that interpose paired-pulse doublets over a constant-frequency background may enhance the metabolic "work" (muscle torque) performed by paralyzed muscle. This study examined the effect of background stimulation frequency on the torque contribution of the doublet before and after fatigue.

Design: Cross-sectional study.

Setting: Research laboratory in an academic medical center.

Participants: Five men with chronic sensory and motor-complete SCI and ten non-SCI controls (6 males, 4 females). SCI subjects were recruited from a long-term study of unilateral plantar-flexor training; both limbs were tested for the present study.

Interventions: Subjects underwent plantar flexor stimulation at 5, 7, 9, and 12 Hz. The four background frequencies were overlaid with 6 ms doublets delivered at the start, middle, or at both the start and middle of each train. The 5 Hz and 12 Hz frequencies were analyzed after fatigue.

Outcome measures: Mean torque, peak torque, torque fusion index, doublet torque.

Results: Trains with doublets at both the start and middle yielded the most consistent enhancement of torque (all P < 0.028). Torque contribution of the doublet was greatest at low stimulus frequencies (all P < 0.016). The low relative fusion of untrained paralyzed muscle preserved the efficacy of the doublet even during fatigue.

Conclusion: Hybrid stimulus trains may be an effective way to increase contractile work in paralyzed muscle, even after fatigue. They may be useful for rehabilitation strategies designed to enhance the metabolic work performed by paralyzed skeletal muscle.

Keywords: Doublet; Exercise; Skeletal muscle; Spinal cord injury.

Figures

Figure 1

Representative examples of electrical stimulation patterns. The least-fused (5 Hz, left panel) and most-fused (12 Hz, right panel) contractile conditions are depicted. The 7 Hz and 9 Hz frequencies (not shown) demonstrated progressively higher levels of between-pulse torque fusion.

Figure 2

Mean (SE) mean torque. After fatigue (right-most panels), only the “Both” condition was administered. Dotted and dashed lines demonstrate the pre-fatigue “Both” value for 5 Hz and for 12 Hz, respectively. All cohorts demonstrated a significant effect of doublet condition; * = different from Constant; ^ = different from Start; # = different from Middle; + = different from pre-fatigue (all P 

Figure 3

Mean (SE) peak torque. After fatigue (right-most panels), only the “Both” condition was administered. Dotted and dashed lines demonstrate the pre-fatigue “Both” value for 5 Hz and for 12 Hz, respectively. The SCI – Untrained Limb cohort demonstrated a significant effect of doublet condition; * = different from Constant; ^ = different from Start; # = different from Middle; + = different from pre-fatigue; & = different from 5 minutes post-fatigue (all P 

Figure 4

Mean (SE) fusion index. Fusion index was calculated from the “Constant” condition pre-fatigue and the “Both” condition post-fatigue. In the post-fatigue data (right-most panels), dotted and dashed lines represent the pre-fatigue 5 Hz and 12 Hz values, respectively. Pre-fatigue, each cohort demonstrated a significant effect of frequency; pairwise differences are indicated with brackets. Significance for all tests: p 

Figure 5

Mean (SE) doublet torque. After fatigue (right-most panels), only the “Both” condition was administered. Dotted and dashed lines denote the pre-fatigue “Both” value for 5 Hz and for 12 Hz, respectively. Pre-fatigue, each cohort demonstrated a significant effect of frequency; pairwise differences are indicated with brackets. The Non-SCI cohort demonstrated a significant effect of doublet condition. * = different from bracketed frequency; # = different from Middle; + = different from pre-fatigue; @ = different from post-fatigue (all P 

Figure 6

Mean (SE) fusion index versus mean (SE) doublet torque for 5 Hz and 12 Hz stimulus frequencies before and after fatigue.