Strategies That Improve Paralyzed Human Quadriceps Femoris Muscle Performance During Repetitive, Nonisometric Contractions

Kebaetse MB, Lee SC, Johnston TE, Binder-Macleod SA. Strategies that improve paralyzed human quadriceps femoris muscle performance during repetitive, nonisometric contractions. To determine the effect of combining different stimulation frequencies on the ability of paralyzed human quadriceps muscle to produce a 50° knee excursion repetitively when starting at 90° of flexion. Repeated-measures design. Clinical research laboratory. Complete data were collected from 9 subjects aged 11 to 25 years (mean ± standard deviation, 17.1±4.5y) with spinal cord injury (SCI). Three protocols were each tested during separate sessions: 20-Hz trains of pulses followed by 66-Hz trains (C20+66), 33-Hz trains followed by 66-Hz trains (C33+66), and 66-Hz trains alone (C66). For each frequency, stimulation was repeated until the knee failed to produce a 50° excursion. This approach allowed us to evaluate the response to stimulation with 20-, 33-, and 66-Hz and combinations of 20- and 66-Hz and 33- and 66-Hz trains. Number of successful contractions. The C20 and C33 did not differ (mean, 41.0±12.6 excursions and 42.0±12.3 excursions, respectively), and each produced more excursions than the C66 protocol. The C20+66 and C33+66 protocols produced 51.4±15.0 and 44.9±13.6 excursions, respectively, and the C20+66 was the best protocol overall (all P≤.05). This study showed that stimulation strategies that start with low frequencies and switch to higher frequencies as the muscle fatigues could improve the ability of functional electric stimulation applications to perform repetitive, nonisometric contractions in subjects with SCI.