Contractile Properties and the Force-Frequency Relationship of the Paralyzed Human Quadriceps Femoris Muscle

Background and Purpose. Following spinal cord injury (SCI), paralyzed muscles undergo physiological changes that alter their force responses to electrical stimulation. The purpose of this study was to investigate the effects of SCI on the contractile properties and force-frequency relationship (FFR) of the paralyzed human quadriceps femoris muscle of adolescents and young adults. Subjects. Thirteen subjects (11 male, 2 female; age range=11–24 years) with motor complete SCIs and 13 matched control subjects (11 male, 2 female; age range=9–23 years) without SCI participated in the study. Methods. Both groups of subjects underwent the same testing protocol using similar equipment. Results. The paralyzed muscles of the subjects with SCI produced 62% of the peak twitch force and had a fatigue ratio that was 65% of that of the control subjects. The paralyzed muscles contracted 14% and 25% faster and relaxed 38% and 46% faster than the nonparalyzed muscles in nonfatigued and fatigued conditions, respectively. Compared with the control subjects, the subjects with SCI had twitch-to-tetanus ratios that were 84% and 127% greater in nonfatigued and fatigued conditions, respectively. Relative to the control subjects, the FFR of the subjects with SCI was shifted to the left in the fatigued condition. Relative to their respective nonfatigued conditions, the FFR of both groups of subjects shifted to the right with fatigue. Discussion and Conclusion. These findings may have important implications for designing stimulation strategies to reduce the rapid fatigue that limits the clinical efficacy of functional electrical stimulation. [Scott WB, Lee SCK, Johnston TE, et al. Contractile properties and the force-frequency relationship of the paralyzed human quadriceps femoris muscle. Phys Ther. 2006;86:788–799.]