The force-velocity relationship of paralyzed quadriceps muscles during functional electrical stimulation cycling

Objectives.  To investigate the nature of the force-velocity relationship on muscle forces and power outputs during functional electrical stimulation (FES)-evoked cycling at different pedaling cadences. Materials and Methods.  Ten patients with T4-T9 spinal cord injuries (ASIA A) performed FES-evoked cycling at 50 rev/min using a motorized isokinetic ergometer for 20 min, after which quadriceps crank torque and power were measured at 10, 30, and 50 rev/min. Results.  Pedal cadence affected both the shape and the magnitudes of the quadriceps torque and power curves. Significantly greater average torque (T) and peak crank torques (PTi) were elicited at lower pedal cadences (T(10)  > T(50) , p < 0.001; PTi(10)  > PTi(50) , p = 0.007). Instantaneous peak power (PPi) and average power output (PO) increased significantly with pedal cadence, such that PPi(50) and PPi(30)  > PPi(10) (p < 0.001) and PO(50) or PO(30)  > PO(10) (p < 0.001). At the higher cadences, peak torque and peak power were developed at significantly later angles (p < 0.001). Conclusions.  The force-velocity relationship of muscle has a significant effect upon the muscle forces produced during FES-evoked cycling. However, muscle force rise times and fatigue within FES-evoked contractions, especially at a low cadence, should be considered when making comparisons between different FES-cycling cadences.