The Effectiveness of Progressively Increasing Stimulation Frequency and Intensity to Maintain Paralyzed Muscle Force During Repetitive Activation in Persons With Spinal Cord Injury

Experimental procedures.

Plots of the peak force responses to the 2 stimulation protocols for a typical subject. Each data point represents a peak force from 1 contraction. (A) The intensity followed by frequency modulation protocol. (B) The frequency followed by intensity modulation protocol. In both protocols, the initial pulse duration was adjusted so that the initial peak forces were equal to a subject's maximal twitch force (solid lines). Stimulation trains were delivered at a rate of 1 train every 1.1 seconds to fatigue the muscles. When the peak forces declined below 90% of the subject's maximal twitch force (dashed lines), stimulation frequency or intensity was progressively increased according to the modulation steps calculated before testing. As shown in the figure, the intensity followed by frequency modulation protocol produced more successful contractions than the frequency followed by intensity modulation protocol.

Plot of the number of successful contractions produced by the 2 stimulation protocols tested. Light gray indicates the number of successful contractions produced by the intensity modulation portion of the protocols; dark gray indicates the number of successful contractions produced by the frequency modulation portion of the protocols. The averaged number of successful contractions produced by the intensity followed by frequency (Inten-Freq) and the frequency followed by intensity (Freq-Inten) modulation (mod) protocols. *P<.05 (see text for details).

Example for the determination of stimulation intensity modulation steps based on the force-intensity relationship curve for a typical subject. The force-intensity relationship curve (black curve) was fitted with equation 1, and the parameter values for A, PD₀, and τ were then determined for each subject. The starting pulse duration was determined by locating the pulse duration that produces peak force equal to the subject's maximal twitch force (MTF) (black point). When the peak force dropped to 90% of the subject's maximal twitch force (gray point), a new curve (gray curve) representing the new relationship between force output and stimulation intensity could then be determined by calculating the new A value. (From the results of a previous study,³⁴ only A value changes with fatigue). By locating the pulse duration that produces peak force equal to the subject's maximal twitch force from the new force-intensity relationship curve, the next intensity modulation step was determined. NOTE. Not drawn to scale. See text for details.

Family of force-intensity relationship curves for a typical subject. The force-intensity relationship shifted down with fatigue. As shown by the arrows, the intensity modulation steps were determined by locating the intersection between the force-intensity curves and the maximal twitch force dashed line.