An improved method for muscle force assessment in neuromuscular disease

Abstract

We describe here the reliability and validity of methods to quantify involuntary muscle torque induced by non-invasive nerve stimulation. A rigid apparatus was used to hold the subject's limb in a predetermined position and confine movement to a specific direction (i.e. ankle dorsiflexion or thumb adduction). An incorporated strain gauge was used to measure isometric torque, and all data were recorded by a data acquisition program. The innervating nerves were stimulated by surface electrodes, using either single stimuli to generate a twitch, or short trains of stimuli to produce tetanic contraction of the individual muscle under study. The average peak tetanic torque generated by the dorsiflexor muscles in healthy control was 20.4 +/- 3.8 Nm and varied by 3.7% with repeated testing. The mean torque generated by the adductor pollicis muscle in controls was 1.5 +/- 0.4 Nm and varied by 4.6% with repeated testing. In patient populations significant changes in activated torque were readily quantified, and the effects of treatment can be easily assessed. Furthermore, several specific parameters of recorded isometric contractions were measured; e.g. time between stimulus and torque onset, peak rate of torque development, time to peak torque, half-relaxation time, and others (none of which are measurable when using voluntary contraction of muscle). Compared to current assessment methods, monitoring muscle torque generated by nerve stimulation improves objectivity, reliability, and quantitative capabilities. The presented method has significant potential both in diagnosing neuromuscular disorders and determining treatment efficacy.

Force assessment of the stimulated arm flexors: quantification of contractile properties

“…The speci ed assessment approach is similar to that previously described to investigate either the ankle dorsi exors or adductor pollicis muscles [14]. Yet, there are two important distinctions to make relative to our previous work: (1) the arm exors can be more reliably activated via motor-point stimulation, which is well de ned; and (2) the arm exors are a much more proximal muscle group.…”

“…Some of our normal subjects even elicited very low peak torque at the end of the nerve stimulation protocol indicating the electrodes may have either moved or that muscle fatigue had occurred, even though we had a two-minute refractory period between each stimulus. In previous studies on other muscles, this two-minute period was considered a suf cient amount of time to prevent fatigue [14].…”

“…The stabilizing device used in this study was a modi cation of a previously constructed device [14]. This device was used to maintain the arm in a stabilized position while allowing for upper arm access needed for stimulation of the musculocutaneous nerve or the middle region of the biceps (i.e.…”

“…This quantitative approach is noninvasive and has been successfully used to study force properties of distal muscles of the legs and the hands, speci cally the anterior tibialis and adductor pollicis [14,23,24]. An added bene t of such an approach is that one can better characterize the phenotypic contractile responses: i.e.…”

Stimulated muscle force assessment of the sternocleidomastoid muscle in humans

“…All applied stimuli were supramaximal with 0.3 ms pulse durations; for the double-pulse stimulations, 5 ms interpulse intervals were employed. This approach was similar to that used by our laboratory for stimulated outcomes assessment of other muscle groups [10,11].…”