Voluntary activation failure is detectable in some myositis patients with persisting quadriceps femoris weakness: an observational study

Abstract

This cross-sectional, observational study was undertaken to examine whether voluntary activation failure could contribute to the persisting weakness observed in some patients with treated idiopathic inflammatory myositis. In 20 patients with myositis of more than six months' duration (5 males, 15 females; mean [+/- 1 SD] age 53 11 years) and 102 normal subjects (44 males, 58 females; mean age 32 8 years), isometric maximum voluntary contractions (MVCs) of the dominant quadriceps femoris (QF) were quantified. Absolute MVC results of normal subjects and patients were then normalised with respect to lean body mass (force per units of lean body mass), giving a result in Newtons per kilogram. Based on mass-normalised force data of normal subjects, patients were arbitrarily stratified into "weak" and "not weak" subgroups. During further MVC attempts, the "twitch interpolation" technique was used to assess whether the QF voluntary activation of patients was complete. This technique relies on the fact that, because muscle activation is incomplete during submaximal voluntary contractions, electrical stimulation of the muscle can induce force increments superimposed on the submaximal voluntary force being generated. No between-gender differences were seen in the mass-normalised MVC results of healthy subjects, so the gender-combined results of 6.6 (1.5) N/kg were used for patient stratification. No between-gender difference was found for mass-normalised MVCs in patients: males 5.4 (3.2) and females 3.0 (1.7) N/kg (p > 0.05). Mass-normalised MVCs of male patients were as great as those of normal subjects (p > 0.05), but mass-normalised MVCs of female patients were significantly smaller than those of the normal subjects (p < 0.001). Only one of the six "not weak" patients exhibited interpolated twitches during electrical stimulation, but six of the 14 "weak" patients did, the biggest twitches being seen in the weakest patient. That interpolated twitches can be induced in some myositis patients with ongoing QF weakness during supposed MVCs clearly suggests that voluntary activation failure does contribute to QF weakness in those patients.

Figure 1

The effect of superimposed twitches on incremental voluntary quadriceps femoris (QF) contractions using the twitch interpolation technique. During a voluntary contraction, 1-Hz electrical twitches are delivered via surface electrodes applied over the motor nerve or muscle motor points. At low levels of voluntary activation (on the y-axis), a large proportion of muscle fibres remain unactivated, so that superimposed stimulation can induce large interpolated twitches. As the level of voluntary activation increases, the proportion of yet unactivated fibres decreases and so the height of the stimulation-induced interpolated twitches decreases until, near or at maximum voluntary contraction (MVC), twitches can no longer be seen (adapted from [32]).

Figure 2

Mass-normalised force results for normal subjects and the "not weak" and "weak" patients with myositis. Males and females are combined in all three groups. The "weak" patients with myositis were obviously weak relative to the other groups, with a mass-normalised force result of 2.2 N/kg compared with 6.6 N/kg for normal subjects and 6.5 N/kg for the "not weak" patients with myositis. Error bars represent + 1 SD from the mean. Abbreviation: F/LBM = force per units of lean body mass.

Figure 3

Force tracing of the male patient whose mass-normalised force was the lowest recorded in this study. Large interpolated twitches can be seen on the force trace before, during, and after a supposed maximum voluntary contraction (MVC).