Involuntary muscle activity reduces passive range of motion at known torque: a stimulation study
Diong J , Héroux ME, Nguyen D, Foo Y, Kastre C, Andersson K, Gandevia SC, Butler JE
1. Sydney Medical School, The University of Sydney, Australia.
2. Neuroscience Research Australia (NeuRA), Sydney, Australia
3. Faculty of Health Sciences, The University of Sydney, Australia.
4. Faculty of Medicine, Linköping University
Assessments of passive joint range of motion either assume that muscles around a joint are relaxed or concomitant involuntary muscle activity of up to 5% of maximal voluntary contraction is small enough to ignore [1,2], but these assumptions have not been tested. We aimed to determine the extent to which small amounts of involuntary muscle activity limit passive range of motion, by measuring passive range of motion in the presence of simulated involuntary muscle activity in antagonist muscles. Thirty able-bodied adults participated in this study. Subjects were first seated with the tested knee flexed at 90 deg and ankle neutral, and predicted maximal plantarflexion torque was determined using twitch interpolation. With the knee flexed at 90 deg and fully extended, soleus was continuously stimulated to generate 0, 1, 2.5, 5, 7.5 and 10% of predicted maximal torque, in random order, while the ankle was passively dorsiflexed by an investigator blinded to the amount of stimulation. Ankle dorsiflexion torque-angle curves at each percent of predicted maximal torque were obtained. On average, for every 1% increase in plantarflexion torque, ankle range of motion at 9 Nm decreases by 2.4 deg (95% CI 2.0 to 2.7 deg; knee flexed 90 deg) and by 2.3 deg (95% CI 2.0 to 2.5 deg; knee fully extended). For a 5% increase in plantarflexion torque, the amount usually considered small enough to ignore, ankle range of motion decreases by ~12 deg. That is, small amounts of involuntary muscle activity substantially limit passive joint range of motion.
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