Small amounts of involuntary muscle activity limit passive joint range of motion

The loss of passive joint range of motion (i.e. contracture) is common in stroke and other neurological conditions. More than half of people with stroke or spinal cord injury will develop at least one contracture (Diong et al., 2012; Kwah et al., 2012). Contracture impairs physical function and can cause profound disability. Assessing and restoring passive range of motion to encourage return to physical function is an important priority in rehabilitation. However, contractures in neurological conditions often persist and are difficult to treat.

Standard clinical assessments of range of motion assume that muscles around a joint are relaxed, or that any involuntary muscle activity present is small enough to ignore. However, activity in elbow flexor and plantarflexor muscles during stretch has been observed to change passive range of motion in people with stroke and cerebral palsy (Genet et al., 2017; McMulkin et al., 2008). Neural (i.e. muscle activity) and non-neural (i.e. mechanical) mechanisms can cause loss of joint range of motion, but clinical assessments of range cannot differentiate between these mechanisms if muscle activity is not measured. Furthermore, interventions that target neural or non-neural mechanisms are different. A key question is: to what extent do small amounts of involuntary muscle activity limit measures of passive joint range of motion?

To answer this question, we used an artificial method to generate small amounts of involuntary muscle activity and measure how it changes passive ankle range of motion. In 30 able-bodied participants, the soleus muscle was continuously stimulated to generate low levels (i.e. 1 to 10%) of plantarflexor muscle contraction as the ankle was passively dorsiflexed by an experimenter. Participants were asked to remain relaxed, and we recorded ankle torque and angle data. Since changing the knee angle changes muscle length of the biarticular gastrocnemius, we performed the testing with the knee flexed 90 degrees and fully extended to determine if differences in gastrocnemius length affected the results.


We found that small amounts of involuntary muscle activity substantially limited passive joint range of motion. Specifically, a 5% muscle contraction decreased ankle angle by 12 degrees on average. The magnitude and precision of this effect was similar with the knee flexed 90 degrees or with it fully extended. So, gastrocnemius length did not affect the results. Clinically, changes in joint angle of approximately 5 degrees are meaningful, so an average decrease of 12 degrees is a big effect.


Our findings imply that assessments of passive range of motion are likely to be inaccurate if performed in the presence of unmeasured muscle activity. Our findings also agree with a recent study that showed small amounts of voluntary muscle activity limit passive range of motion (Le Sant et al., 2019). These results suggest that findings from clinical or laboratory studies in which muscle activity was not measured, or where muscle activity up to 5% maximal contraction was ignored, are problematic.

Further, our findings highlight the importance of accurately measuring small amounts of muscle activity, and adjusting for this activity, when measuring passive joint range of motion.


Diong, J., Gandevia, S.C., Nguyen, D., Foo, Y., Kastre, C., Andersson, K., Butler, J.E., and Héroux, M.E. (2019) Small amounts of involuntary muscle activity reduce passive joint range of motion. J Appl Physiol (in press). doi: 10.1152/japplphysiol.00168.2019.

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Diong, J., Harvey, L. A., Kwah, L. K., Eyles, J., Ling, M. J., Ben, M., and Herbert, R. D. (2012). Incidence and predictors of contracture after spinal cord injury – a prospective cohort study.Spinal Cord, 50(8), 579–584.

Genet, F., Schnitzler, A., Droz-Bartholet, F., Salga, M., Tatu, L., Debaud, C., Denormandie, P., and Parratte, B. (2017). Successive motor nerve blocks to identify the muscles causing a spasticity pattern: example of the arm flexion pattern. J Anat, 230(1), 106–116.

Kwah, L. K., Harvey, L. A., Diong, J. H., and Herbert, R. D. (2012). Half of the adults who present to hospital with stroke develop at least one contracture within six months: an observational study. J Physiother, 58(1), 41–47. h

Le Sant, G., Gross, R., Hug, F., and Nordez, A. (2019). Influence of low muscle activation levels on the ankle torque and muscle shear modulus during plantar flexor stretching. J Biomech (in press). doi: 10.1016/j.jbiomech.2019.06.018. 

McMulkin, M. L., Gordon, A. B., Caskey, P. M., Ferguson, R. L., and Baird, G. O. (2008). Range of motion measures under anesthesia compared with clinical measures for children with cerebral palsy. J  Pediatr Orthop. Part B, 17(6), 277–280.


Dr Joanna Diong is an Honorary Research Associate at NeuRA and Senior Lecturer at The University of Sydney. Her research focuses on the mechanisms of impaired human movement in stroke. She regularly co-authors a blog on doing good science at

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