Neuromechanics Weekly: Pain Matters

We know that joint swelling (and thus inflammation) inhibits the contraction of the muscle which crosses the joint from the landmark work of Iles and Stokes back in the late 80’s. Now here is a paper stating that pain does the same thing. 

This tells us that there is an axon collateral from the primary pain neuron (the “C” fiber) that somehow inhibits the alpha moto neuron, similar to a flexor reflex, as pictured. his is most likely through affecting the gamma moto neuron (which goes to the spindles) rather than the alpha motoneuron; so the “sensitivity” of the muscle is changed (remember, spindles detect length changes, golgi’s tension).

So what does this mean to us and gait? It tells us that pain will inhibit the activity (voluntary and involuntary) of the ability for one to use their muscles, especially those crossing the joint in questions. Be aware of inflammation (painful or non painful) or the painful joint, which can contribute to a compensation pattern.

Ivo and Shawn…The Gait Guys. Making your life less painful and more functional….

Muscle Nerve. 2000 Aug;23(8):1219-26.

Inhibition of motor unit firing during experimental muscle pain in humans.

Sohn MK, Graven-Nielsen T, Arendt-Nielsen L, Svensson P.

Source

Center for Sensory-Motor Interaction, Orofacial Pain Laboratory, Aalborg University, Denmark.

Abstract

Electromyographic activity was recorded in the masseter muscle to investigate whether the firing characteristics of single motor units (SMUs) were affected by muscle pain. Capsaicin was injected into the masseter to induce pain. The interspike interval (ISI) and recruitment threshold of SMUs were measured while subjects performed isometric contractions at 5, 7.5, 10, 15, and 20% of maximum voluntary contraction. All subjects were able to maintain a stable isometric force during pain, but the mean ISI was significantly increased without changing the recruitment threshold. In all the experimental conditions, the firing frequency increased with increasing force, and SMUs recruited at low force fired at higher rates for all force levels. These results suggest that acute stimulation of nociceptive muscle afferents inhibits SMU activity without changing recruitment order in the homonymous muscle.

Copyright 2000 John Wiley & Sons, Inc.