Study Of Pain

Voluntary and involuntary activation of painful muscles in humans.

Milan Zedka, Ming Chan, Arthur Prochazka

Painful muscles don’t move normally. Recently we found that activity of trunk muscles infused with hypertonic saline to make them painful was altered during voluntary movements while their involuntary (stretch reflex) activation remained unchanged (Zedka et al., 1999).

In a more recent study we therefore focused on the descending voluntary commands (Zedka et al., 2000). We reasoned that as the brain generates voluntary motor commands, muscle twitches evoked by stimulating the motor cortex magnetically should be attenuated during muscle pain.

We applied a series of stimuli of constant intensity to the region of the motor cortex of 6 subjects with a magnetic coil placed over the cranial vertex. EMG and force output during submaximal and maximal contractions of the left biceps brachii or the lower back muscles (erector spinae) were recorded in each subject first during a control session and then during muscle pain. Constant, diffuse muscle was induced by a continuous infusion of 5% NaCl (subjects rated pain intensity between 4 and 7 on a 0 - 10 scale).

As expected, voluntary movements during pain changed, but contrary to our hypothesis, the amplitudes of muscle responses evoked by magnetic stimulation of the brain did not change.

The lack of pain effect on the excitability of the spinal motoneuronal pool and the cortico-spinalpathways leads us to the conclusion that the altered voluntary movement patterns characteristically seen during muscle pain result from altered input to the motor cortex, or altered extrapyramidal motor commands. Subjects reported the feeling of consciously wanting to minimize the voluntary activation of painful muscles. This avoidance evidently has its origin outside the corticospinal system.

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