Voluntary and reflex control of human back muscles during induced pain

1. Back pain is known to change motor patterns of the trunk. The purpose of this study was to examine the motor output of the erector spinae (ES) musc les during pain in the lumbar region. First, their voluntary activation was assessed during flexion and. re-extension of the trunk. Second, effects of cutaneous and muscle pain on the ES stretch reflex were measured, since in creased stretch reflex gain has been suggested to underlie increased muscle tone in painful muscles. 2. The trunk movement and electromyographic al (EMG) signals from the right and left ES during pain were compared with values before pain. Controlled muscle pain uas induced by infusion of 5 % saline into the right lumbar ES. Cutaneous pain was elicited by mechanical or electrical stimulation of the dorsal lumbar skin. The stretch reflex was evoked by rapidly indenting the right lumbar ES with a servo-motor prodder. 3. The results from the voluntary task show that muscle pain decreased the modulation depth of EX EMG; activity. This pattern was associated with a de creased range and velocity of motion of the painful body segment, which wou ld normally serve to avoid further injury Interestingly when subjects overc ame this guarding tendency and made exactly the same movements during pain as before pain, the EMG modulation depth was still reduced. The results see m to reconcile the controversy of previous studies, in which both hyper- an d hypoactivity of back muscles in pain have been reported. 4. In the tapped muscle, the EMG response consisted of two peaks (latency 1 9.3 +/- 2.1 and 44.6 +/- 2.5 ms, respectively) followed by a trough. On the contralateral side the first response was a trough (26.2 +/- 3.2 ms) while the second (46.1 +/- 4.3 ms) was a peak, similar to the second peak on the tapped side. Cutaneous pain had no effect on the short-latency response bu t significantly increased the second response on the tapped side. Surprisin gly, deep muscle pain had no effect on the stretch reflex. A short-latency reciprocal inhibition exists between the right and left human EX. 5. It is concluded that deep back pain does not influence the stretch refle xes in the back muscles but modulates the voluntary activation of these mus cles.