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Chronic low back pain (CLBP) is a major clinical problem with a substantial socio-economical impact. Today, diagnosis and therapy are insufficient, and knowledge concerning interaction between musculoskeletal pain and motor performance is lacking. Most studies in this field have been performed under static conditions which may not represent CLBP patients' daily-life routines. A standardized way to study the sensory-motor interaction under controlled motor performances is to induce experimental muscle pain by i.m. injection of hypertonic saline. The aim of the present controlled study was to analyze and compare electromyographic (EMG) activity of and coordination between lumbar muscles (8 paraspinal recordings) during gait in 10 patients with CLBP and in 10 volunteers exposed to experimental back muscle pain induced by bolus injection of 5% hypertonic saline. When the results are compared to sex- and age-matched controls, the CLBP patients showed significantly increased EMG activity in the swing phase; a phase where the lumbar muscles are normally silent. These changes correlated significantly to the intensity of the back pain. Similar EMG patterns were found in the experimental study together with a reduced peak EMG activity in the period during double stance where the back muscles are normally active. Generally, these changes were localized ipsilaterally to the site of pain induction. The clinical and experimental findings indicate that musculoskeletal pain modulates motor performance during gait probably via reflex pathways. Initially, these EMG changes may be interpreted as a functional adaptation to muscle pain, but the consequences of chronic altered muscle performance are not known. New possibilities to monitor and investigate altered motor performance may help to develop more rational therapies for CLBP patients.

SUMMARY1. Twitch contractions were elicited in human anterior tibial muscle by intramuscular microstimulation of single motor axons with a bipolar needle electrode. The population of stimulated motor units studied was fairly representative for the muscle.2. The conduction velocity of the fibres in the motor unit was calculated as the ratio between the electrode separation (15 mm) in a tripolar array of surface electrodes and the conduction delay of the motor unit potential along the electrode array. The motor unit conduction velocity ranged from 2-6 to 5-3 m/s with a mean of 3-7 m/s.3. The contractile properties of the motor units were obtained by averaging the torque developed around the ankle joint. Twitch torques ranged from less than 1o-3 to 16 x 10-3 N m, with a mean of 5-7 x 10-3 N m. The twitch torque of the whole anterior tibial muscle was approximtely 5 N m. Rise times were 47-80 m/s with a mean of 61 m/s, and half-relaxation times were 40-78 ms with a mean of 60 ms.4. The mechanical properties of individual motor units were highly correlated (rise time and twitch torque: r = -0-81; rise time and half-relaxation time: r = 0-75; twitch torque and half-relaxation time: r = -0-81).5. The motor unit conduction velocity was highly correlated to twitch torque (r = 0 87), rise time (r = -0 75) and half-relaxation time (r = -0 66). This indicates that the motor unit conduction velocity can be included in the family of interrelated 'size principle parameters'.