COACTIVATION OF IPSILATERAL AND CONTRALATERAL MUSCLE GROUPS DURING CONTRACTION OF ANKLE DORSIFLEXORS

Seventeen adult, healthy subjects, age 38.4 +/- 0.24 years (mean +/- SEM) 7 of which were females, were studied. Each subject was seated on a specially designed chair with trunk and legs fixed and the foot strapped to a rigid plate that was attached to a load cell. The position of the strap was adjusted so as to lie across the foot at the level of the metatarsal bones. The knee and ankle joints were adjusted to 90 degrees. To record EMG activity, pairs of surface electrodes were placed over the belly of both the right and left tibialis anterior, quadriceps, hamstring and contralateral triceps surae muscles. Two experimental paradigms were used, A and B. In A the subject was asked to sustain maximum voluntary contraction (MVC) of the ankle dorsiflexors until the force decreased to 50% of the initial value; in B the subject was asked to carry out contractions of the ankle dorsiflexors for 6 seconds followed by 4 sec relaxation periods. The initial contraction was 20% of MVC followed by 40, 60, 80 and 100% of MVC which represented one cycle. The subject was asked to repeat this cycle 10 times. Voluntary contraction of ankle dorsiflexors was regularly accompanied by activation of other muscles, usually first in the same leg, later in the contralateral leg during MVC of ankle dorsiflexors. When intermittent contractions with step wise increments of force developed by the ankle dorsiflexors were carried out, co-activation of ipsilateral and contralateral muscle groups occurred before the force of the contracting muscles decreased. Our study shows a relatively regular pattern of co-activation of the muscle groups and this may be an indication that the source of the increased excitatory drive is a motor pattern generator. Under the conditions used here motor pattern generators might reveal features which are not apparent when excitability is not increased by voluntary contraction. The study of co-activation of other muscle groups during an isolated movement might be relevant for understanding the changes of motor control in patients with motor dysfunction.