ORGANIZATION OF INPUTS TO MOTONEURON POOLS IN MAN

1. Surface EMGs were recorded from pairs of muscles involved in moveme nts of the wrist and/or digits in the upper limb and from pairs of int rinsic foot muscles in the lower limb during voluntary isometric contr actions. 2. EMGs were also recorded from lower limb and trunk muscles during three different tasks: lying, standing and balancing. 3. To inv estigate if the co-contraction of muscles was due to the presence of a common drive to each of the two motoneurone pools, cross-correlation analysis of the two multiunit EMG signals was used. 4. Evidence for a common drive was seen between pairs of muscles that share a common joi nt or joint complex (such as the metacarpophalangeal joints); no evide nce was found for a common drive to co-contracting muscles that did no t share a common joint. 5. When considering analogous hand and foot mu scle pairs, the degree of synchrony was significantly greater for lowe r limb pairs. 6. Where a common drive was detected with lower limb mus cle pairs, the degree of synchrony was significantly larger during bal ancing than during either lying or standing. 7. The origin of the comm on drive is discussed. It is concluded that activity in both last-orde r branched presynaptic fibres and presynaptic synchronization is invol ved.