Evidence for positive force feedback during involuntary aftercontractions.

We examined the involuntary isotonic muscle aftercontraction which follows a 1-min voluntary contraction at 60% of maximum voluntary force. Aftercontractions provide a unique paradigm for investigating the automatic postural control of the upper limb in man. Previous work has shown that the strength of these contractions in m. deltoid is linearly dependent on the inclination of the subject's body in the gravitational field. The current experiments were designed to test if this was due to the gravitational loading on the arm as this also changes with body inclination. The subject sat with the head stationary to prevent changes in vestibular orientation. Aftercontractions in m. deltoid were evoked with the normal weight of the arm present and also with arm loadings equal to 0, 25, 50 and 75% of this value. These reductions were produced by a counter lever apparatus. The results showed that the EMG amplitude of this involuntary contraction depended linearly on the loading of the arm. This is interpreted as being the result of positive load feedback. Aftercontraction EMG amplitude at 100% load was 2.7 times greater than at 0% load. The results are discussed in the context of recent work on positive load feedback in man and animals.