Postural after contractions are involuntary contractions that occur predominantly in proximal muscles following a prolonged, strong, voluntary, isometric contraction of the same muscle (Forbes et al. 1926). Aftercontractions can be easily elicited in m. deltoid when the subject is standing vertically (Brice & McDonagh, 2001). The mechanism underlying these contractions is unknown. One hypothesis is that aftercontractions are driven by a control system that seeks to maintain the arm position against the gravitational torque experienced at that position. An alternative view is that gravitational influences are irrelevant. We reasoned that if the latter were true, aftercontractions should be of the same magnitude with the subject standing or horizontal.

Nine subjects, five of whom were female, took part in the experiment which had local ethical committee approval and the written consent of the subjects (means ± S.D.: age 21 ± 0.9 years; height 173.4 ± 8.2 cm; weight 73.5 ± 10.2 kg). Subjects were asked to produce a 1 min long isometric contraction of m. deltoid equal to 60 % of their maximal voluntary contraction (MVC) against a force transducer. They could see the force they were producing on one channel of a monitor screen and were asked to match this to a target equal to 60 % of their MVC that was also shown on the screen. When necessary the subjects wore prism glasses so that they could still view the screen. At the end of the voluntary contraction the subjects closed their eyes. Then a pneumatic ram withdrew the force transducer, and involuntary abduction of the arm ensued. The subjects lay supine on a tilt table that allowed experiments to be carried out with the subjects body at inclinations of 0, 15, 30, 45, 60, 75 and 90 (head uppermost) deg to the horizontal. At inclinations less than 90 deg the subjects’ arms were supported by slings such that arm abduction was free to move only in the coronal plane of the body.

The results showed that aftercontractions were almost four times stronger with the subject standing than they were with the subject horizontal. There was an approximately linear relationship between aftercontraction EMG amplitude and the angle at which the body was inclined. To normalise the data across subjects, the maximum rectified surface EMG amplitudes from m. deltoid during the aftercontraction were expressed as a ratio of the EMG amplitude found during the preceding voluntary contraction. The mean EMG data for the group of nine subjects at each body inclination angle were: means ± S.D.: 0 deg 12.7 ± 23.6%; 15 deg 12.1 ± 2.4%*; 30 deg 23.3 ± 9.7%*; 45 deg 29.5 ± 21.9%**; 60 deg 30.3 ± 21.0%**;75 deg 39.1 ± 22.7%**; 90 deg 46.6 ± 24.5%**. Values significantly greater than the value at zero degrees of body inclination are shown as *P < 0.05, **P < 0.01 (repeated measures ANOVA with LSD post hoc).

We conclude that the inclination of the body in the gravitational field has a substantial influence on the magnitude of postural aftercontractions.