Postural aftercontractions are involuntary contractions that occur following a strong voluntary isometric contraction (Adamson and McDonagh, 2004). The strength of these aftercontractions in m.Deltoid is proportional to the inclination of the body in the gravitational field (Lemon et al., 2003). It is greatest with the body upright and smallest with the body supine and horizontal. This effect could be due to the change in vestibular apparatus position or to the change in gravitational load on the muscle. In the present experiments head position was fixed and only load was varied. Ten subjects took part, three of whom were female (means +/- SD: age 25+/-11 years; height 172.1+/-8.3 cm; weight 73.2+/-12.5 kg). The load on the muscle was reduced by using a first class lever to counterbalance the weight of the arm. The axis of the lever was concentric with the flexion-extension axis of the glenohumeral joint. One arm of the lever was attached to the lateral aspect of the extended upper limb and the other lever arm had a counterbalancing weight which could be moved to provide reduced loads on the arm. The subjects sat with their left arm extended forwards and downwards at an angle of 40 degrees to the vertical. They then pressed up on a force transducer for one minute with 60 % of their maximal isometric force. Following this an aftercontraction ensued which produced a flexion of the extended limb at the shoulder joint. The activation of the aftercontraction was proportional to the loading on the arm. There was a positive linear relationship between arm load and the rectified emg amplitude (repeated measures ANOVA: p=0.001). This amplitude was expressed as a percentage of the average rectified emg signal recorded during the prior voluntary effort. The results across the 10 subjects at each load (as a fraction of the normal weight of the arm) were: means +/-SD: 0.00 load 24.8, +/-15.1%; 0.25 load 33.6, +/-17.3%; 0.50 load 46.4, +/-18.4%*; 0.75 load 57.3, +/-15.0%*; 1.00(normal) load 67.6, +/-24.8%* (data taken at a joint angle of 70 degrees, *p<0.004 pairwise comparisons of each load with 0 load condition). In conclusion load strongly increases the activation of involuntary aftercontractions independently of vestibular changes. This mechanism may underlie the normal control of body segment position in relation to gravitational load (e.g. Dietz, 1998).