Proceedings of The Physiological Society

University College London December 2005 (2006) Proc Physiol Soc 1, C1

Oral Communications

Asymmetrical activation of trunk muscles following unpredictable loading of an outstretched arm

Klungarvuth, Lee; Mullington, Christopher J; Catley, Maria; McGregor, Alison H; Strutton, Paul H;

1. Biosurgery and Surgical Technology, Imperial College London, London, United Kingdom.


When an individual moves an arm, trunk muscles are activated to oppose the reactive forces from the movement and stabilise the trunk. When the movement is predictable, there is a specific pattern of activation in the trunk muscles, with some trunk muscles being activated prior to the movement of the arm. However, when the movement of the arm is unpredictable there is a different pattern of activation of specific trunk muscles. In addition, the corticospinal drive to the erector spinae muscles on one side of the back is increased when the opposite arm is abducted (Davey et al. 2002) and there is evidence to suggest that the cortical drive to trunk muscles is asymmetric (Strutton et al. 2004; Kuppuswamy et al. 2005). Here we examine whether there is asymmetry in responses of trunk muscles to an unpredictable loading of an abducted arm. Eighteen healthy right-handed human subjects were recruited. Bilateral surface electromyographic (EMG) recordings were made from the deltoid, erector spinae at T12 and L4 vertebral levels and the rectus abdominis muscles. A modified bucket with a microswitch in the handle was used for the weight drop protocol. Subjects were blindfolded and stood upright with their right arm abducted to 90 deg holding a tube connected to the bucket handle by a length of string. The bucket was dropped, either loaded with a 1.25 kg weight or empty (randomly selected to prevent a learning effect), at intervals of at least 5 s. Subjects were instructed to hold their arm stable to arrest the fall of the bucket. The microswitch was triggered when the bucket was dropped and this triggered the data acquisition system to record EMG activity. The protocol was then repeated with the left arm abducted. After a brief rest period, the entire protocol was repeated again. The amplitudes and latencies of the responses (to the weighted bucket drop) in each of the trunk muscles opposite the abducted arm were examined for differences. The sizes of the responses were significantly larger (P < 0.001; one way ANOVA with Holm-Sidak test) in ES T12 than ES L4 and RA in both right and left arm abduction scenarios (L ES T12 0.10

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