Transcranial magnetic stimulation (TMS) has been a powerful tool to understand the function of the motor cortex. It has been used to investigate its role in decision-making (Michelet et al., 2010), movement preparation (Soto et al., 2010) and, in a recent study by Gritsenko and colleagues in the Journal of Neuroscience, knowledge of limb dynamics (Gritsenko et al., 2011).
Knowledge of limb dynamics is essential for the control of reaching movements. Indeed, shoulder motion produces interaction torques at the elbow and movements of the elbow generate torques at the shoulder joint. This complex inter-segmental dynamics must clearly be taken into account during movement planning and execution in order to produce the typical straight paths that characterizes natural reaching movements (Hollerbach and Flash, 1982). The source of interaction torque compensation has been a matter of debate for years. Some argue that compensation takes place at the level of the spinal cord (Bizzi et al., 2000) whereas others argue for a more direct control of muscle activation mediated by the primary motor cortex (M1) (Todorov, 2000; Scott, 2004). Whether the descending motor commands accounts for inter-segmental dynamics remains a significant open question at the centre of spinal versus cortical control hypotheses.