The observation of a single limb movement induces multi-limb motor resonance in the observer

Aim: Observation of hand movement induces a motor resonance (MR) in hand cortical and spinal motor pathways, which reproduces the motor commands needed to execute the observed movement. Recent fMRI evidence suggests that, during observation of upper or lower limb movements, the pre-motor cortex is activated in a somatotopic manner. Aim of this study is to investigate whether MR is evoked in foot motor pathways by the observation of foot movements,and whether observation of hand or foot movements can induce MR in a reciprocal manner. Methods: A modulation of the H-reflex in the right Soleus (Sol) or Flexor Carpi Radialis (FCR) muscle was recorded in human observers (n=9 and n=7, respectively), watching a 1Hz sinusoidal oscillation of the right foot (HSol-foot mov and HFCR-foot mov). In addition, a modulation of the H-reflex in the right Sol muscle was also recorded in 5 observers, watching a 1Hz oscillation of the right hand (HSol-hand mov). Results: In all experimental conditions, observation induced a significant (p<0.05) 1 Hz H-reflex modulation which could be fitted by a sinusoidal function with the same period as the observed movement. Phase relation between H-reflex modulation and movement (Delta Phi), varied in the different experimental conditions (average Delta Phi = -127° ± 37 for HSol-foot mov; 58° ± 48 for HFCR-foot mov; and - 43° ± 113 for HSol-hand mov. Conclusion: 1) observation of foot movement can reliably evoke MR in foot motor pathways; 2) MR can also be reciprocally evoked in both foot and hand motor pathways; 3) the great difference in Delta Phi and in its variability during the resonant responses, may reflect different temporal relations between motor command and movement, should this be executed as in the above experimental conditions. Thus we conclude that MR may involve not only the observed moving limb, but also other limbs linked by postural chains sustaining the observed movement.