Asymmetric standing posture after stroke is related to a biased egocentric coordinate system

Weakness and somatosensory deficits have long been known to be involved in the postural instability of subjects with stroke. Recently, it has been shown that impaired representations of the orientation of the longitudinal axis of the body (LBA, egocentric reference) and of verticality (allocentric reference) may also play a role. The objective of the present study was to determine whether these two references were independently linked to postural asymmetry in standing stroke patients.

Twenty-two subjects were tested after a first hemispheric stroke (13 +/- 7.5 weeks). The LBA perception was investigated in the supine position by adjusting the orientation of a luminous rod in the frontal plane to correspond to the subjective LBA. The subjective visual vertical (SVV) was assessed by adjusting the orientation of a luminous line in the frontal plane to correspond to the SVV in upright patients. Weight distribution was measured in the standing position for about 2 minutes and 45 seconds by two separate force platforms under the feet.

LBA and SVV were strongly associated (r = 0.7; p < 0.001). The estimate of the LBA was a better predictor (r = -0.52: p < 0.02) of weight bearing asymmetry than was SVV (r = -0.41; p = 0.074) when adjusted for motor weakness and hypoesthesia.

Contralesional rotation of the longitudinal axis of the body could lead to unequal distribution of loading on the feet. This novel interpretation of weight bearing asymmetry underlines the complexity of control of the erect stance following stroke and brings new perspectives for rehabilitation programs.