Vestibular control of deep and superficial lumbar muscles

The active control of the lumbar musculature provides a stable platform critical for postures and goal-directed movements. Voluntary and perturbation-evoked motor commands can recruit individual lumbar muscles in a task-specific manner according to their presumed biomechanics. Here, we investigated the vestibular control of the deep and superficial lumbar musculature. Ten healthy participants were exposed to noisy electrical vestibular stimulation while balancing upright with their head facing forward, left or right to characterize the differential modulation in the vestibular-evoked lumbar extensor responses in generating multidirectional whole-body motion. We quantified the lumbar muscles activation on the right side using indwelling (deep multifidus, superficial multifidus, caudal longissimus (L4) and cranial longissimus (L1)) and high-density surface recordings. We characterized the vestibular-evoked responses using coherence and peak-to-peak cross-covariance amplitude between the vestibular and electromyographic signals. Participants exhibited responses in all lumbar muscles. The vestibular control of the lumbar musculature exhibited muscle-specific modulations: responses were larger in the longissimus (combined cranio-caudal) compared to the multifidus (combined deep-superficial) when participants faced forward (p<0.001) and right (p=0.011) but not when they faced left. The high-density surface recordings partly supported this observation: the location of the responses was more lateral when facing right compared to left (p<0.001). The vestibular control of muscle sub-regions within the longissimus or the multifidus was similar. Our results demonstrate muscle-specific vestibular control of the lumbar muscles in response to perturbations of vestibular origin. The lack of differential activation of lumbar muscle sub-regions suggests the vestibular control of these sub-regions is co-regulated for standing balance.