title: Synergies and strategies underlying normal and vestibulary deficient control of balance: implication for neuroprosthetic control.
abstract: Future developments of neuroprosthetic control will probably permit locomotion and posture to be maintained without the aid of crutches and will therefore require some form of balance control. Three fundamental questions will arise. First, the question of the location of imbalance-sensing transducers must be assessed. Secondly, the synergy, which is the relative amplitude and timing of muscle activity, and/or the strategy of joint torques required to re-establish a stable posture for different types of balance disturbances must be addressed. Thirdly, the control laws that map either trunk muscle activity or imbalance-sensing transducer outputs into multi-joint postural control of standing by paraplegic individuals must be generated. The most appropriate means of gathering the relevant information applicable to neuroprosthetic control systems is through the detailed analysis of normal and non-normal human models. In order to gain such detailed insights into normal balance control and its dependence on head angular and linear accelerations, the synergy and strategy of balance corrections in normal subjects or patients with vestibular deficits were investigated for two types of support surface perturbation, a dorsiflexion rotation (ROT) and a rearward translation (TRANS). These experimentally induced perturbations to upright stance were adjusted to cause equal amplitudes of ankle dorsiflexion, thus providing additional information about the role of lower leg proprioception on balance control. Synergies defined on the basis of peak cross-correlations of each recorded muscle's EMG to that of the largest muscle response were significantly different for TRANS and ROT. Translation synergies consisted of a sequential coactivation at several levels (soleus and abdominals some 30 msec before hamstrings, and trapezius some 15 msec before paraspinals), whereas the sequential activation of paraspinals and tibialis anterior dominated the balance synergy to ROT. Likewise, response strategies, defined using cross-correlations of joint torques, differed. That for TRANS was organised as a multi-link strategy with neck torques leading those of all other joints by 40 msec or more; hip joint lead ankle torques by 30 msec. That for ROT was organised around hip and ankle torques without a major correlation to neck torques. Vestibulary deficient subjects developed weaker synergies with respect to subjects with normal balance systems under eyes-open conditions and there was no clear synergy with eyes closed. Consequently, hip torques were delayed some 180 msec with respect to ankle torques, and correlations to neck torques were completely out of phase under eyes-closed conditions. Fundamental changes in TRANS synergies and strategies also occurred in vestibulary deficient subjects for eyes-open and eyes-closed conditions.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of ORL, University Hospital, Basel, Switzerland.
authors.forename: J H
authors.initials: JH
authors.lastname: Allum
authors.forename: F
authors.initials: F
authors.lastname: Honegger
mesh_terms: D000842:Ankle / Q000502:physiology; D001696:Biomechanical Phenomena; D004558:Electric Stimulation; D006615:Hip / Q000502:physiology; D006801:Humans; D007596:Joints / Q000502:physiology; D007717:Knee / Q000502:physiology; D007866:Leg / Q000502:physiology; D009068:Movement / Q000502:physiology; D009132:Muscles / Q000502:physiology*; D009334:Neck Muscles / Q000502:physiology; D009424:Nervous System Physiological Phenomena; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D012399:Rotation; D014159:Transducers; D015837:Vestibular Diseases / Q000503:physiopathology*; D014722:Vestibule, Labyrinth / Q000502:physiology* / Q000503:physiopathology*