Runge, C. F. | Search NASA

NASA NTRS1999

Ankle and hip postural strategies defined by joint torques

Previous studies have identified two discrete strategies for the control of posture in the sagittal plane based on EMG activations, body kinematics, and ground reaction forces. The ankle strategy was characterized by body sway resembling a single-segment-inverted pendulum and was elicited on flat support surfaces. In contrast, the hip strategy was characterized by body sway resembling a double-segment inverted pendulum divided at the hip and was elicited on short or compliant support surfaces. However, biomechanical optimization models have suggested that hip strategy should be observed in response to fast translations on a flat surface also, provided the feet are constrained to remain in contact with the floor and the knee is constrained to remain straight. The purpose of this study was to examine the experimental evidence for hip strategy in postural responses to backward translations of a flat support surface and to determine whether analyses of joint torques would provide evidence for two separate postural strategies. Normal subjects standing on a flat support surface were translated backward with a range of velocities from fast (55 cm/s) to slow (5 cm/s). EMG activations and joint kinematics showed pattern changes consistent with previous experimental descriptions of mixed hip and ankle strategy with increasing platform velocity. Joint torque analyses revealed the addition of a hip flexor torque to the ankle plantarflexor torque during fast translations. This finding indicates the addition of hip strategy to ankle strategy to produce a continuum of postural responses. Hip torque without accompanying ankle torque (pure hip strategy) was not observed. Although postural control strategies have previously been defined by how the body moves, we conclude that joint torques, which indicate how body movements are produced, are useful in defining postural control strategies. These results also illustrate how the biomechanics of the body can transform discrete control patterns into a continuum of postural corrections.

Non-NASA Center↗

NASA NTRS1998

Role of vestibular information in initiation of rapid postural responses

Patients with bilateral vestibular loss have difficulty maintaining balance without stepping when standing in tandem, on compliant surfaces, across narrow beams, or on one foot, especially with eyes closed. Normal individuals (with no sensory impairment) maintain balance in these tasks by employing quick, active hip rotation (a "hip strategy"). The absence of a hip strategy in vestibular patients responding to translations of a short support surface has previously been taken as evidence that the use of hip strategy requires an intact vestibular system. However, many tasks requiring hip strategy alter one or a combination of important system characteristics, such as initial state of the body (tandem stance), dynamics (compliant surfaces), or biomechanical limits of stability (narrow beams). Therefore, the balance deficit in these tasks may result from a failure to account for these support surface alterations when planning and executing sensorimotor responses. In this study, we tested the hypothesis that vestibular information is critical to trigger a hip strategy even on an unaltered support surface, which imposes no changes on the system characteristics. We recorded the postural responses of vestibular patients and control subjects with eyes closed to rearward support surface translations of varying velocity, in erect stance on a firm, flat surface. Subjects were instructed to maintain balance without stepping, if possible. Faster translation velocities (25 cm/s or more) produced a consistent pattern of early hip torque (first 400 ms) in control subjects (i.e., a hip strategy). Most of the patients with bilateral vestibular loss responded to the same translation velocities with similar torques. Contrary to our hypothesis, we conclude that vestibular function is not necessary to trigger a hip strategy. We postulate, therefore, that the balance deficit previously observed in vestibular patients during postural tasks that elicit a hip strategy may have been due to the sensorimotor consequences of the system alterations imposed by the postural tasks used in those studies. Preliminary results from two younger patients who lost vestibular function as infants indicate that age, duration of vestibular loss, and/or the timing of the loss may also be factors that can influence the use of hip strategy as a rapid postural response.

NASA Discipline Neuroscience↗