Platform Perturbations Research Articles

An Examination of Reaction Time and Sagittal Joint Angle Responses to Platform Perturbations During Bipedal Stance in Normal and Individuals Post-Stroke

Department of Biokinesiology & Physical Therapy, University of Southern California, Los Angeles, CA.

Dynamic Posturography in Assessment of Polyneuropathic Disease

Twenty-eight patients with polyneuropathy aged 49-82 years (mean 67 years) were assessed by dynamic posturography. The patient group was compared to a control group comprising 29 healthy controls aged 70 through 75 years (mean 73 years). The dynamic posturography method comprises a sensory organization part in which the platform and visual surround are either stable or referenced to the patient's sway; furthermore, the eyes are open or closed. In a movement coordination part the platform makes active movements. In the sensory organization part of the dynamic posturography the patient group showed significantly lower equilibrium performance compared to the control group in the test conditions with absent vision, sway-referenced surrounding, and, finally, sway referenced platform and surrounding. In these conditions the influx of somatosensory information is of paramount importance for stable posture. In the movement coordination test, the patient group showed prolonged muscular response latencies in both forward and backward platform perturbations compared to the control group. It is concluded that dynamic posturography is a valuable diagnostic tool in assessment of the dynamic equilibrium performance in patients with polyneuropathy.

Interactions Between Automatic Postural Adjustments and Anticipatory Postural Patterns Accompanying Voluntary Movement

In order to examine interactions between centrally initiated postural activity preceding voluntary arm movements and compensatory postural activity, we studied patterns of postural muscle activity preceding a vigorous bilateral reach and grasp task or triggered by support surface motion. The reaching task required movement onset to be coincident with a predictable stimulus, and in some trials a brief backward platform perturbation was timed to occur before, during, or after the reach onset. Centrally initiated anticipatory postural activity was subject-specific and was very often absent when perturbation-induced postural activity was elicited just prior to movement onset. Likewise, compensatory postural activity patterns elicited by the platform perturbation did not occur when they would have coincided with the anticipatory postural activity. These data support the idea that the central neural processes which determine the specific activation pattern of the supporting limb musculature are influenced by both the intended dynamic outcome and the current dynamic status of the body.

Electromyographic Analysis of Postural Adjustments in Two Methods of Balance Testing

The primary purpose of this study was to compare postural responses during two methods of balance testing: 1) a platform perturbation test (PP), with destabilizing forces of uniform magnitude given at the base of support, and 2) a postural stress test (PST), with destabilizing forces of increasing magnitude given at the waist. A secondary purpose was to examine the relationship between balance strategy scores and the patterns of muscle responses as determined by electromyographic analysis on the PST. Postural responses to backward sway of 17 healthy subjects, aged 60 to 79 years (means = 69.0, s = 5.3), were characterized bilaterally by EMG recording of the tibialis anterior, quadriceps femoris, gastrocnemius, and hamstring muscles. We examined three measures of bilateral postural responses: 1) latency to first muscle response (FR) among the four lower extremity muscle groups, 2) tibialis anterior muscle latency (TA), and 3) patterns of muscle responses. Our results demonstrated significantly shorter (p less than .001) TAs and FRs (in milliseconds) on the PST than on the PP (PST TA: = means = 80.3, s = 12.4; PP TA: = means = 142.6, s = 13.4; PST FR: means = 75.9, s = 10.4; PP FR: = means = 140.4, s = 13.6). The ankle strategy was the most frequently used pattern of muscle response during both the PP and the PST, but it occurred less frequently on the PST than on the PP (PP = 72.5%, PST = 50.9%) (p less than .05). No consistent relationship was observed between the balance strategy scores as determined by videotape analysis of the PST and the patterns of muscle responses as determined by EMG analysis.

Aging and Postural Control

Two different balance testing methods were compared: (1) measurement of spontaneous postural sway during quiet standing, and (2) measurement of induced postural sway in response to an applied postural perturbation. Eyes-open tests were performed in 64 healthy young and elderly adults and in five elderly subjects with a history of falling. In both balance tests, the sway was defined in terms of the displacement of the center of pressure on the feet. Spontaneous sway was quantified using a number of different amplitude- and frequency-based parameters. Induced sway was measured in response to anterior-posterior acceleration of a platform on which the subject stood. The induced-sway test was specially designed to be safe and nonthreatening for elderly subjects; thus, the platform perturbation was confined to small accelerations and a gentle pseudorandom motion was used. To derive a measure of postural stability, the data from this test were fitted with a model that was then used to predict the response to sudden (transient) perturbations, thereby simulating the response in actual falls. Although both induced- and spontaneous-sway measures demonstrated significant aging-related decreases in stability, the differences were more pronounced for the induced-sway data. Conversely, some of the spontaneous-sway measures were much more successful in distinguishing the fallers from the nonfallers. There was a significant correlation between induced-sway and certain spontaneous-sway measures in the normal young adults; however, in the elderly normals and fallers, the data from the two types of balance tests either showed no correlation or, for certain spontaneous-sway measures, tended to show an inverse relationship.

Neck, trunk and limb muscle responses during postural perturbations in humans.

This study examined the EMG onsets of leg, trunk, and neck muscles in 10 standing human subjects in response to support surface anterior and posterior translations, and to plantar and dorsiflexion rotations. The objective of the study was to test the hypothesis that the responses radiating upward from distal leg muscles represent part of a large ascending synergy encompassing axial muscles along the entire length of the body. If these responses are not ascending, then the muscles of the neck, and possibly the trunk, can be independently activated by vestibular, proprioceptive or visual inputs. We analysed the timing of postural muscle responses within and between body segments in order to determine whether they maintained a consistent temporal relationship under translational and rotational platform movement paradigms. Our results did not strongly support an ascending pattern of activation in all directions of platform perturbation. Temporal differences between activation patterns to platform perturbations in the forward or backward directions were revealed. In response to posterior platform translations we observed an ascending pattern of muscle responses along the extensor surface of the body. In addition, responses elicited in the neck flexor and abdominal muscles occurred as early as those of the stretched ankle muscles. This pattern of upward radiation from stretched ankle muscles was not as clear for anterior platform displacements, where early neck flexor muscle responses were observed during the ascending sequence on the flexor surface of the body. Platform rotations caused fewer responses in the neck and upper trunk muscles than translations, and all muscles responses occurred simultaneously rather than sequentially. Probable differences in the stimulation of vestibular and neck proprioceptive inputs and the mechanical demands of the rotation and translation paradigms are discussed.

Lower extremity antagonist muscle response following standing perturbation in subjects with cerebrovascular disease

The onset, amplitude, and relative timing of opposing muscle groups during forward and backward body sway were investigated in standing subjects with hemiplegia and a normal control group. The agonist was the first muscle stretched by platform perturbation and the antagonist was the opposing muscle passively shortened by the movement stimulus. Tibialis anterior (T), quadriceps (Q), gastrocnemius (G) and hamstrings (H) were simultaneously monitored bilaterally for burst activity. During rotational (toe up/down) and horizontal perturbations in normals, two distinct muscle responses were observed—an initial long-latency response (LLR) in the agonist and a subsequent response in the antagonist muscle group. Hemiplegics showed early activation of the antagonist response (AR) with respect to the initial LLR and frequent coactivation of synergists (T-Q or G-H). Although the AR was tightly coupled to the initial LLR in both hemiplegic lower limbs, initiation of the distal LLR-AR sequence was significantly delayed in the paretic extremity by 25–40 ms. In addition, the distal AR amplitude was suppressed in the paretic limb. The number of AR defaults in hemiplegics for all perturbation modes was significantly greater than normal. Therefore, unilateral cerebrovascular disease may reduce the occurrence of antagonist muscle activation and alter the latency, amplitude and phasing of passively shortened antagonists muscles.

Inhibition of the Achilles tendon reflex by antagonist long-latency postural responses in humans

The present study demonstrated inhibition of the Achilles tendon reflex in freely standing humans during platform perturbations which elicited a long-latency postural response (90 to 110 ms) in the stretched antagonist tibialis anterior musculature. The inhibition was apparent under those conditions in which the long-latency postural response had adapted to subthreshold within the antagonist motoneuron pool. The inhibition was also apparent under those conditions in which the long-latency postural response occurred in the ankle muscles shortened rather than stretched by the perturbation, and therefore did not require prior activation of antagonist muscle spindle afferent neurons. This modulation was more efficacious than equivalent phasic or tonic voluntary activation of the antagonist musculature and this implied that pathways activated during long-latency postural responses and voluntary responses were not equivalent. The experiments also demonstrated that the myotatic stretch reflex (40 to 45 ms latency) remained in the homonymous muscle under conditions in which the long-latency postural response adapted to subthreshold. This finding supports the hypothesis that postural response adaptation involves mechanisms other than presynaptic inhibition of the Ia afferent fibers or inhibition of the homonymous motoneuron pool.