Rambling–trembling analysis separates the center of pressure into two components: the rambling component (RM: supraspinal) and trembling component (TM: muscle stiffness/reflex). We hypothesized that persons with spinal cord injury (SCI) would demonstrate decreased RM resulting from altered supraspinal processing and increased TM resulting from increased muscle stiffness. We also anticipated that the TM component would be greater in SCI patients with Harrington rods than in those without them. The seated postural control was assessed in 18 persons with SCI, comprising 12 with and six without Harrington rods, and 18 age-matched controls. The SCI group had greater center of pressure sway, RM, and TM components than the controls, with no difference in the postural sway between the SCI subgroups, suggesting that the impairment of seated postural control in individuals with SCI results from disturbed supraspinal and peripheral mechanisms, but that the control itself is unaffected by internal fixation with Harrington rods. These were not entirely consistent with our hypothesis.
Sunghoon Shin and Jacob J. Sosnoff
Jacob J. Sosnoff, Sae Young Jae, Kevin Heffernan and Bo Fernhall
The purpose of the current investigation was to examine the relation between cardioballistic impulse and the fluctuations in continuous isometric force production. Subjects produced isometric force via index finger flexion to constant force targets (0.5, 1 and 2 N) with and without visual feedback while beat to beat blood pressure of their middle finger was recorded. Force fluctuations were quantified using distributional statistics. The association between blood pressure oscillations and fluctuations in force output were quantified with coherence analysis. Overall, it was found that force variability (i.e., SD) increased with force level and removal of visual feedback. Coherence values between blood pressure oscillations and force fluctuations were significant and the greatest in the 8–12 Hz bandwidth. There was no effect of force magnitude on the coupling strength between blood pressure oscillations and force production. This coupling was greater in the visual condition. These data suggest that peripheral alterations in blood pressure are related to fluctuations in isometric force production independent of force level and that this interaction is influenced by visual feedback.
Sunghoon Shin, Robert W. Motl and Jacob J. Sosnoff
The rambling-trembling analysis of postural control maintains that the center of pressure (COP) trajectory can be decomposed into deviations resulting from supraspinal (i.e., rambling; RM) and spinal processes (i.e., trembling; TM). The purpose of this investigation was to test the rambling and trembling hypothesis by comparing persons with multiple sclerosis (MS) who had either elevated or normal spinal reflexes to healthy controls. 16 subjects with MS and 16 age and gender matched control subjects completed a postural control task. The persons with MS were divided into groups with high (MShigh) or low (MSlow) H-reflex amplitude. The MShigh group had an elevated ratio of TM to COP compared with healthy controls, but no differences in the ratio between RM and COP. The findings are congruent with the assumptions of the rambling-trembling hypothesis. Further work is needed to determine if RM and TM represent distinct spinal and supraspinal mechanisms to postural control.
Katherine L. Hsieh, Yaejin Moon, Vignesh Ramkrishnan, Rama Ratnam and Jacob J. Sosnoff
Virtual time to contact (VTC) is a measure of postural stability that estimates the virtual time it would take to reach an individual’s stability boundary. This study aimed to validate VTC as measured by a depth sensor and determine whether VTC from the depth sensor distinguishes between older adult fallers and nonfallers compared with a force platform. VTC was assessed in 10 young and 20 older adults by having participants lean in a circular direction followed by 5 balance tests: eyes open, dual task, eyes open foam, eyes closed, and eyes closed foam. Spearman rank correlations and Bland–Altman plots were conducted to determine validity, and receiver operating characteristic curves were constructed to discriminate between fallers and nonfallers. Significant correlations were found in the dual task (P = .03), eyes open foam (P < .01), and eyes closed foam conditions (P = .05). The depth sensor discriminated between fallers and nonfallers in the eyes open (P = .02), dual task (P = .03), and eyes open foam conditions (P = .04). VTC was in agreement between the 2 devices, and VTC derived from a depth sensor can be used to discriminate between older adult fallers and nonfallers during challenging balance conditions.
Amanda M. Ward, Torrey M. Loucks, Edward Ofori and Jacob J. Sosnoff
Audiomotor and visuomotor short-term memory are required for an important variety of skilled movements but have not been compared in a direct manner previously. Audiomotor memory capacity might be greater to accommodate auditory goals that are less directly related to movement outcome than for visually guided tasks. Subjects produced continuous isometric force with the right index finger under auditory and visual feedback. During the first 10 s of each trial, subjects received continuous auditory or visual feedback. For the following 15 s, feedback was removed but the force had to be maintained accurately. An internal effort condition was included to test memory capacity in the same manner but without external feedback. Similar decay times of ~5–6 s were found for vision and audition but the decay time for internal effort was ~4 s. External feedback thus provides an advantage in maintaining a force level after feedback removal, but may not exclude some contribution from a sense of effort. Short-term memory capacity appears longer than certain previous reports but there may not be strong distinctions in capacity across different sensory modalities, at least for isometric force.