Simple model of arch support: Relevance to Charcot Neuroarthropathy.

BACKGROUND: Charcot neuropathy is a common complication resulting from poorly controlled diabetes and peripheral neuropathy leading to the collapse, and ultimately the breakdown, of the midfoot. Mechanically, it is likely that a compromised arch support in this, or any other patient group that experiences foot flattening, would be associated with slippage at the distal and proximal interface regions of the plantar surface of the foot and the adjacent support surface. This slippage, although difficult to quantify with standard motion capture systems used in a gait laboratory, could potentially be assessed with systems for monitoring interface shear stresses. However, before investing in such systems, a correlation between arch flattening and interface shear stresses needs to be verified. METHODS: For this purpose, a sagittal plane model of a foot was developed using a multi-body dynamics package (MSC Adams). This model mimicked a subject swaying back and forth, and was constructed to show the dependence of interface stresses on altered arch support. FINDINGS: The model's predictions matched typical FootSTEPS data: lengthening of the arch of 1-2 mm, sway oscillations of 0.22-0.33 s and frictional force differences (calcaneus relative to forefoot) of 60 N. Of clinical relevance, when the stiffness of the plantar spring (representing aponeurosis and intrinsic muscles) was reduced by 10%, the frictional force difference increased by about 6.5%. INTERPRETATION: The clinical implications of this study are that, while arch lengthening of less than 2 mm might be difficult to measure reliably in a gait lab, using shear sensors under the forefoot and hindfoot should allow arch support to be assessed in a repeatable manner.

Preliminary electrophysiological characterization of functionally vestigial muscles of the head: potential for command signaling.

In devastating neurological disorders, such as quadriplegia resulting from high-level spinal cord injury, it is essential to focus on functions that have been spared and optimally exploit them to enhance the individual's quality of life. It follows that certain muscles, which prior to the paralysis of much of the rest of the body seemed to have no useful function, might be used to provide unique signals to control assistive devices. This report presents preliminary electrophysiological data demonstrating potentially useful myoelectrical signals from 3 functionally vestigial muscles in humans; the posterior, anterior, and superior auricular muscles. In phylogenetically lower species, these muscles serve to position the ear to enhance hearing. The auricular muscles receive their major innervation from cranial nerve VII and should not be compromised by even high-level spinal cord lesions. In this study, it was found that the muscles could be voluntarily activated and, by standard surface-electrode recording, had potentials ranging to 680 microV in amplitude. Posterior auricular muscle potentials were used to command a paddle in a computer ping-pong task that employed a CyberLink interface. The t values for accuracy scores and ball hits were both significant at the p = .0001 level. These facts indicate that the auricular muscles may be useful for controlling assistive devices.

Effects of increased ambient CO2 on brain tissue oxygenation and performance in the hypoxic rhesus.

Alterations of cerebral gas tensions and performance in response to hypoxia, with or without 5% CO2 in the ambient inspirate, were studied in eight conscious rhesus monkeys. The animals were trained to perform a lever press (Sidman) avoidance task. Physiological and performance data were obtained during exposures to normal (21% O2) and hypoxic (12, 10, and 8% O2--all with N2 balance) breathing atmospheres, with or without the addition of 5% CO2. With hypoxia, cerebral PO2 and PCO2 declined steadily, but a significant performance decrement was noted only at the 8% O2 level. With the addition of 5% CO2 to the inspired atmospheres, cerebral PO2 was elevated relatively but still declined as hypoxia intensified. Cerebral PCO2 and the avoidance task performance were sustained at near baseline values with the 5% CO2 inspirate.

A head enclosure for exposing monkeys to selected gas mixtures.

An acrylic head enclosure, permitting rapid exchanges of atmospheric gases, was developed for investigations of physiologic and performance responses of small nonhuman primates to graded hypoxic hypoxia and hypercapnia. The total system included a neck yoke plate which served both as a physical barrier to arm and hand movements in the head area and as a base on which to rest the head enclosure. The transparent, airtight, head enclosure permitted rapid exchanges of controlled gas mixtures and the execution of performance tasks in an apparently comfortable experimental environment.