Motor-unit synchronization is not responsible for larger motor-unit forces in old adults.

Motor-unit synchronization, which is a measure of the near simultaneous discharge of action potentials by motor units, has the potential to influence spike-triggered average force and the steadiness of a low-force isometric contraction. The purpose of the study was to estimate the contribution of motor-unit synchronization to the larger spike-triggered average forces and the decreased steadiness exhibited by old adults. Eleven young (age 19-30 yr) and 14 old (age 63-81 yr) adults participated in the study. Motor-unit activity was recorded with two fine-wire intramuscular electrodes in the first dorsal interosseus muscle during isometric contractions that caused the index finger to exert an abduction force. In a separate session, steadiness measurements were obtained during constant-force isometric contractions at target forces of 2.5, 5, 7. 5, and 10% of the maximum voluntary contraction (MVC) force. Mean (+/-SD) motor-unit forces measured by spike-triggered averaging were larger in old (15.5 +/- 12.1 mN) compared with young (7.3 +/- 5.7 mN) adults, and the differences were more pronounced between young (8.7 +/- 6.4 mN) and old (19.9 +/- 12.2 mN) men. Furthermore, the old adults had a reduced ability to maintain a steady force during an isometric contraction, particularly at low target forces (2.5 and 5% MVC). Mean (+/-SD) motor-unit synchronization, expressed as the frequency of extra synchronous discharges above chance in the cross-correlogram, was similar in young [0.66 +/- 0.4 impulses/s (imp/s); range, 0.35-1.51 imp/s; 53 pairs) and old adults (0.72 +/- 0.5 imp/s; range, 0.27-1.38 imp/s; 56 pairs). The duration of synchronous peaks in the cross-correlogram was similar for each group (approximately 16 ms). These data suggest that motor-unit synchronization is not responsible for larger spike-triggered average forces in old adults and that motor-unit synchronization does not contribute to the decreased steadiness of low-force isometric contractions observed in old adults.

Effects of vision on head stability and torques during voluntary pulls made by standing humans.

This study tested the hypothesis that vision would improve pitch-plane stabilization of the head by increasing how well neck muscle torque compensates for torques associated with body motion, in a task where standing human subjects made rapid voluntary pulls. Ten subjects performed abrupt horizontal pulls on a handle to two peak force targets, with the eyes open and closed. We evaluated head angular velocity with respect to space. Inverse dynamics were used to subdivide the torque acting on the head into gravito-interactive and muscle components. A torque compensation ratio was computed from those two components. Head angular velocity was lower and the compensation ratio was higher during pulls made with the eyes open, for both force targets. The data suggest that vision enhances head stability by increasing the effectiveness with which muscle torques oppose gravito-interactive torques during voluntary pulls made while standing.

A review of prosthetic interface stress investigations.

Over the last decade, numerous experimental and numerical analyses have been conducted to investigate the stress distribution between the residual limb and prosthetic socket of persons with lower limb amputation. The objectives of these analyses have been to improve our understanding of the residual limb/prosthetic socket system, to evaluate the influence of prosthetic design parameters and alignment variations on the interface stress distribution, and to evaluate prosthetic fit. The purpose of this paper is to summarize these experimental investigations and identify associated limitations. In addition, this paper presents an overview of various computer models used to investigate the residual limb interface, and discusses the differences and potential ramifications of the various modeling formulations. Finally, the potential and future applications of these experimental and numerical analyses in prosthetic design are presented.

Effects of labial margin design on stress distribution of a porcelain-fused-to-metal crown.

PURPOSE: This study compared the stress distribution of three porcelain-fused-to-metal labial margins on a central incisor under simulated occlusal force. MATERIALS AND METHODS: The margin designs were rounded-shoulder, rounded-shoulder with a bevel, and a chamfer. RESULTS: Evaluation of mean equivalent Mises tensile stress did not show any difference at the cement-dentin and metal-cement interfaces of the three margins. The mean equivalent Mises tensile stress at the porcelain-metal interface was found to be significantly higher for rounded-shoulder when compared with the chamfer. There was no significant difference in the axial stresses along the three interfaces among the three margin designs. Statistically significant differences in radial stresses at some interfaces of the three margin designs were found. CONCLUSION: The change of cement thickness of the chamfer margin is significantly greater than that of either the rounded-shoulder or the bevelled-shoulder margin.

Rigidity of initial fixation with uncemented tibial knee implants.

This study quantifies the in vitro motion occurring between bone and cemented and noncemented tibial components. Liquid metal strain gauges were used to measure the motion between the tibial component and bone at four locations in eight cadaver tibia at near-point cyclic loads ranging from 10 to 2,000 N. Two types of motion were observed: inducible displacement, which is reversible, followed the oscillating load and occurred in both cemented and uncemented tests, and liftoff or separation of the component and bone, which occurred only for the noncemented cases and remained even after removal of the load. For both motion types, noncemented tests exhibited significant (P less than .05) and dramatic increased interface motion compared to the cemented cases for all load types. The results suggest that the magnitudes of implant-bone interface separation at loads in the low physiologic range for noncemented implants can be sufficiently large to inhibit bony ingrowth into a prosthesis with an average pore size of 300 microns.

Effect of cement pressure and bone strength on polymethylmethacrylate fixation.

The effect of the quality of the bone and of the cement pressurization magnitude and duration on the fixation achieved with polymethylmethacrylate (PMMA) bone cement is studied in vitro. Seventy-one cement-bone interface specimens, prepared under various conditions of pressurization of low-viscosity bone cement, are tested in tension. The load at failure and the maximum cement penetration are measured to assess the fixation achieved, and the quality of the bone is assessed by determining the compressive strength of each of the bone specimens. Statistical analysis of the data indicates that the pressure magnitude is the most influential of the factors considered in the cement penetration behavior and in the development of failure load capacity. The duration of the pressure does not appear to be a significant factor. The cement penetration is a decreasing function of the bone strength, reflecting a decrease in the porosity and an increase in the area fraction. Although not directly measured in these tests, these latter bone properties are indirectly measured by the bone compressive strength. The effect of increasing bone strength on the failure load is nonlinear. The development of adequate failure load capacity is the result of a balance between the cement penetration allowed by the porosity of the bone and the inherent strength of the cancellous bone itself. Weak bone, although adequately penetrated by cement, cannot provide strong fixation. Stronger, denser bone limits cement penetration, but pressurization enhances development of failure load capacity through more complete infusion and interlocking of the cement in the available pore space. The strength of the fixation achievable for any bone is limited by the intrinsic strength of the bone.(ABSTRACT TRUNCATED AT 250 WORDS)