The investigation of motor-unit fatigability in reduced-animal models has been dominated by a single fatigue test, one fatigue index, and an emphasis on changes in the magnitude of (usually peak) force. Although the standard fatigue test has been reported to elicit changes in the dynamic phases of an isometric tetanus, this has not been systematically studied in single motor units. Furthermore, changes in the profile of individual tetani during the fatigue test have led some investigators to suggest that other force parameters (i.e., integrated force) or fatigue indices may provide additional information about motor-unit performance during the test. The purposes of this project were to: (1) evaluate the time courses of a variety of force parameters characterizing both the magnitude of force and the dynamic aspects of force during a 4-min fatigue test of functionally isolated cat, tibialis posterior motor units; and (2) determine if motor units could be classified into the conventional motor-unit types based on these new parameters. There was considerable variability in the average time course of the magnitude of force during the fatigue test. The variability within the type FR and F(int) motor-unit groups resulted in several units whose characteristics bordered those which, by definition, separate unit types. The classification of these units depended on the force parameter and fatigue index used to quantify their fatigability. The time course of the magnitude of force also revealed differences in the behavior of potentiating and non-potentiating groups. There were many differences between motor-unit types in terms of dynamic-force parameters before, during and after the fatigue test. Comparison of initial and 2-min values revealed a preferential effect of stimulation on force development in type S and FR units (i.e., increased rate) and on force decay in type F(int) and FF units (i.e., prolonged duration and decreased rate). The time courses of these effects further revealed qualitative differences between different combinations of motor-unit types. Groups of units (or lack thereof) revealed by dynamic-force parameters were compared to conventional motor-unit types by discriminant analysis. The results were not always consistent with conventional types.

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M.Sc. (Physiotherapy), Faculty of Health Sciences, University of the Witwatersrand, 2008. / Paediatric Human Immunodeficiency Virus (HIV) remains a significant challenge to children and caregivers in South Africa. Although the availability of antiretroviral (ARV) therapy has improved, it is not yet universally accessible. Rates of transmission from mother to child thus remain high and the virus widely uncontrolled. One aspect affecting children infected with HIV is that of muscle strength. For children weakness has been inferred by way of developmental studies in young children infected with HIV. Impaired performance in activities such as standing, walking, stair-climbing and jumping have been noted. These gross motor activities require higher muscle outputs and strength against gravity. This study sought to ascertain the feasibility of a full study on muscle strength in children infected with HIV. It analysed the effect of HIV on muscle strength, height and weight of those children receiving and not receiving highly active antiretroviral therapy (HAART). Children were recruited from Harriet Shezi Children’s HIV Clinic at Chris Hani Baragwanath Hospital, Soweto, Gauteng Province, South Africa. The study population included a group of children receiving HAART (n=16) and a group of children not receiving HAART (n=16). A once off test of muscle strength was administered to each child using a hand-held dynamometer. A demographic questionnaire and the Household Economic and Social Status Index (HESSI) were administered to their primary caregiver. Results showed the sample population to be of low socio-economic status (average score=54%) and the children to be underweight and short for their age (p<0.001). The CD4 count of the group on HAART was significantly higher than the group not receiving HAART (p<0.05). The group not receiving HAART was significantly stronger than the HAART group (p<0.05). Length of time having received HAART and muscle strength showed no significant correlation (p=0.647). No significant correlation was shown between CD4 count and muscle strength in the group receiving HAART (p>0.1). A significant negative correlation was shown between CD4 count and muscle strength in the group not receiving HAART (p<0.05). As statistically significant normative muscle strength data for children not infected with HIV in this age group fails to exist, the study was unable to ascertain a quantitative measure of weakness in these children. Comparison of those values available, however, showed normative values to be double that of children who participated in the study. The implications of these findings are that as one observes this group of children’s CD4 count drop, so too does their muscle strength. HAART, once initiated, stems the decrease in muscle strength over a period of time but does not reverse it. Furthermore, children and caregivers who participated in this study were faced with the adversities of poor socioeconomic status, limited access to medication and ARV treatment and inadequate nutritional intake, most of which were largely beyond their immediate control. This pilot study has indicated the feasibility and importance of a full study to investigate the muscle strength of children infected with HIV. Further research is needed to establish the impact of earlier administration of HAART on muscle strength. The effect of exercise on the muscle strength of children who are infected with HIV has yet to be documented. The implication of these factors on gross motor development in children infected with HIV has yet to be investigated.

Motor unit synchronization is phenomenon driven by a common input that results in the near-simultaneous firing of two or more motor units, which is referred to as short-term synchronization. The relationship between motor unit synchronization and force steadiness is still unclear, even after numerous experiments and simulations. Our main hypothesis was that the decreased force tetanus brought on by motor unit synchronization would be correlated to reduced steadiness at very low hand muscle forces. To determine if this correlation existed, young, healthy adults performed a submaximal, isometric pinch at four forces to determine if motor unit synchronization increased with a progressive decrease in force steadiness driven by reduced force levels. However, before performing synchronization analyses, we had to establish the best technique for measuring motor unit coherence, which quantifies the strength and frequency of a periodic common input. We used a pool of simulated spike trains with various firing rates, coefficients of variation (CV), common input frequencies and trial durations to explore the effects of data segmentation and spike train properties on coherence. We found that tapered segments overlapped by at least 50% maximized coherence measurements, regardless of taper type and that increasing common input frequency CV from 0.15-0.50 made coherence measurements unusable, even at high synchronization levels. During an isometric pinch at 2, 4, 8, and 12% of maximum digit force, we recorded thumb and index finger forces and EMG from the first dorsal interosseous (FDI) and adductor pollicis (AdP) muscles. As expected, the force CV dropped as each digit force increased. Pooled coherence revealed a dominant peak for the 2-10 Hz, but power for both digits' forces was limited to the 0-2 Hz bandwidth. There was a weak correlation for thumb force CV and coherence for within-AdP pairs, but no significant correlations were found for within-FDI pair coherence and finger force CV. Therefore, motor unit synchronization was not a strong driver of force steadiness for this protocol. To ensure that inherent firing rate nonstationarity of spike train data did not affect coherence measurements, we produced a new set of spike train pairs with firing rates and variances that approximated those for physiological motor units, which varied from 0-25%. Stationarity level was not significantly correlated to peak coherence (max R² = 0.082). Therefore, coherence measurements of spike train data with characteristics similar to those of the simulated trains were not significantly affected by nonstationarity. The establishment of the best method for computing coherence, the lack of a strong correlation between force steadiness and motor unit synchronization for submaximal isometric forces, and the knowledge that spike train nonstationarity has no significant effect on coherence measurements are all important discoveries needed for progress in the areas of basic neuromuscular function, motor unit synchronization, and pathological force unsteadiness.

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