Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Age-related changes in the numbers and physiological properties of human motor units (MUs) were examined in four studies.;In the first study, spike-triggered averaging was used to estimate the numbers of MUs in the biceps brachii and brachialis muscles. The maximum isometric twitch contraction (MTC) and maximum voluntary contraction (MVC) of the elbow flexors were also measured. In older subjects, the estimated numbers of MUs were significantly reduced (47%) whereas, average surface-detected motor unit action potential (S-MUAP) sizes were significantly larger (23%). Significant but less marked age-associated reductions were found for the MTC (33%) and MVC (33%).;In the second study Multiple Point Stimulation (MPS) was used to collect a sample of the lowest threshold, all-or-northing, thenar S-MUAPs. The average negative peak area and peak to peak amplitude of the sample of S-MUAPs from each subject were calculated and divided into the corresponding value for the maximum M-potential to derive the motor unit estimate (MUE). In younger subjects, the mean estimated number of MUs was 288 {dollar}\pm{dollar} 95 (ISD) based on negative peak area. Older subjects had significantly smaller M-potentials (33%) and significantly larger S-MUAPs (37%). Older subjects had about 50% fewer MUs with a mean value of 139 {dollar}\pm{dollar} 68.;In the third study, trains of 300 stimuli at intensities evoking M-potentials between 10 and 30% of the peak to peak amplitude of the maximum M-potential, were derived to the median nerve. In the young, observed firing probabilities of S-MUAPs extracted from the F-response ranged from less than 1% to 10%, the S-MUAPs varied in size from 0.015-5.3% of the maximum M-potential negative peak area, and were similar in size to the population of S-MUAPs collected by MPS. The conduction velocities (CVs) of MUs in the young ranged from 48-68 m{dollar}\cdot{dollar}s{dollar}\sp{lcub}-1{rcub}{dollar} (mean 59 {dollar}\pm{dollar} 4). In older subjects, the S-MUAP sizes were significantly larger (39%), and the range and distribution of motor unit CVs (38-61 m{dollar}\cdot{dollar}s{dollar}\sp{lcub}-1{rcub}{dollar}; mean 52 {dollar}\pm{dollar} 3) were markedly shifted to reflect a slower population of motor fibers.;In the fourth study, MPS was used to collect a sample of all-or-nothing, thenar MUs from younger and older subjects. (Abstract shortened by UMI.)



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