Age effects on properties of motor unit action potentials: ADEMG analysis

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Abstract
We have measured the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) from the brachial biceps, brachial triceps, and anterior tibial muscles in 10 young (20–40 years), 10 middle‐aged (40–60 years), and 10 elderly (60–80 years) normal individuals, using an automatic method for Decemberomposition of the electromyo‐graphic (EMG) interference pattern (ADEMG). Recordings were made during stable isometric contractions at threshold, 10%, and 30% of maximum voluntary contraction using standard concentric needle electrodes. At supra‐threshold forces, an average of 5.9 simultaneously active MUAPs were identified at each recording site. Mean amplitudes, durations, and numbers of turns all increased linearly with age in both low‐threshold and high‐threshold MUAPs (p < 0.01), suggesting an ongoing process of progressive denervation and compensatory reinnervation. Mean MUAP firing rates Decemberreased with age (p = 0.01) when force was measured proportionately, but not when measured absolutely. In a subgroup of 12 age‐matched gender pairs, men had larger mean MUAP amplitudes, rise rates, and numbers of turns (p < 0.05), probably reflecting larger muscle fiber diameters. These findings amplify previous observations from traditional analysis of lowest‐threshold single MUAPs; establish a base of normative adult data for ADEMG; and further validate the clinical applicability of rapid, automatic EMG Decemberomposition.