Discharge properties of human diaphragm motor units with ageing

Ageing is associated with changes in the respiratory system including in the lungs, rib cage and muscles. Neural drive to the diaphragm, the principal inspiratory muscle, has been reported to increase during quiet breathing with ageing. We demonstrated that low-threshold motor units of the human diaphragm recruited during quiet breathing have similar discharge frequencies across age groups and shorter discharge times in older age. With ageing, motor unit action potential area increased. We propose that there are minimal functionally significant changes in the discharge properties of diaphragm motor units with ageing despite remodelling of the motor unit in the periphery.

There are changes in the skeletal, pulmonary and respiratory neuromuscular systems with healthy ageing. During eupnoea, one study has shown relatively higher crural diaphragm electromyographic activity (EMG) in healthy older adults (>51 years) than in younger adults, but these measures may be affected by the normalisation process used. A more direct method to assess neural drive involves the measurement of discharge properties of motor units. Here, to assess age-related changes in neural drive to the diaphragm during eupnoea, EMG was recorded from the costal diaphragm using a monopolar needle electrode in participants from three age groups (n ≥ 7 each): older (65-80 years); middle-aged (43-55 years) and young (23-26 years). In each group, 154, 174 and 110 single motor units were discriminated, respectively. A mixed-effects linear model showed no significant differences between age groups for onset (group mean range 9.5-10.2 Hz), peak (14.1-15.0 Hz) or offset (7.8-8.5 Hz) discharge frequencies during eupnoea. The motor unit recruitment was delayed in the older group (by ∼15% of inspiratory time; p = 0.02 cf. middle-aged group) and had an earlier offset time (by ∼15% of inspiratory time; p = 0.04 cf. young group). However, the onset of multiunit activity was similar across groups, consistent with no global increase in neural drive to the diaphragm with ageing. The area of diaphragm motor unit potentials was ∼40% larger in the middle-aged and older groups (P < 0.02), which indicates axonal sprouting and re-innervation of muscle fibres associated with ageing, even in middle-aged participants.