The diaphragm is the major inspiratory muscle, but the intercostal and scalene muscles also generate inspiratory pressure to draw air into the lungs. There is synergistic activation of the inspiratory muscles to ventilate the lungs, but coordinated activity across these muscles is also important to reduce the work of breathing. In recent history, the mechanics of the inspiratory muscles for breathing is quantified as a fractional change in muscle length per passive increase in lung volume, i.e. mechanical advantage (De Troyer et al., 2005). The relative contribution of the inspiratory muscles, and even different regions within each muscle, to generate inspiratory pressure differs. How does the central nervous system deal with the diversity and redundancy in respiratory muscle mechanics?

In humans, single motor unit studies have demonstrated that the parasternal intercostal muscles are activated in a precise pattern, with earlier and greater activity in the rostral interspaces compared to the caudal spaces during eupnoea. This finding is robust, having been replicated twice (Hudson et al., 2019). This rostrocaudal pattern of neural drive mirrors the relative inspiratory mechanical advantages of these muscles. Similarly, a rostrocaudal gradient of neural drive parallels that of mechanical advantage in the external intercostal muscles. In addition, in the external intercostals, there are within interspace differences in neural drive and mechanical advantage, both being greatest in the dorsal portion of muscle and declining along the interspace. The coefficient of correlation between neural drive and mechanical advantage is 0.99 for both intercostal muscle groups (Hudson et al., 2019). This remarkable relationship between neural drive and mechanics is a strategy that minimises the metabolic cost of muscle activation (De Troyer et al., 2005) and led to the discovery of the “principle of motor unit recruitment by neuromechanical matching” (Hudson et al., 2019).

For the diaphragm, neural drive is greater to the costal than the crural portion, with increases in either voluntary or involuntary drive to breathe (Nguyen et al., 2020). Given the costal portion generates more thoracic expansion, via the zone of apposition (Domnik et al., 2020), this suggests motor unit recruitment according to neuromechanical matching occurs across portions of the major inspiratory muscle in humans.

The ‘respiratory muscles’ have other motor functions, and their neural control adapts according to their mechanics in the motor task. In targeted voluntary breaths, where the mechanics of the intercostal muscles are comparable to eupnoea, the rostrocaudal pattern of neural drive is maintained (see Hudson et al., 2019). However, in ipsilateral trunk rotation, for populations of the same intercostal motor units, the pattern of recruitment across interspaces is reversed compared to the rostrocaudal gradient during eupnoea. This is likely to reflect different mechanics for the parasternal intercostal muscles in these tasks (Hudson et al., 2017).

Motor unit recruitment by neuromechanical matching is the most efficient way to recruit the respiratory muscles for breathing and other tasks in health. Non-invasive methods to assess patterns of inspiratory muscle activity will facilitate discoveries on neuromechanical matching in clinical populations and is the focus of new research.