CONTRACTILITY OF THE VENTILATORY PUMP MUSCLES

The ventilatory muscles are striated skeletal muscles, and their in si tu function is governed by the same relationships that determine the c ontractile force of muscles in vitro. The ventilatory muscles, however , are functionally distinct from limb skeletal muscles in several aspe cts, the most notable being that the ventilatory muscles are the only skeletal muscles upon which life depends. Among the muscles that parti cipate in ventilation, the diaphragm is closest to its optimal resting length at functional residual capacity (FRC) and has the greatest cap acity for shortening and volume displacement, making it the primary mu scle of inspiration. All inspiratory muscles shorten when the lung is inflated above FRC, but interactions among the various inspiratory mus cles make for a wider range of high force output than could be achieve d by any one muscle group acting in isolation. The velocity of inspira tory muscle shortening, especially diaphragmatic shortening, causes ma ximal dynamic inspiratory pressures to be substantially lower than max imal static pressures. This effect is especially pronounced during max imal voluntary ventilation, maximal exercise, and maximal inspiratory flow volume maneuvers over the full vital capacity. During quiet breat hing, the ventilatory muscles operate well below the limits of their n eural activation and contractile performance. During intense activity, however, the diaphragmatic excursion approaches its limits over the e ntire vital capacity, and respiratory pressures may near their dynamic maximum. Because the system map operate near its available capacities during increased ventilatory demands, multiple strategies are availab le to compensate for deficits. For example, if the diaphragm is acutel y shortened, it can still generate the required respiratory pressure i f it receives more neural drive. Alternatively, other muscles can be r ecruited to take over for an impaired diaphragm. Thus, the whole syste m is highly versatile.