Starling's law (the energy of muscle contraction is proportional to th
e initial fiber length) has been applied to contraction of inspiratory
muscles. Its application to the expiratory muscles is difficult becau
se both maximal length and maximal pressure development occur at total
lung capacity (TLC). We hypothesize that decrease of both inspiratory
(Pimax) and expiratory (Pemax) muscle strength in chronic ascites (CA
) will reflect generalized muscle weakness and stretching of both the
diaphragm and abdominal wall as well. To test this hypothesis, we eval
uated Pimax and diaphragm length (at functional residual capacity) in
22 patients. Pemax, external oblique and transversus abdominus muscle
lengths, and anterior abdominal wall muscle thickness were measured at
TLC. We found Pimax(78 +/- 19% predicted), Pemax (61 +/- 17%), and -
as an index of general muscle strength - handgrip strength (75 +/- 22%
) all to be minimally reduced. Respiratory muscle strength did not cor
relate with any measurement of inspiratory/expiratory muscle length or
thickness. With fluid removal, abdominal muscles shortened; diaphragm
atic curvature decreased although diaphragm length was unchanged. Neve
rtheless, neither Pimax nor Pemax increased. Respiratory muscle streng
th depends upon generalized muscle strength more so than upon muscle l
ength in CA patients.