ABSTRACT
The anterior latissimus dorsi (ALD) muscles of the chicken are mostly composed of multiply innervated slow fibres, and the posterior latissimus dorsi (PLD) muscles consist mostly of focally innervated fast fibres (Ginsborg, 1960). The rate of development of isometric tension is 8–10 times higher in the PLD than in the ALD, and the unloaded speed of shortening is 4–5 times higher (Canfield, 1971). On tetanic stimulation ALD fibres are able to maintain a steady isometric tension for more than 2 min, whereas the PLD begins to fatigue after only 1 s (Canfield, 1971). The fatigue resistance and economy (Goldspink, Larson & Davies, 1970) of the ALD when contracting isometrically are probably related to its postural role in holding the wings against the body. The rate of relaxation of PLD fibres following short tetani is eight times faster than for the ALD (Canfield, 1971). This difference is attributable in part to the less highly developed sarcoplasmic reticulum in the ALD (Page, 1969). The present study investigates the pCa-tension relationship of these fibres.
