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Immunocytochemical characteristics of elbow, knee and ankle muscles of the five‐toed jerboa (Allactaga elater)

Biochemical adaptations of limb myofibres to intensive bipedal hopping were investigated using the five‐toed jerboa Allactaga elater as a model in comparison with the rat. Immunofluorescence methods included immunoreactivity to anti‐fast and anti‐slow MHC and troponin I. There is no specialization of triceps caput mediale for postural function in the minute non‐locomotor forelimbs, unlike quadruped mammals. The various elbow extensor heads and the flexor muscles are alike with regard to fibre type population and cross‐sectional areas of each type of fibre. The extensor muscle in the elongated hindlimbs of the five‐toed jerboa, at both the knee and the ankle joints, differ from each other extensively. One head, made up of an extremely high percentage of type I, fatigue‐resistant fibres, is suited to postural function. Two extensor heads at each joint contain a very high percentage of type IIB fibres (having the greatest maximal velocity of contraction) and are able to produce the powerful acceleration needed to trigger the leap. The relative cross‐sectional areas of the myofibres are characteristic of hopping locomotion: predominance in number of one type of myofibre in a muscle accompanies greater cross‐sectional area, which increases muscle efficiency in either postural or accelerative function of the muscle.