![]() The tasks with and without counter movement were compared. In the present study, we investigated the muscle fibre behaviour of the gastrocnemius during a plantar flexion exercise in humans, by directly measuring changes in the fascicle length in vivo. The gastrocnemius muscle, one of the major plantar flexors in humans, has long tendons and aponeuroses that possess substantial compliance ( Alexander & Bennet-Clark, 1977 Soest et al. Komi, 1992) therefore, it has not been possible to separately observe the behaviour of contractile and elastic components, and the interactions between muscle fibres and tendinous tissues in counter movement have not been observed. In humans, however, muscle-tendon unit length has only been estimated from ‘external’ joint kinematics (e.g. 1997) in vivo has revealed significant elastic recoil of tendinous structures that contributes to joint performance. 1989) and running turkeys ( Roberts et al. In fact, direct observation of fascicle length change in walking cats ( Hoffer et al. In the latter case, tendinous tissues have been thought to act as a spring ( Alexander & Bennet-Clark, 1977 Morgan et al. Mechanisms for the enhancement of performance of the final action have been proposed, including the potentiation of force exerted by prestretched muscle fibres and the additional power provided by the elastic potential of the muscle-tendon unit ( Huijing, 1992 Komi, 1992). Enhanced performance of human movement accompanied by a preceding counter action has also been widely reported ( Thys et al. Such a counter-movement action is frequently used in animal locomotion for efficient power production ( Alexander & Bennet-Clark, 1977 Morgan et al. Shortening of muscle preceded by active lengthening has been shown to be more powerful than that resulting from shortening alone ( Cavagna et al. This makes it difficult to estimate muscle fibre behaviour solely from observation of joint performance ( Fukunaga et al. 1978), the force generated by a muscle-tendon unit is not a simple function of its length and velocity as a whole. Therefore, although the force-producing capability of a muscle fibre itself is a function of its length and velocity of shortening or lengthening ( Katz, 1939 Hill, 1951 Close, 1972 Edman et al. Previous studies revealed muscle deformation upon contraction: muscle fibres shorten even during fixed-end isometric contractions while tendinous tissues are elongated to take up muscle fibre shortening ( Griffiths, 1991 Fukunaga et al. 1991 Kawakami & Lieber, 2000) and are hence a major constituent of the series elastic component (SEC) of a muscle model initially proposed by Hill (1951). The tendinous tissues-sheet-like structures such as aponeuroses in particular-possess elastic characteristics ( Alexander & Bennet-Clark, 1977 Proske & Morgan, 1987 Lieber et al. The force exerted by skeletal muscle fibres is transmitted to the tendinous tissues (tendon and aponeurosis) before producing torque around a joint. It was concluded that during CM muscle fibres optimally work almost isometrically, by leaving the task of storing and releasing elastic energy for enhancing exercise performance to the tendon. In NoCM, fascicle length decreased throughout the movement and the fascicle length at the onset of movement was longer than that of the corresponding phase in CM. In CM, in the dorsiflexion phase, fascicle length initially increased with little electromyographic activity, then remained constant while the whole muscle-tendon unit lengthened, before decreasing in the final plantar flexion phase. The estimated peak force, average power, and work at the Achilles’ tendon during the plantar flexion phase in CM were significantly greater than those in NoCM. ![]() From real-time ultrasonography of the gastrocnemius medialis muscle during the movement, the fascicle length was determined. ![]() The reaction force at the foot and ankle joint angle were measured using a force plate and a goniometer, respectively. ![]() Six men performed a single ankle plantar flexion exercise in the supine position with the maximal effort with counter movement (CM, plantar flexion preceded by dorsiflexion) and without counter movement (NoCM, plantar flexion only) produced by a sliding table that controlled applied load to the ankle (40 % of the maximal voluntary force).
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