Stretch-shortening cycle: a powerful model to study normal and fatigued muscle
Introduction
The true nature of muscle function is difficult to assess from isolated forms of isometric, concentric or eccentric contractions. In real life, exercise seldom involves a pure form of these types of isolated muscle actions. The natural variation of muscle function is more often a stretch and shortening cycle and thus this model provides a good basis from which to study both normal and fatigued muscle. Two important aspects of this phenomenon are: (1) pre-activation and (2) variable activation of the muscles preceding the functional phase of a given movement (e.g. ground contact for the leg extensor muscles during running). Other important concepts that need to be addressed are length changes in muscle versus tendon during the contact phase and the role of the stretch reflex in the stretch-shortening cycle.
The stretch-shortening cycle (SSC) of muscle function comes from the observation that body segments are periodically subjected to impact or stretch forces. Running, walking and hopping are typical examples in human locomotion of how external forces (e.g. gravity) lengthen the muscle. In this lengthening phase the muscle is acting eccentrically, then a concentric (shortening) action follows. The true definition of eccentric action indicates that the muscles must be active during stretch. This combination of eccentric and concentric actions forms a natural type of muscle function called the stretch-shortening cycle or SSC (Norman and Komi, 1979; Komi, 1984; Komi and Nicol, 2000). (Fig. 1). This type of sequence in muscle function also involves the important features of preactivation and variable activation. SSC muscle function has a well-recognized purpose: enhancement of performance during the final phase (concentric action) when compared to the isolated concentric action. This can be demonstrated in isolated preparations with constant electrical stimulation (e.g. Cavagna et al., 1965, Cavagna et al., 1968), in animal experiments with natural and variable muscle activation (e.g. Gregor et al., 1988) and in maximal effort conditions of human SSC actions (Cavagna et al., 1968; Komi, 1983). Considerable effort has been devoted to explain the mechanisms for force and power potentiation during a SSC. Cavagna et al. (1965) was one of the first to argue that this enhancement is primarily from stored elastic energy. Since that time many additional alternative explanations (e.g. Huijing, 1992; Van Ingen–Schenau et al., 1997; Komi and Gollhofer, 1997) have been presented. However, no convincing evidence has been presented that negates elasticity as an important element in force potentiation during a SSC.
The schematic presentation of Fig. 1 takes into consideration the common assumption that in a SSC the contractile and tensile elements are stretched during the eccentric phase. There are, however, arguments in the literature suggesting that the contractile component may maintain a constant length (Hoff et al., 1983; Belli and Bosco, 1992) or even shorten (Griffiths, 1991) during the early phase of ground contact.
The present report reviews the work of SSC muscle actions performed during human experiments primarily in our laboratory. The main focus will be to demonstrate with in vivo measurements the recoil nature of a SSC and how the stretch-reflex can play an important role in force potentiation. The SSC model will then be introduced for fatigue experiments where it's unique loading characteristics can be used to examine neuromuscular fatigue in a very comprehensive way.
Section snippets
Use of in vivo force measurements to characterize the SSC in human locomotion
Two techniques can be applied to record directly, and in vivo, tendon forces in humans: a buckle transducer method and an optic fiber technique. From these methods, the buckle technique is a more invasive one and it was used solely for Achilles tendon (AT) force recordings (e.g. Komi et al., 1987; Komi, 1990; Fukashiro et al., 1993, Fukashiro et al., 1995). The buckle is surgically implanted around the AT under local anesthesia, but the subject is able to perform 2–3 h of unrestricted locomotion
Can stretch-reflexes contribute to force enhancement during SSC?
Hopping and running, activities which are often used as models of a human SSC, seem very suitable for possible interaction from stretch reflexes. These activities seem very effective due to the following fundamental conditions (Komi and Gollhofer, 1997): (1) the muscles are preactivated before touch down (and the braking phase) (see Fig. 2); (2) the eccentric (lengthening phase) is short and fast, and (3) there is an immediate transition (Short delay) between stretch (eccentric) and shortening
SSC is a unique model to study neuromuscular fatigue
The mechanisms presented above are not only relevant in non-fatigued situations and can be put under severe stress during SSC fatigue. In traditional fatigue experiments, with either isometric or concentric actions, the fatigue effects can be discussed primarily from a metabolic point of view. In SSC fatigue impact loads are repeated over a certain time period with the exercise taxing all the major elements: metabolic, mechanical, and neural. It is SSC fatigue models, in particular, which cause
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