Whether comparing an SME or an AEL approach with ST, all groups improved concentric strength although not always to the same degree as reported previously. Such was the case with Godard et al. (31) when they studied the effect of SME (120% 1RM) on quadriceps strength. They found that after 10 weeks of training 2 times per week, participants did not improve concentric strength in one group significantly more than the other. Brandenburg and Docherty (13) reported similar findings with the elbow flexors (SME increased 10% and ST increased 9%).
One of the issues with some of the current research is that despite an AEL or SME approach, eccentric strength is rarely assessed. Considering the well-documented principle of training specificity, eccentric strength has been shown to be best enhanced by eccentric-specific training (69). Hortobagyi et al. (36) recognized this limitation and addressed this in his study as well as any potential neurologic benefits to AEL training. As expected, eccentric strength gains for the AEL (27%) group were double that of the ST (11%) group; and any increase in concentric 1RM was accompanied by a directly proportional increase in electromyographic activity.
Neither SME nor AEL has demonstrated any advantage over ST with regard to hypertrophy. Ojasto and Häkkinen (58) subjected healthy men to a hypertrophy training protocol using AEL and found that it was not more favorable for hypertrophy when compared with ST. Other studies have shown that both AEL and SME present no clear benefit in increasing muscle cross-sectional area. Eccentric training at high speeds (180° per second) has been shown to be more effective for strength and hypertrophy than comparable concentric training (13,31,56,81). Higbie et al. (33) demonstrated that eccentric strength was best developed by eccentric-based training, whereas concentric strength was best for developing concentric strength.
It is very difficult to make conclusions as to the benefit of AEL or SME for sport performance enhancement. Based on the current research as shown in Table 3, there does appear to be certain advantages to an AEL or SME protocol for untrained populations, those requiring acute benefits from strength training, training specific muscle groups and for athletes required to perform at levels above lactate threshold or for specific muscle groups (13,36,39,80,81).
It should be noted that the research tabulated and discussed in this brief treatise is not without limitations. With the exception of Brandenburg and Docherty (13), all research used a nonathletic population. It is well known that untrained subjects respond differently to well-trained individuals. Furthermore, all studies, with the exception of the Yarrow et al. (80,81) and Ojasto and Häkkinen (58), chose single joint exercises such as knee extension, leg curls, and elbow flexion and extension. Although these exercises have merit in certain situations, increased performance in them is not indicative of potential for athletic performance, which requires multi-joint movement. Furthermore, the training protocols chosen for the studies, such as training every day only for 1 week or 1 set per day twice a week, are often inconsistent with that which are typically seen in sports performance–based training programs.
There is a clear need for future research. It is suggested that SME be used with multi-joint exercises such as the squat and dead lift on participants who are actively involved in competitive sport and/or resistance training. The program chosen needs to be volume adjusted to compensate for the increased eccentric load, and the set/rep/rest/frequency variables should be consistent with a sports performance–based program.
When a muscle is required to overcome resistance, or contract concentrically, its ability to do so may be determined by whether that concentric contraction was preceded by an eccentric muscle action. Research has shown that concentric force production in isolation is relatively low compared with concentric contractions that are coupled with an initial eccentric muscle action (44). This pairing is termed the SSC as defined previously. The SSC may have large or small amounts of angular displacement of the relative joints, and it is composed of both voluntary and involuntary (stretch reflex [SR]) actions (46,68). For optimal SSC potentiation (i.e., a more forceful concentric contraction), a number of factors are thought critical:
Elasticity refers to the ability of an object to return to form after it has been altered, and elastic energy is the work done during this process (26). A number of tissues can store elastic energy during an SSC including the muscle's connective tissues (e.g., perimysium, epimysium, endomysium), structures in series with the muscle fibers (e.g., tendon, titin), and the contractile elements themselves. The latter occurs within the cross-bridges between filaments when the actual muscle lengthens without the “popping” of the actin–myosin cross-bridge. This energy stored in the various tissues, however, is finite in duration with a half-life of 0.85 second and a 55% decrease by 1.0 second; therefore, to make the most of the stored elastic energy, the coupling times need to be minimal and the SSC should last less than 0.25 milliseconds (66,78). That is, the force generated during the concentric phase will tend to be higher when the duration of the SSC is shorter. As the SSC duration lengthens, the benefits of stored energy dissipate (79).
When preactivation occurs and when the athlete makes contact with the ground, a reflexive action results called an SR (54). The SR is a by-product of a signal sent by the spindles in the muscles to the central nervous system. Muscle spindles are receptors in the muscles, and they provide information about length and velocity of length change. In the drop jump, as the athlete makes contact with the ground, the muscle spindle senses the lengthening of the affected muscles (ankle plantar flexors and knee and hip extensors) and a signal is sent to the spinal cord via sensory motor neurons. A synapse occurs in the spinal cord, and excitatory messages are sent to the muscles via alpha motor neurons, which produce a concentric contraction in the muscles (to return the spindle to its initial length). The higher the velocity of the stretch, the greater potentiation of a reflexive forceful concentric contraction; this reflex is dependent on the level of motor neuron excitation and the amplitude of the movement, that is, small relative joint motion.
Research has shown that the optimal amount of energy stored during an SSC is largely determined by the amplitude of the relative joints. That is, some joint movement is necessary; however, too much angular displacement will decrease the number of actin and myosin cross-bridges interacting with each other and decrease reflex potentiation, which ultimately affects the storage and utilization of elastic energy within the muscle, thereby reducing its force production capability (63). Rack and Chu (63) demonstrated that drop jumps where the subjects maintained knee angles of less than 75° allowed them to keep their foot contact time to 416 ± 41 milliseconds, which created greater concentric force production than jumps where the subjects had knee angles greater than 85°. Longer contact times are indicative of larger stretch amplitudes of the relative muscles, and when muscles are stretched beyond a certain point, the resulting concentric contraction no longer benefits from the SR (49).
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