The Stretch-Shortening Cycle - Neuromuscular Fatigue

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The Stretch-Shortening Cycle: A Model to Study Naturally Occurring

Neuromuscular Fatigue.

Review Article

Sports Medicine. 36(11):977-999, 2006.

Nicol, Caroline 1; Avela, Janne 2; Komi, Paavo V 2

Abstract:

Neuromuscular fatigue has traditionally been examined using isolated

forms of either isometric, concentric or eccentric actions. However,

none of these actions are naturally occurring in human (or animal)

ground locomotion. The basic muscle function is defined as the

stretch-shortening cycle (SSC), where the preactivated muscle is

first stretched (eccentric action) and then followed by the

shortening (concentric) action. As the SSC taxes the skeletal muscles

very strongly mechanically, its influence on the reflex activation

becomes apparent and very different from the isolated forms of muscle

actions mentioned above. The ground contact phases of running,

jumping and hopping etc. are examples of the SSC for leg extensor

muscles; similar phases can also be found for the upper-body

activities. Consequently, it is normal and expected that the fatigue

phenomena should be explored during SSC activities.

The fatigue responses of repeated SSC actions are very versatile and

complex because the fatigue does not depend only on the metabolic

loading, which is reportedly different among muscle actions. The

complexity of SSC fatigue is well reflected by the recovery patterns

of many neuromechanical parameters. The basic pattern of SSC fatigue

response (e.g. when using the complete exhaustion model of hopping or

jumping) is the bimodality showing an immediate reduction in

performance during exercise, quick recovery within 1-2 hours,

followed by a secondary reduction, which may often show the lowest

values on the second day post-exercise when the symptoms of muscle

soreness/damage are also greatest. The full recovery may take 4-8

days depending on the parameter and on the severity of exercise. Each

subject may have their own time-dependent bimodality curve.

Based on the reviewed literature, it is recommended that the fatigue

protocol is 'completely' exhaustive to reduce the important influence

of inter-subject variability in the fatigue responses. The bimodality

concept is especially apparent for stretch reflex responses, measured

either in passive or active conditions. Interestingly, the reflex

responses follow parallel changes with some of the pure mechanical

parameters, such as yielding of the braking force during an initial

ground contact of running or hopping. The mechanism of SSC fatigue

and especially the bimodal response of performance deterioration and

its recovery are often difficult to explain. The immediate post-

exercise reduction in most of the measured parameters and their

partial recovery 1-2 hours post-exercise can be explained primarily

to be due to metabolic fatigue induced by exercise. The secondary

reduction in these parameters takes place when the muscle soreness is

highest.

The literature gives several suggestions including the possible

structural damage of not only the extrafusal muscle fibres, but also

the intrafusal ones. Temporary changes in structural proteins and

muscle-tendon interaction may be related to the fatigue-induced force

reduction. Neural adjustments in the supraspinal level could

naturally be operative, although many studies quoted in this article

emphasise more the influences of exhaustive SSC fatigue on the

fusimotor-muscle spindle system. It is, however, still puzzling why

the functional recovery lasts several days after the disappearance of

muscle soreness. Unfortunately, this and many other possible

mechanisms need more thorough testing in animal models provided that

the SSC actions can be truly performed as they appear in normal human

locomotion.

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Carruthers

Wakefield, UK