Keynote and Invited Abstracts |
|
 |
Eccentric exercise: Injury, repair and the effects of training U
Proske*, D Morgan, T Allen, J Talbot, J Rundell, C
Brockett & N Whitehead While after most exercise we are just tired, after eccentric exercise we are not only tired but our muscles are sore the next day. It has been known since the turn of the century that some forms of exercise makes us sore (Hough, 1902) but a comprehensive explanation for why eccentric exercise leads to soreness has become available only recently (Morgan, 1990). During eccentric contractions muscles are forcibly lengthened, during concentric contractions they shorten. Sports which involve eccentric contractions include downhill walking, skiing and horse riding. Sports during which contractions are predominantly concentric include swimming and cycling. It is generally agreed that an eccentric contraction leads to membrane damage in muscle fibres, a loss of calcium homeostasis, and consequent injury or even death of the muscle fibre. It has been proposed by Morgan, that the primary mechanical event triggering the damage is an overextension of some sarcomeres in muscle fibres. In a contracting muscle fibre not all sarcomeres are at the same length, or of the same strength, particularly at longer muscle lengths where some sarcomeres are likely to be on the descending limb of their length-tension relation. During an eccentric contraction the weaker sarcomeres are stretched more than their stronger neighbours. If they are stretched to the point where they reach the descending limb of their length-tension relation they become progressively weaker, extending rapidly, uncontrollably. A point is reached where they no longer have any overlap between the myofilaments. During relaxation most overextended sarcomeres are able to re-establish their normal interdigitating pattern while some become disrupted and represent islands of weakness scattered along the lengths of muscle fibres. During repeated eccentric contractions the damage spreads until membrane elements become implicated. Evidence of damage in the muscle is a drop in whole-muscle force and an increase in series compliance resulting from the non-contracting, disrupted segments of muscle fibres in series with the still normal portions (Jones et al, 1997). It is a common experience that after a period of soreness from eccentric exercise, a second bout of exercise a week later leads to much less soreness. Some adaptation has taken place, limiting the amount of damage from the second period of exercise. We propose that following repair of the damage, muscle fibres incorporate extra sarcomeres and that this acts as a form of protection. That is, because there are more of them, sarcomeres are working at shorter lengths and the instabilities inherent in the descending limb of the length-tension relation are avoided. These various propositions have formed the basis of a number of studies, looking for changes in muscle mechanical properties as indicators of damage. Studies are underway to test different kinds of training regimes with the aim of designing programs which provide protection against the damage of eccentric exercise while at the same time ensuring that there is adequate strength training of the exercising muscles to allow maintenance of high performance levels.
References:
|
