All muscle training falls into three categories:
In isotonic contractions, the muscle contracts and shortens, giving movement. Nearly all the training you do is isotonic.
In isometric contractions, the muscle contracts but does not shorten, giving any movement. The Plank is a good example of an isometric contraction.
Isometric training is not sufficient on its own. You need to combine it with isotonic training.
In isokinetic contractions, the muscle contracts and shortens at a constant speed. An isotonic contraction is different to an isokinetic contraction because it is usually slowest at the start.
For isokinetic training, you need special equipment that detects when a muscle is speeding up and increases the load to slow it down again.
Concentric contraction occurs when a muscle shortens in length and develops tension e.g. the upward movement of a dumbbell in a biceps curl or as you spring back from a jump landing, extending your knees and jumping back up in the air, the quadriceps are shortening as they create a force to push you off.
Eccentric contraction involves the development of tension while the muscle is being lengthened, e.g. the downward movement of a dumbbell in a biceps curl or when you land on two feet from a jump and bend your knees the quadriceps are lengthening.
Eccentric movements recruit most fast-twitch fibres
Research, McHugh et al. (2002), investigated the difference in activation patterns between eccentric and concentric quadriceps contractions. The researchers measured the amount of muscle activity as revealed by electromyography (EMG) and the EMG signal's mean frequency. As a rule, the larger the EMG signal recorded, the more muscle fibres are being recruited, while the signal's frequency is an indication of how fast they are being recruited. Research has shown that higher frequency EMG is consistent with greater fast-twitch fibre recruitment.
They found that the total EMG signal was greater during the concentric phase, suggesting more muscle fibres are active at this time, while the mean frequency of the EMG signal was greater during the eccentric phase, suggesting more fast-twitch fibres are being recruited at this time. They concluded that there is less total muscle fibre recruitment during a maximal eccentric contraction, with fast-twitch fibres recruited in preference to slow-twitch ones. In contrast, all the muscle fibres are used during a maximal concentric contraction.
This finding is significant for power athletes as the research suggests that if you want to train your fast-twitch fibres, it would seem that eccentric contraction movements are more useful than concentric ones. Plyometric exercises, which involve high-force eccentric movements, would be particularly useful for this purpose. A good example is the depth jump, which involves jumping off a box, bending at the knee and hip to control the landing softly, then jumping back up. Power athletes may also want to consider performing strength exercises using the eccentric phase only. By this means you may target just the fast-twitch fibres and perform less total work, potentially making the training more efficient. You will need a training partner or coach to assist you with each concentric phase, leaving you to complete the effort on each eccentric phase alone.
Muscle soreness that occurs 24 to 48 hours after intense exercise, usually involves eccentric contractions. This causes increases in intracellular pressure that irritates the nerve endings, producing swelling and local pain. The soreness can indicate potential muscle adaptation to follow, but it could indicate overtraining or considerable muscular tissue damage if it persists or is debilitating.
An appropriate warm-up and cool-down may help to avoid or reduce DOMS.
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