Quadriceps: concentric and eccentric muscle contractions
Concentric and Eccentric Quadriceps Contractions
Eccentric moves recruit most fast twitch fibres. A new piece of research has investigated the differences in activation patterns between concentric and eccentric quadriceps contractions. In particular, the researchers were concerned with measuring the amount of muscle activity as revealed by electromyography (EMG) and the mean frequency of the EMG signal. As a rule, the larger the EMG signal recorded the more muscle fibres are being recruited, while the frequency of the signal 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. Concentric contraction involves force created when the muscle fibres shorten, while eccentric contraction involves force created when they lengthen. For example, when you land on two feet from a jump and bend your knees the quadriceps are lengthening, but also creating a force to control the landing. As you spring back from the landing, extending your knees and jumping back up in the air, the quadriceps are shortening as they create force to push you off. In this experiment the subjects performed maximal concentric and eccentric contractions of the quadriceps, while the researchers measured the EMG activity and frequency of signals. 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 during a maximal eccentric contraction there is less total muscle fibre recruitment, with fast twitch fibres recruited in preference to slow twitch ones, whereas during a maximal concentric contraction all the muscles fibres are used.
This finding is significant for power athletes: 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. The landing phase is the eccentric contraction – and the bigger the depth jump, the greater the eccentric forces. Power athletes may also want to consider performing strength exercises using the eccentric phase only. By this means you may be able to 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.
Journal of Sports Sciences, 20(2), p83-91
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