The electrochemical process of generating tension within a muscle. You would be forgiven for thinking that when a muscle contracts it shortens. This does happen in some types of contraction (concentric contractions), but muscles can also lengthen during a contraction (eccentric contractions), or stay the same length (isometric contractions). Consequently, many exercise physiologists prefer to use the phrase ‘muscle action’, because this does not imply a change in muscle length.
Concentric contractions occur when a muscle develops sufficient muscle tension to overcome a resistance. The muscle shortens visibly to move a body part. Concentric contractions may be isokinetic or isotonic.
During isokinetic contractions a muscle shortens at constant speed (or constant angular velocity) over the full range of motion. During a full isokinetic contraction, the tension developed by a muscle is at its maximum throughout its whole range. To perform a controlled isokinetic contraction, special equipment is needed which contains a speed governor so that the speed of movement is constant no matter how much tension is produced by the contracting muscle. Training that uses isokinetic contractions is thought to increase the blood supply to skeletal and cardiac muscle, and therefore it improves muscle strength, endurance, and cardiac fitness. Because high muscle tensions are exerted at each phase of movement, isokinetic contractions strengthen the whole muscle. In addition, isokinetic exercises can be designed to mimic the actual speeds of sports-specific activities. This is thought to improve neuromuscular coordination so that more muscle fibres can be recruited and muscles can contract more efficiently. The major disadvantages of isokinetic exercises are that they can only be performed properly on machines which are usually expensive, and the types of movement that can be performed are rather limited.
Isotonic contractions are the most common type of contraction (figure 48). The muscle changes length and takes a joint through a specific range of motion against a fixed resistance; the muscle may shorten (concentric contraction) or lengthen (eccentric contraction). Isotonic exercises often involve raising and lowering a weight. The speed of movement is controlled by the exerciser but the load remains constant. Isotonic exercises are good for developing strength and cardiovascular endurance. Unfortunately, the efficiency of joints vary with joint angles; consequently, unlike isokinetic contractions, a fixed resistance may not provide a sufficient workload over the complete range of motion to give maximum training benefits.
An eccentric contraction causes a muscle to lengthen under tension. Such contractions are used to resist external forces such as gravity. The quadriceps muscles, for example, undergo eccentric contractions when a person walks down steps, runs downhill, or lowers a weight. Eccentric contractions also occur during the deceleration phases of running.
Training in which eccentric contractions predominate tends to cause significantly greater muscle soreness than other forms of training, though it may protect against future muscle soreness. Strong muscle fibres may replace weak ones, and neuromuscular coordination improve, so that forces are distributed more effectively. To get the maximum benefit from weight training using eccentric contractions, the weights must be lowered slowly and in a controlled manner.
Subjects: Medicine and Health.