Every movement begins with a muscle contraction, which pulls on a joint to move your skeleton. These muscle contractions can be categorized into one of three distinct types:
1. Concentric. During concentric contractions the muscle shortens while generating force. In general, concentric muscle actions are responsible for the 'lifting' part of an exercise, such as the biceps shortening as you lift a dumb-bell to shoulder height during a biceps curl.
2. Eccentric. During eccentric contractions - usually the 'lowering' part of an exercise - the muscle lengthens and is around ten per cent stronger than it is during concentric contractions. It is these heavy eccentric loads that cause the maximum amount of damage to your muscles. It's for this reason you should always lower a weight slowly and under control: not only does this minimize the risk of injury it also makes each rep more effective.
3. Isometric. During isometric contractions a muscle generates force without changing length. Examples include your entire abdominal region during a plank, or the muscles of the hand and wrist when you grip an object. Although research by NASA into preventing muscle mass breakdown found isometric contractions aren't as effective for building muscle mass as concentric and eccentric contractions, they should still form part of your workout, especially for abs and core work. But, isometric contractions can cause a rapid rise in blood pressure so should be avoided if you have a heart condition.