Muscle force production and adaptation are influenced by a range of factors but important to these are the length, and rate of change of length, of the active fibres involved in the activity (Rassierand Pun, 2010; Rassier and Herzog, 2002). When fibres are extended - usually by an external joint moment caused by a ground reaction force -the activity is termed eccentric. Similarly, when the muscles are shortening the activity is termed concentric.

However, despite the known health benefits of physical activity and the fact that it is potentially accessible to all, it is estimated that over 60% of the world's population are not active enough. (1) In 1997 the Institute of European Food Studies conducted a pan-EU survey of consumer attitudes towards physical activity, body weight and health, in 15,339 adults from 15 member states.