ABSTRACT
The work done can be classified as either positive work or negative work depending on whether the force moves in the direction of movement of the object or in the opposite direction. In the case of the bench press example, lifting the barbell upwards results in positive work being done on the barbell as the force of the hand applied to the barbell is in the same direction as the movement. The outcome of this positive work is that the barbell has changed its position against gravity. On the downward movement the force applied by the hand is in the same upward direction, but the direction of movement of the barbell is downwards. This introduces a negative sign into equation D1.1 and so the work done becomes negative. This means that when the barbell is being lowered, the barbell is doing work on the person, rather than when it is being lifted where the person is doing work on the barbell. Where does this negative work go? Often it is dissipated as heat in the muscles so it is lost. Of interest in sports performance is the fact that some of this work can be used to deform structures of the body (most notably the muscles and tendons) which, given the right technique, can be recovered again during their shortening. This phenomenon can explain a range of observations in sport from why people “cheat” when doing tasks such as a barbell bench press (by bouncing the bar off the chest) to the greater efficiency of running compared with walking at certain speeds (due to the stretching and recoil of the tendons of the leg).
