Isokinetic Contractions are muscle contractions in which the speed of the movement stays the same.
At the beginnig andof a contraction due to the state of overlap of actin and myosin filaments the movement is generally slower than in the middle of the movement. To create uniform speed, more force has to be exerted at the beginning and end of movement.
For example in a tennis serve, at the beginning of the shoulder extension, when the extensor muscles are extremelz stretched, the muscle force has to be greater to achieve a movement with consistent speed.