As the torso moves ahead of the foot, the drive is initiated and the achilles and calf are placed under great stress. It is therefore important that stretching and strengthening of this area be incorporated into training. Muscle fibres in the calf respond to a reflex action as they are placed in near full stretch and contract quickly, thus apparently straightening the foot, forcing the athlete back up higher on their fore-foot. (This makes the foot a further lever, often forgotten by many runners). The foot "grips" the ground as the torso moves ahead, forcing the leg into full extension. Once again, strength and flexibility of the hamstrings are important.