The 1st rocker starts as the heel contacts the ground. At this point a ground reaction force is created; the ground pushes on the heel causing the ankle to plantarflex and the foot to pronate. This helps to absorb the impact of the step and allows the foot to become flexible and adapt to the shape of the ground. At the same time the knee flexes helping to absorb the impact of the step. Due to the position of the pelvis and pronation occurring the entire limb from hip to foot internally rotates.
Ankle plantarflexion and knee flexion stop when the forefoot contacts the ground and the 2nd rocker begins. Pronation and internal rotation of the limb continue to occur until mid stance, when the tibia is perpendicular to the ground.
This starts when the forefoot contacts the ground, the tibia, propelled by the opposite limb starts to move forward causing dorsiflexion at the ankle. At the same time the knee starts to extend. During the 2nd rocker the hip internally rotates on the stance limb causing external rotation of the leg which causes the foot to stop pronating and start supinating. This reduces the range of motion within the foot joints turing the foot into a rigid lever.
When the ankle reaches its maximum range of dorsiflexion the heel starts to lift off the ground causing dorsiflexion at the metatarsalphalangeal joints (MPJ, ball of the foot) which is the 3rd rocker. The knee continues to extend and the hip continues to internally rotate therefore increase the stride length of the opposite leg. Due to MPJ dorsiflexion the plantar fascia is pulled tight due to a windlass mechanism which helps to supinate and stabilise the foot ready for toe off, which indicates the end of stance phase.