Center of pressure trajectory. The shift of the COP occurs in three phases: braking (S1), transitional (S2) and stabilization (S3). The S1 phase represents the braking phase of GT, during which a shift occurs from the penultimate step to the heel of the lead limb as it hits the ground and finally toward the ball of the foot of the leading limb. The transitional phase, S2, occurs as the pressure is transferred fully under the lead limb, while the trailing limb is in swing phase, until the trailing limb strikes the force plate and bilateral stance is established. During the stabilization phase, S3, there is a shift in the COP back to the middle as both feet are planted.