Final answer:
The knee flexors are primarily active during the swing phase of gait for advancing the leg, and secondary activity occurs in initial contact and mid-stance to absorb impact and prepare for the swing.
Step-by-step explanation:
The knee flexors are primarily active during the swing phase of gait. This is because the swing phase involves moving the leg forward to step ahead and this action requires bending (flexing) the knee. The knee flexors are also active to a lesser extent during the initial contact and mid-stance to modulate the limb's motion and support the body's weight. This process includes a slight flexion of the knee during initial contact to absorb impact and control limb stability, and during mid-stance, the knee begins to flex in preparation for the swing phase.
In the gait cycle, the knee flexors are active during two phases: initial contact and swing phase. During the initial contact phase, the knee flexors help absorb the impact of the foot hitting the ground. During the swing phase, the knee flexors contract to raise the foot off the ground and move it forward.