Final answer:
Knee extensors are active during the Initial Contact, Mid Stance, and Terminal Stance phases of gait to support and stabilize the knee, but they are less active during the Pre-Swing phase.
Step-by-step explanation:
Knee extensors are muscles that help straighten the leg at the knee. These muscles are particularly active during certain phases of the gait cycle to support the body's weight and provide stability. The phases of gait where knee extensors are notably active include Initial Contact, Mid Stance, and Terminal Stance. During Initial Contact, the knee extensors are active to prevent the knee from collapsing as the foot hits the ground. Mid Stance is when the body's weight is directly over the supporting foot, and knee extensors work to keep the knee stable and extended as it supports the body's weight. The action of knee extensors during Terminal Stance helps prepare the leg for the push-off phase, facilitating a smooth transition to the next step.
In contrast, during the Pre-Swing phase, which helps to propel the limb forward, the role of knee extensors is less significant, as this phase mainly involves the knee flexors to initiate the leg's forward motion for the swing phase.
An understanding of muscle activity during the gait cycle is important for fields such as physiotherapy, biomechanics, and sports science to assess movement and devise rehabilitation or training programs accordingly.