Final answer:
Leaning the trunk backward while using a weight machine makes it easier to extend the knee joint by shifting the center of gravity backward, increasing stability and reducing the force required.
Step-by-step explanation:
When leaning the trunk backward, it becomes easier to extend the knee joint in a weight machine because this position shifts the center of gravity backward, increasing stability and reducing the amount of force required to extend the knee.
When the center of gravity is shifted backward, it creates a larger base of support, which improves balance and stability. Additionally, leaning the trunk backward helps to align the axis of the hips, reducing strain on the knee joint during extension.