Exercise 3.4
You can perform these two jumps (Fig. ): Straight-leg jump — keep knees and ankles stiff. Normal jump — bend knees and ankles naturally. How did your ankle, knee and hip positions differ between the two jumps?
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
In the straight-leg jump all three joints stay nearly straight the whole time; in the normal jump all three bend first and then straighten together.
Straight-leg jump. The knee stays locked straight before, during and after take-off, the hip stays extended with the trunk upright, and only the ankle can push. With one small joint doing all the work the jump is low, and on landing the stiff leg has nothing left to bend, so the shock passes straight up through the bones as a hard jolt.
Normal jump — the crouch. The ankle bends forward, the knee bends, and the hip bends so the body leans slightly forward. All three joints are flexed at the same time.
Normal jump — take-off. The three straighten in quick succession — hip, then knee, then ankle — each adding to the push, so the jump is far higher.
Normal jump — landing. The sequence reverses: ankle, knee and hip bend again and absorb the shock gradually, so the landing is soft.
Why the joints behave differently. The knee is a hinge joint and can only bend and straighten in one plane; the hip is a ball and socket joint, so it can also swing sideways and rotate as the body leans.
What actually does the bending. The chapter is explicit that joints allow movement but cannot move the bones on their own — muscles contract and pull on the bones through tendons, and the movement appears at the joint.