Long jump: faster approach, worse jump?
A 16-year-old female long jumper has a personal best of 4.85 m from a 32 m approach. Her coach lengthens the approach to 38 m to raise horizontal velocity. In the next three competitions she fouls five of six jumps and her best legal mark falls to 4.62 m. Timing gates show her final 5 m velocity on the runway is 7.8 m·s−1, but her horizontal velocity at take-off is 7.2 m·s−1 and her take-off angle is 27°, compared with 7.5 m·s−1 and 21° from the shorter approach. Her penultimate stride is visibly longer and her hips drop before take-off. On landing she lands with the knee falling inward and complains of vague knee pain. She is 17 months post-menarche and has recently grown 6 cm. The coach asks whether to keep the longer approach and add more plyometrics.
1. Using impulse–momentum and projectile principles, explain why the longer approach did not increase jump distance. Refer to the numbers.
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Jump distance depends on take-off velocity, take-off angle, height of the centre of mass at take-off and landing, and air resistance. A longer approach can raise runway velocity, but the athlete must convert horizontal momentum into resultant momentum during a very short ground contact. By impulse–momentum, the resultant impulse from the take-off leg changes momentum; braking forces reduce horizontal momentum.
The data show runway velocity 7.8 m·s−1 but take-off horizontal velocity only 7.2 m·s−1, so she lost 0.6 m·s−1 before take-off. With the shorter approach she had 7.5 m·s−1 horizontal at 21°; resultant ≈ 8.0 m·s−1. With the longer approach, 7.2 m·s−1 at 27° gives resultant ≈ 8.1 m·s−1 but a much larger vertical component. The higher angle increases vertical velocity but reduces horizontal velocity: 7.2 cos 27° ≈ 6.4 m·s−1 versus 7.5 cos 21° ≈ 7.0 m·s−1.
The overlong penultimate stride and lowered hips increase braking and ground-contact time, so the extra runway speed is not transferred. The optimum angle is not fixed; it falls as approach speed rises, so 27° is likely too steep for her.
2. Advise on the penultimate stride, check-mark and take-off mechanics. Give two coaching cues and one drill.
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The penultimate stride should be shorter and faster, with the foot landing closer under the hips rather than reaching forward. This reduces braking and keeps horizontal velocity high while lowering the centre of mass slightly in preparation. The check mark should be adjusted so the take-off foot lands under the body; if she is over-striding, move the check mark back or shorten the approach until rhythm is restored.
Take-off: drive the free knee and swing the arms, extend the take-off leg fully, and maintain a tall posture. Cues: 'fast, fast, tall' for the last two strides; 'step down and punch' for the penultimate-to-take-off action.
Drill: 5-stride approach jumps with timing gates and a coach checking penultimate foot placement, plus wicket runs to set stride rhythm. Increase approach length only when she can maintain horizontal velocity and a consistent check mark.
3. Evaluate the landing knee valgus and the coach's plan to add plyometrics. What would you do before increasing plyometric load?
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Landing with the knee falling inward is dynamic knee valgus. It increases valgus loading and ACL strain, especially in a growing athlete with recent menarche and a 6 cm growth spurt, when coordination and strength may lag behind limb length. Vague knee pain needs screening before loading.
Before adding plyometrics, assess landing mechanics from a drop jump, single-leg squat and hop, and test hip abductor and external rotator strength, ankle dorsiflexion, quadriceps and hamstring strength. Teach a soft landing with the knee over the second toe, hips back and no inward collapse. Strengthen gluteus medius, hamstrings, calf and trunk; use low-level plyometrics such as pogo and low box drops only after good control. Keep volume low and monitor pain.
The coach should not simply add more plyometrics to compensate for a longer approach; technique and landing control come first.