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sion in this case. There are several biome-chanical reasons why it is likely more beneficial to work on shot trajectory than increasing range of motion. First, using the desirable trajectory increases the angle of entry and the probability of a made shot. Second, this slightly higher trajectory requires less ball speed than a very flat one. Third, the young player is likely to increase her strength while the desirable trajectory will remain the same. The interaction of biomechanics and performer characteristics suggests to the teacher that subsequent practice should focus on a slightly higher shot trajectory.

EXERCISE/ACTIVITY PRESCRIPTION

Another important content area of physical education is fitness. Physical educators planning to increase student physical fitness must employ biomechanical knowledge to determine the most effective exercises for various parts of the body and fitness components. Like strength and conditioning professionals, physical educators qualitatively analyze exercise technique to be sure that students are safely training their bodies.

Figure 9.4. An elementary student shooting a free throw at an 8-foot-high hoop. Time between images is 0.07 s.

During the first week of your high school weight-training unit, you notice many students performing their curl-up exercises like the student depicted in Figure 9.5. You want to immediately provide some group feedback to help many students with this exercise technique and reinforce some of the technique points you made earlier. Make a list of the critical features or technique points that are important in the curl-up exercise. What biomechanical principles are most related to the objectives of doing curl-ups for health-related fitness (muscular endurance)? Which of the biomechanical principle(s) seem to be weakly applied in the concentric phase of the curl-up for the student shown in Figure 9.5?

The purpose of curl-up exercises is to focus a conditioning stimulus on the abdominal muscles by limiting the contribution of hip flexors and other muscles. The biomechanical principles that are important in this objective are Force-Motion, Range of Motion, Inertia, and Force-Time. The inertia of the body provides the resistance for the exercise, and the range of motion for the exercise should focus the stress (force-motion) on the abdominal muscles. The repetitions should be slow and controlled (ForceTime) for safety and to promote training for muscular endurance.

The student in Figure 9.5 has several weaknesses in his curl-up technique. He uses too much range of motion, performing more of a sit-up (hip flexion) than a trunk curl. In a curl-up exercise, the abdominal muscles should raise the shoulders to about a 30 to 40° angle with the hip (Knudson, 1996), just lifting the shoulder blades off the ground. Hip flexion is required if the shoulders are to be raised further. The student also decreases the resistance or inertia by

Figure 9.5. Concentric phase technique of a curl-up for a high school student. Time between images is 0.17 s.

keeping the weight of the arms close to the transverse axis of rotation for trunk flexion. The third weakness is in stabilizing his feet with the weight bench. This affects both the Force-Motion Principle and the Principle of Inertia. By stabilizing the feet with the bench, the performer has essentially unlimited inertia for the lower extremities. This allows hip flexor activation to contribute to trunk flexion through the kinematic chain of the lower extremity, so the Force-Motion Principle is not applied well for the training objective of isolating the abdominal muscles. Performing the curl-up without foot stabilization would require greater abdominal activation and stabilization to lift the trunk without hip flexors. The time information in the caption for Figure 9.5 suggests that the student was applying the Force-Time Principle well; in other words, he did not perform the exercise too fast.

The best intervention in this situation is to provide group intervention, reminding all students to perform curl-ups without lower-extremity stabilization. This exercise may feel more difficult, but the teacher can use this opportunity to reinforce the idea that the students are training and teaching their abdominal muscles an important trunk-stabilizing task. Focusing on using more abdominal muscles for a longer time (Force-Time Principle) better simulates the nearly isometric actions of the muscles in stabilizing the trunk and pelvis. There is a large body of physical therapy literature focused on training specific abdominal muscles so as to stabilize the trunk (McGill, 1998; Vezina & Hubley-Kozey, 2000). The teacher could then provide some individualized intervention for the student. One good strategy would be to compliment (reinforce) the good exercise cadence, but challenge the student to place his hands on top of his head and keep the arms back to increase the resistance for the exercise.

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