During Absolute Speed Figure 4, when the front leg lands and bears weight, knee flexion is about how many degrees for the length-tension relationship?

• A. 0 degrees
• B. 10 degrees
• C. 20 degrees
• D. 40 degrees

Answer: (C)

The length-tension relationship explains why muscle force depends on how stretched or shortened the muscle is at a given joint angle. In this landing phase, the knee extensors (the quadriceps) must eccentrically control knee flexion to absorb impact and keep the knee stable for the next movement. About 20 degrees of knee flexion places the quadriceps at a length where they can generate a strong, effective force to decelerate the leg and manage load, while still allowing the knee to extend quickly for propulsion. If the knee is nearly straight, the muscle is at a shorter length and its force potential is reduced; if the knee is more bent (around 40 degrees), the muscle lengthening moves past the optimal range and force production drops as well. So, 20 degrees hits the balance to maximize absorptive capacity and readiness to drive off.