When you lower to a chair, the __ acts in eccentric knee flexion to prevent plopping into the chair. A Rectus femoris B Gastrocnemius C Biceps femoris D Rectus abdominus
The correct answer and explanation is:
The correct answer is C) Biceps femoris.
The biceps femoris is a muscle located in the posterior thigh and is part of the hamstring group, which plays a crucial role in controlling movements involving the knee and hip. When lowering yourself to a chair, the biceps femoris helps control the motion of knee flexion by acting eccentrically.
Eccentric contraction refers to the lengthening of a muscle as it contracts. In the context of sitting down, as the body descends toward the chair, the biceps femoris is actively lengthening while still generating force to control the knee’s flexion and prevent a rapid or uncontrolled descent, known as “plopping.”
This control is important because if the muscle were to contract concentrically (shortening) instead of eccentrically, the knee would buckle, and the movement would be less controlled. Eccentric contractions allow the muscle to resist gravity and manage the load in a smooth and controlled manner. The hamstrings, including the biceps femoris, are key in decelerating the body during activities like sitting down or descending stairs.
While other muscles listed, such as the rectus femoris and gastrocnemius, are involved in knee movement, they are primarily responsible for other types of knee actions. The rectus femoris, for example, is involved in knee extension (not flexion), and the gastrocnemius is more involved in ankle plantarflexion. The rectus abdominis, on the other hand, is a core muscle that stabilizes the trunk but does not have a direct role in knee flexion.
Thus, the biceps femoris is the muscle primarily responsible for controlling eccentric knee flexion during the motion of sitting.