7.10 Antagonistic Arrangement of Skeletal Muscles

Skeletal muscles work in opposing pairs to produce controlled movement at joints, using the knee joint as a specific example.

Muscle Attachment to Bone

• Origin: The end of a muscle attached to a bone that remains stationary during contraction.
• Insertion: The end of a muscle attached to a bone that moves when the muscle contracts. The insertion is typically pulled toward the origin.
• Belly: The thick, fleshy, central part of a muscle between the origin and insertion that performs the contraction.

Antagonistic Muscle Arrangement

• Muscles can only pull (contract); they cannot push (extend).
• To allow for movement in two opposite directions (e.g., bending and straightening a limb), muscles are arranged in opposing pairs.
• When one muscle in the pair contracts (the agonist), the other muscle relaxes (the antagonist). This coordinated action allows for smooth and controlled movement.

Types of Muscles in an Antagonistic Pair

• Flexor muscle: A muscle that, upon contraction, bends a joint, decreasing the angle between the bones.
• Extensor muscle: A muscle that, upon contraction, straightens a joint, increasing the angle between the bones.

Movement in the Knee Joint

The knee joint (tibio-femoral joint) is a complex hinge joint between the femur (thigh bone) and the tibia (shin bone). Its primary movements are flexion and extension.

Details of Knee Muscles

• Biceps Femoris (Hamstring):
  • Origin: Pelvic girdle and the top of the femur.
  • Insertion: Upper parts of the tibia and fibula.
• Quadriceps Femoris (Extensor):
  • Origin: Ilium (part of the pelvis) and femur.
  • Insertion: Tibia, via a common tendon that surrounds the patella (kneecap).