Erector Spinae
An agonist muscle is a muscle that plays a part in the extension of a muscle. The agonist muscles for a knee extension are the quadriceps and hamstrings.
The over-extension of a muscle might feel like burning or pain in the muscle. After the exercise, the muscle can feel sore or even swollen.
Inn other words muscle flexing can be called muscle shortening and extension when muscle is elongating. For example when you bend you arm towards your shoulder your bicep muscle is shortening and you can feel a bump. When you extend your arm you can feel that the bump is stretching out and disappearing, that is muscle extension.
Quadriceps muscle or thigh muscle.
The primary function of the knee extension muscle is to straighten the leg by extending the knee joint.
Flexion and extension.
The leg extension exercise primarily targets the quadriceps muscle group in the front of the thigh.
The supinator muscle is a muscle in the forearm that is not directly responsible for flexion or extension of the arm. Its primary function is to rotate the forearm in a movement called supination, where the palm faces up.
Muscle extension is when a muscle lengthens, while flexion is when a muscle shortens. Extension helps with stretching and increasing flexibility, while flexion helps with bending and contracting. Both movements are important for maintaining range of motion and strength in the body.
This muscle is called the extensor muscle. There are specific names for different joints.
Flexor carpi
Erector Spinae