extensors / extensor muscles
A flexor is a muscle that causes a joint to bend. In contrast, an extension is a muscle that straightens a joint.
latissimus dorsi
When a muscle contracts it causes flexion and when muscles relax they cause extension
Contractions of the gluteus maximus muscle causes (hyper)extension of the thigh.
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 latissimus dorsi muscle is responsible for adducting the shoulder and causing extension of the shoulder joint. It is a large triangular muscle located in the back that helps with various movements of the shoulder and arm.
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.