Stretching serves as a beneficial tool to minimize injury through steady muscle contraction. However, the muscle tissue does not become shortened in most stretches. Instead, contraction in muscle fiber generally means you apply a force that results in some sort of movement.
Stretching and Muscle Contraction
Rather than shortening, like the definition of contraction generally describes, muscle tissue and many fibers passively lengthen during most stretches. The term passive contraction simply means that you do not actually place force or act directly on the muscles that receive a stretch, but rather they remain in a steady lengthening state because of your body position.
An example of a body position that results in a passively contracting stretch occurs when you sit on the floor and reach out to touch both feet. Rather than the arms, fingers and hands, the body parts that perform the action (receiving a thorough stretch), the hamstrings and glute muscles, feel a lengthening sensation. This example proves that even though the muscles that receive the stretch do not require direct action, we know how to target muscles during stretches through passive muscle contraction.
Contraction Types – Shortening
Some types of stretches actually do make the muscle fiber contract in a shortening manner. When you flip your hands palms up and attempt to touch the top of each shoulder, the bicep muscles become shorter. This compacting motion often does not provide a literal stretch, though the muscle contraction loosens the fiber and tissue, allowing more oxygen to travel to those areas.
Concentric muscle contraction also occurs when the body folds at the waist, with legs fully extended. Although you will probably feel this stretch mostly in the hamstrings and lower back, the abdominal muscles and the core actually shorten and literally contract so that the muscle fibers become temporarily shorter. Other abdominal stretches such as sitting knee tucks temporarily contract the muscle group and then lengthen the fibers too.
Contraction Types – Lengthening
Other stretches, like lightly weighted squats, involve eccentric or muscle contractions that lengthen muscle fiber. When you hold a dumbbell between both legs with arms fully extended, the muscles in both arms continually stretch to a maximum distance. In the same position, if you continue to squat and then spring back up, the leg muscles will also feel a stretching sensation because an eccentric contraction occurs.
When the knees become bent, as in running and walking, the quadriceps lengthen and then return to normal length once the leg becomes fully extended once again. Repeating this process in sprints and squats plays a large role in the burn that you feel and the soreness that occurs in the days that follow some workouts.
Contraction Types – Isometric
You have probably experienced the struggle of holding a heavy bag off of the ground with one arm, such as during grocery shopping or at school. Regardless of the objects weight, suspending anything off of the ground in a fixed position requires muscles to hold at a fixed length. This provides the fibers with an isometric muscle contraction. Some stretches that implement this technique include holding partial pull-up or partial push-up positions.