Difference Between Stretching & Flexing a Muscle

The human body contains three main types of muscles: cardiac, smooth and skeletal. The first two types of muscles are involuntary, meaning that you cannot control their contractions. You control the skeletal muscles, the muscles responsible for moving your body voluntarily. When your brain sends a signal down the spinal cord to the muscle, microscopic structures within the muscle respond. Your approximately 640 skeletal muscles can stretch or flex in response to your brain’s signals.

A muscle consists of numerous muscle fibers that have been bundled together to form fascicles. Each of the muscle fibers contains tiny thread-like structures called myofybrils. Within each of the myofybrils are sarcomeres. The sarcormeres have myofilaments, thick and thin filaments that overlap. The degree of overlap between the filaments changes in response to signals from your brain and causes the myofybrils to contract or elongate.

When a muscle contracts, the ends of adjacent myofilaments in the sarcomeres slide closer together, increasing the amount of overlap between the filaments. This type of sliding action occurs with every type of skeletal muscle contraction, even those that do not produce a joint movement. The informal term “flexing,” however, typically refers to a specific type of contraction: concentric contractions. In this type of contraction, the entire muscle decreases in length as it produces movement at a joint. For example, imagine that you want to bring a coffee cup to your mouth. Your brain sends a signal to the biceps brachii -- one of the muscles in your upper arm -- to contract. This muscle bends the elbow, bringing your coffee cup toward your mouth. This action is accomplished by the myofilaments sliding together.

When you stretch a muscle, the opposite occurs. The myofilaments in the sarcomeres slide apart. This decreases the amount of overlap between adjacent myofilaments. When you stretch, however, you are not only stretching muscles. Once the sarcomeres have been fully stretched, your connective tissue, such as tendons and ligaments, begin to stretch to produce the movement.

Many people tend to think about muscles either stretching or contracting during movement. In some cases, the muscles do a combination of the two. For example, when you stretch a muscle, your brain may send a signal to the muscle to contract. This phenomenon, known as the stretch reflex, helps protect your muscles from damage. As another example, in one type of muscle contraction, eccentric contractions, the muscle actually lengthens. This type of contraction slows down the movement, typically by resisting the force of gravity, and helps protect joints and muscles from damage.