Joint ROM results from a combination of factors which could be classified as internal or external. Internal structures relate to the physical structures of body materials and tissue. External factors are non-structural and include environmental temperature, gender, age, excess fat mass, muscle mass, and restrictions in clothing or equipment.
Internal factors include joint structure/joint mechanics and the connective and soft tissue surrounding the joint. Because muscular actions such as muscular contractions and stretching are controlled by the nervous system, another internal factor can be attributed to the neuromuscular system and how the stretching and tension is managed.
A joint is a location on the body in which two or more bones intersect and interact. For example, the humerus (upper arm) intersects with the radius and ulna (lower arm) at the point of the elbow. The bony formation of each joint structurally limits the ROM. For example, the shoulder joint which is structurally a ball-in-socket joint, can rotate in multiple directions. In other words, it has a wide range of motion. However, the knee joint is a modified hinge joint which is limited to essentially a forward-backward direction of movement. Additionally, excessive fat mass surrounding a joint or even large muscle mass may limit the ROM for a particular joint. Although weight loss could affect amounts of fat mass surrounding a joint, or loss of muscle, joint structure cannot be altered. As a result, little can be done in this area to improve flexibility.
Not only is range of motion related to the joint structure, but flexibility exercises are joint-specific. In other words, you can’t stretch your hamstring and expect your shoulders to improve. Likewise, you can be flexible in your shoulder but very “stiff” in your fingers or ankles. So, a complete stretching program must include multiple stretches for various joints.
Connective and Muscle Tissue
Joints are surrounded and connected by muscles, tendons, ligaments, and skin. For example, the head of the humerus fits into a small cavity to create the shoulder joint. However, those bones can only remain in place as a result of the muscles, tendons, and ligaments that keep the joint tight an in place. In addition, muscle tissue is surrounded with connective tissue, primarily collagen and elastin. As a joint moves through it’s normal range of motion, all of this soft tissue must stretch to accommodate the movement. Therefore, static and dynamic flexibility is probably most limited by the flexibility of the surrounding soft tissue, specifically the connective tissue.
While the exact biomechanics of how flexibility is changed isn’t well understood, it does appear to be related to the elastic and plastic properties of the connective tissue. Elasticity is defined as the ability to return to resting length after passive stretching (i.e. elastic recoil). Like a spring, soft tissues stretch and then recoil to their resting position. Plasticity is the tendency to assume a greater length after passive stretching (i.e. plastic deformation). In other words, taking that stretchy spring and changing the resting position to a new longer length. The goal of a flexibility program, is to repeatedly overload the elastic properties of the muscle to elicit plastic deformation over time. Several studies have suggested that a slow, sustained stretch of 30-90 seconds is necessary to produce chronic plastic deformation.
Modern cars come equipped with a central computer and sensors throughout to troubleshoot problems with the vehicle. Sensors in the engine determine temperature. Sensors on the wheels determine tire pressure while sensors in the gas tank tell you when the gas tank is low in fuel. Much like car, our bodies are equipped with sensors, called proprioceptors, that help us manage movement and prevent injury.
Muscles have two specific types of proprioceptors that determine the length and tension of the muscle. These proprioceptors are called muscle spindles and Golgi tendon organs (GTO’s).
Muscles spindles, lie parallel to the regular muscle and help determine the length of muscles when they are being stretched. When a muscle is stretched, they send signals to the central nervous system causing the stretched muscle to contract. In other words, there is some resistance to the stretch generated by the nervous system’s reflexive stimulus sent to the stretching muscle. This is called the myotatic or stretch reflex. Additionally, that same signal also causes the antagonist (the opposing) muscle to relax, called reciprocal inhibition. So, when you stretch your upper thigh (quadriceps) your hamstrings (antagonist to the quadriceps) are relaxed.
The GTOs are located near the musculotendon junction, the end points of the muscle, and relay messages to the central nervous system regarding muscle lengthening and tension of the muscle. When activated, these signals will override the stretch reflex causing a sudden relaxation of the stretching muscle. This is called autogenic inhibition or the inverse myotatic reflex. This inhibitory reflex can only occur after the muscle has been stretched for 5 seconds or longer. This is why, to effectively stretch, movements must be sustained for long, slow increments of time. Otherwise, the resistance encountered from the stretch reflex will not be overridden and lengthening cannot occur. Whether signaling the muscles to contract or relax, the neuromuscular system manipulates the stretched muscle, presumably as a protective mechanism to prevent injury.