Flexibility is defined as the available range of motion around a joint. Almost everyone can increase their flexibility, as this attribute is highly adaptable to consistent effort. Understanding the science behind these adaptations and the limits imposed by your unique body structure is the pathway to achieving lasting mobility.
The Physiology of Stretching
Flexibility improvements are less about physically lengthening a muscle permanently and more about teaching the nervous system to tolerate a deeper stretch. When a muscle is stretched, sensory receptors called muscle spindles detect the change in length and speed of the stretch. These spindles trigger the protective stretch reflex, which automatically signals the muscle to contract, preventing overstretching and limiting your range of motion.
Consistent and prolonged stretching works by gradually recalibrating the sensitivity of this protective reflex. Over time, the nervous system becomes accustomed to the new, extended muscle length and shifts its threshold, allowing the joint to move further before the reflex is activated. This adaptation is neurological habituation, where the body learns that the new range of motion is safe.
Physical changes also occur within the soft tissues, though they are slower than the neurological adaptations. Regular stretching can cause muscle fibers to adapt by adding tiny contractile units, called sarcomeres, in series, which incrementally increases the muscle’s resting length. Connective tissues like fascia and tendons also become less stiff and more pliable as they respond to the mechanical tension of consistent stretching.
Inherent Limitations on Flexibility
While nearly everyone can become more flexible, the ultimate extent is determined by factors beyond muscle and nerve adaptation. The most absolute limitation is the body’s skeletal structure, which no amount of stretching can alter. For instance, the depth and orientation of the hip socket (acetabulum) dictate how far the thigh bone can move before the bones physically contact each other, creating a hard stop.
If a stretch feels like an abrupt, bony block rather than a gradual, soft tissue pull, it is likely a structural limitation. Genetic factors also play a substantial role in determining a person’s natural propensity for flexibility. The composition and elasticity of connective tissues are heavily influenced by inherited genes, such as COL5A1, which affects collagen production, leading to denser, stiffer fibers and a greater challenge in achieving extreme ranges of motion.
Age and previous injury further complicate the potential for flexibility gains. As a person ages, collagen fibers naturally undergo cross-linking and the tissues lose water content, which reduces elasticity and makes adaptation slower. Scar tissue formed from past injuries is also significantly less elastic than healthy muscle tissue, creating localized restrictions that resist lengthening.
Training Principles for Lasting Flexibility
Achieving lasting flexibility requires a training approach built on consistency and progressive adaptation. Flexibility gains are temporary and must be maintained through regular, frequent practice (ideally daily or at least five times per week). Consistency in practice is more impactful than occasional, intense sessions.
For static stretching to signal the necessary nervous system and tissue adaptations, time under tension is a defining factor. Research suggests that holding a stretch for at least 30 seconds is the minimum threshold required to elicit a significant change in flexibility. Repeating each stretch two to four times, accumulating approximately 60 seconds per muscle group, is an effective strategy for most people.
It is also important to ensure the muscles are warm before beginning any deep stretching, typically by engaging in five to ten minutes of light aerobic activity. The intensity of the stretch should be challenging but never painful, aiming for a safe level of discomfort that allows for deep breathing and relaxation. Finally, like strength training, flexibility requires progressive overload, where the stretch depth or duration is gradually increased over time to continually challenge the tissues and sustain long-term improvements.