How to improve flexibility: the science of stretching

Flexibility is not simply a matter of tight or loose muscles. It involves the nervous system, connective tissue, and specific brain structures that determine how far a muscle can lengthen before the body activates a protective mechanism. Andrew Huberman, neuroscientist at Stanford University, explains the science behind stretching and which practices produce lasting improvements in range of motion.

The nervous system controls flexibility

Within the spinal cord, two types of neurons are central to movement and flexibility:

Motor neurons: send signals that contract muscles by releasing acetylcholine.
Sensory neurons (muscle spindles): detect the stretch of muscle fibers and send signals back to the spinal cord. If a muscle is elongating too much, they trigger contraction to bring the limb back into a safe range of motion.

At the end of each muscle, tendons contain Golgi tendon organs (GTOs), which detect load on the muscle. When load is excessive, they inhibit motor neuron activation as a safety mechanism to prevent injury.

The role of the insula and von Economo neurons

Within the brain, the insular cortex integrates information about the body's internal state, including pain, discomfort, and the position of limbs. Within the posterior insula, von Economo neurons are exceptionally large neurons, likely unique to humans, that allow us to interpret pain and make conscious decisions about whether to continue despite it.

These neurons are the neural substrate of "relaxing into a stretch." They can modulate the response of muscle spindles and reduce the perception of discomfort when the brain understands that the stretch is safe and intentional.

Types of stretching

There are four main categories:

Dynamic: controlled movement through the range of motion, with minimal momentum.
Ballistic: movement that uses momentum or swinging, especially at the end range.
Static: holding the position at the end range without momentum.
PNF (proprioceptive neuromuscular facilitation): a combination of muscle contraction and relaxation with or without assistance, using straps, weights, or a training partner.

Static stretching is the most effective for long-term gains

A review of the scientific literature on stretching typology found that all methods produce improvements, but static stretching generates significantly greater gains in range of motion over a long-term period compared to ballistic or PNF protocols.

The key parameter is total weekly time: at least 5 minutes per week per muscle group are needed to produce meaningful changes. This translates to 3 sets of 30-second holds, 5 days per week per target muscle. This protocol is most effective when performed after a warm-up or at the end of a training session.

Recommended protocol for hamstrings:

3 static stretch sets of 30 seconds each
Brief rest between sets
5 days per week

Intensity matters: less hurts more

A study comparing two groups of dancers over 6 weeks found:

Low-intensity group (micro-stretching at 30-40% of the pain threshold): 60-second holds at very low intensity, with no discomfort.
Moderate-intensity group (80% of the pain threshold): same exercises and duration, but at much higher intensity.

The low-intensity group achieved greater improvements in active range of motion. This is counterintuitive but important: you do not need to push to the point of pain to gain flexibility. A relaxed, sustained stretch at 30-40% of your pain threshold is more effective than an intense one.

Static stretching before training may reduce performance

Evidence suggests that static stretching before strength or cardiovascular training can temporarily reduce performance in those activities. The general recommendation:

Perform static stretching after your training session.
If stretching before training, warm up first with 5 to 10 minutes of light cardio or calisthenics.
Dynamic stretching before training can be useful to prepare neural circuits, joints, and connective tissue.

Yoga and structural brain changes

A study published in Cerebral Cortex showed that yoga practitioners have double the pain tolerance compared to non-practitioners. They also show greater gray matter volume in the insula, reflecting an expanded capacity to interpret and manage pain from within.

Yoga trains not just muscular flexibility but the nervous system's relationship to discomfort, with benefits that extend beyond the yoga mat.

Summary protocol

Type: low-intensity static stretching (30-40% of pain threshold).
Duration per set: 30 to 60 seconds.
Sets per muscle: 3 per session.
Frequency: 5 to 7 days per week.
Optimal timing: after training or following a warm-up.
Weekly minimum: 5 minutes per muscle group.

With consistency, this protocol is sufficient to offset age-related flexibility loss and improve range of motion durably.