The connection between flexibility and muscle length

In fitness and sports medicine, many people mistakenly believe that improving flexibility – moving a joint through a wider range of motion – doesn’t mean that muscles get longer. This idea, though common, oversimplifies the complex connection between how much a muscle can stretch and flexibility. To fully understand what's happening, we need to examine the basics of how muscles stretch and recognise that this process does, in fact, affect muscle length.

Flexibility is usually defined as a joint's ability to move through a range of motion, which cannot be separated from how much the muscles around the joint can stretch. When we talk about muscle extensibility, we mean the muscle’s capacity to lengthen when a force is applied. So, when discussing flexibility, we are also talking about how far a muscle can stretch before reaching its current limit. Therefore, increasing flexibility is directly related to increasing muscle extensibility, which means the muscle can stretch further, effectively becoming longer.

Let's look at the hamstrings as an example. These muscles are crucial for movements like extending the hip and bending the knee. If someone has tight hamstrings, they might only be able to bend their hip to a certain point before feeling resistance, which limits their movement. This happens because the muscle can't stretch any further at that point. However, by doing specific stretching exercises to improve flexibility, the hamstrings can stretch more before hitting that resistance. This shows that increasing flexibility does indeed lead to the muscle being able to stretch to a longer length.

How to quantify change in muscle length

To measure changes in how long a muscle is, we use the idea of muscle strain. Muscle strain tells us how much the muscle has changed compared to its original length. It is a way to measure how much the muscle has stretched or shortened. The formula to find muscle strain (ε) is:

Muscle Strain(ε)= ΔL/L0

ΔL represents the change in muscle length, and L0 is the muscle's original length. This formula reveals that any change in muscle length (ΔL) resulting from stretching manifests as muscle strain. When a muscle is regularly stretched, its extensibility improves, leading to a more significant change in length (ΔL) and, consequently, a higher strain value. Over time, this repeated application of strain through stretching causes a semi-permanent increase in muscle length as the tissue adapts.

Why are changes in flexibility only semi-permanent?

Changes in flexibility are semi-permanent, meaning they don't last forever because they need regular effort to maintain. When you stretch regularly, your muscles lengthen, and your connective tissues become more elastic, which helps improve flexibility. You will stay flexible for as long as you continue doing the things that made you flexible in the first place. (The good news is that far less effort is required to maintain flexibility than to develop it.) However, if you stop stretching, these tissues return to their original state, reducing flexibility. This return to the original state is known as reversibility.

The concept of reversibility can be explained by a few key factors. One is how the body naturally adjusts at the biological level. Muscles and tissues in the body are highly responsive to what we ask, changing both structure and function to suit activities like stretching. But when regular stretching is stopped, the body stops maintaining these changes, causing a loss of flexibility.

Second, connective tissues, which have a lot of collagen, are constantly renewed. If you don't stretch regularly, the new collagen tightens up, making the tissues less flexible.

In the end, changes happen in the nervous system. Flexibility isn't just about the muscles but also involves the nerves that control them. With regular stretching, the nervous system adjusts by lowering the reflexes that usually stop muscles from stretching too far. If you discontinue stretching, these changes in the nervous system can disappear, leading to less flexibility. It's important to include stretching in a regular exercise plan to keep the flexibility we've gained.

Physiological basis for increased muscle length

As previously noted, muscles adapt to stretching through several bodily processes. A major factor is the restructuring of sarcomeres, the tiny units responsible for muscle contraction. When stretching occurs over a short period, it can temporarily lengthen muscle fibres by stretching and reorganising the existing sarcomeres. However, regular stretching encourages the muscle fibres to add more sarcomeres in sequence, a process called sarcomerogenesis. This growth increases the overall length of the muscle, enabling it to stretch further before reaching its maximum, thereby improving flexibility [1-3].

Stretching also affects the muscle's connective tissues: the endomysium, perimysium, and epimysium. These tissues are somewhat elastic and help the muscle-tendon unit stretch. Regular stretching decreases muscle stiffness by encouraging the connective tissue to remodel its collagen. This process involves breaking down and rebuilding collagen fibres due to the stress of stretching. Over time, this leads to collagen reorganisation, making the tissues more flexible and less stiff. Stretching might also encourage the production of more flexible collagen types, further reducing stiffness. As a result, the muscle can stretch farther before resistance is felt. This decrease in stiffness is often experienced as an increase in the muscle’s ability to lengthen, linking better flexibility with longer muscle length.

Neural adaptations to stretching

In addition to physical changes in the muscles, adjustments in the nervous system are likely the most essential factor in improving flexibility. The nervous system controls how tight muscles are and how far they can stretch by using reflexes and feedback from the body. At first, when a muscle is stretched, the nervous system might react to protect the muscle from overstretching, which is often felt as tightness. However, with regular stretching, the point at which this protective response happens is pushed further along the range of motion, allowing the muscle to stretch more without tightening up. These changes in the nervous system work alongside the physical changes in the muscle, helping to increase overall flexibility and muscle length.

Regular stretching can significantly affect how the brain senses pain by changing the pathways that control pain perception. When you stretch, sensors in your muscles and joints send signals to the brain that help reduce the central nervous system's sensitivity to pain. This process, called "pain gating," allows the brain to focus more on the feeling of stretching rather than on pain signals. Over time, as you keep stretching, it causes changes in the brain that make it less sensitive to pain, reducing discomfort and allowing your muscles to stretch further. These brain changes also explain why regular stretching can help ease chronic pain by increasing your overall tolerance to pain. An important point to remember here is that even if the nervous system was the only reason we get more flexible with stretching – and it isn't – muscle length still increases as flexibility improves.

Key takeaways

Research shows that regular stretching makes muscles more flexible by changing their structure. But even without these changes, it's logical that improving flexibility means making muscles more stretchable. This, by definition, means the muscles must be able to stretch further, which naturally leads to an increase in muscle length. When we understand how muscle strain measures this change and consider the physical changes in both the muscle and its connective tissues, it's clear that flexibility training does make muscles longer. Therefore, when discussing improving flexibility, we're talking about the muscle's ability to stretch and lengthen, disproving the idea that flexibility doesn't affect muscle length.

References

1. Zöllner, A. et al. (2012) 'Stretching skeletal muscle: Chronic muscle lengthening through sarcomerogenesis.' PLoS ONE, volume 7, number 10, article e45661.
2. Simpson, C. et al. (2017) 'Stretch training induces unequal adaptation in muscle fascicles and thickness in medial and lateral gastrocnemii.' Scandinavian Journal of Medicine & Science in Sports, volume 27, number 12, pages 1597 - 1604.
3. Lecharte, T. et al. (2020) 'Effect of chronic stretching interventions on mechanical properties of muscles in patients with stroke: A systematic review.' Annals of Physical and Rehabilitation Medicine, volume 63, number 3, pages 222-229.
4. Marshall, P. et al. (2011) 'A randomised controlled trial for the effect of passive stretching on measures of hamstring extensibility, passive stiffness, strength, and stretch tolerance.' Journal of Science and Medicine in Sport, volume 14, number 6, pages 535-540.
5. Nakamura, M. et al. (2020) 'Effects of static stretching programs performed at different volume-equated weekly frequencies on passive properties of muscle-tendon unit. Journal of biomechanics, volume 103, article 109670.
6. Takeuchi, K. et al. (2023) 'Long‐term static stretching can decrease muscle stiffness: A systematic review and meta‐analysis.' Scandinavian Journal of Medicine & Science in Sports, volume 33, number 8, pages 1294-1306.