What role does genetics play in flexibility?

Introduction

Flexibility is an essential element of physical fitness, ranking alongside other vital components like cardiorespiratory endurance and muscle strength. There is a common belief that flexibility may influence the likelihood of muscle injuries and improve athletic performance. Yet, there is no unanimous agreement on the significance of flexibility in sports where an extensive range of motion (ROM) isn't critical to success. This ambiguity might stem, at least in part, from the absence of universally accepted definitions and ways of measuring flexibility, coupled with a broad lack of scientific insight into the genetic factors that determine flexibility. This article outlines the basic ideas surrounding flexibility and how researchers measure it in human studies. It focuses on genetics, which is considered a significant factor influencing flexibility.

Flexibility, Hypermobility, Laxity

Flexibility can be sorted into four types - click here for more information - but for ease of understanding, in this article I will categorise it into two main types: static flexibility and dynamic flexibility. Static flexibility refers to the ROM in a joint or several joints, focusing not on speed but the extent of movement. Generally, to measure static flexibility, the assessor asks the subject to relax as much as possible. Several methods exist to evaluate this type of flexibility, with standard tests including the toe-touch and the sit-and-reach tests (though these tests involve active muscle contraction in the trunk). Another standard test is the unilateral passive straight-leg-raise.

On the other hand, dynamic flexibility is the ability to move a joint or multiple joints during physical activities at regular or high speeds [1]. A related concept is ballistic flexibility - technically a sub-type of dynamic flexibility - which involves bouncing and rhythmic movements; however, tests for dynamic flexibility do not always require such fast-paced or bouncing movements. Dynamic flexibility is complex because it involves various factors, such as muscle strength and coordination, as well as an individual's static flexibility. Therefore, interpreting the results of dynamic flexibility tests demands careful consideration.

Joint hypermobility and joint laxity are concepts related to how flexible a person is, but they don't mean the same thing. Flexibility is about how stretchable specific tissues are and how far joints can move normally or as they should physiologically. On the other hand, joint laxity concerns the stability of a joint and mainly has to do with the joint capsule and ligaments. A joint can be overly lax due to long-term injuries or conditions present from birth or passed down through families. Simply put, flexibility refers to how much a joint can move normally, whereas joint laxity is how much a joint moves in ways it shouldn't.

The term joint hypermobility specifically describes when the movement of a joint exceeds what is typically seen. It can be caused by abnormally extensible connective tissues or bone shapes that don’t resist movement as much as we would see in the average person. Hypermobility is common in healthy individuals who don't experience any problems; it is estimated to affect between 10% and 15% of the population, with notable differences across different races, genders, and ages, and it happens three times more often in women than in men [2-4]. Hypermobility isn’t considered a medical issue and might even be beneficial in sports that require a lot of flexibility, like artistic and rhythmic gymnastics, karate, and figure skating. However, as will be discussed later, hypermobility can turn into a medical issue if it starts to cause symptoms such as pain and instability, which are significant concerns [5].

Factors Influencing Flexibility

The ROM in a joint or multiple joints is affected by the limits of what the person testing or being tested can feel (sensory perception) and the physical (viscoelastic) reactions of the whole muscle-joint system. While the feelings of the person conducting the test or the subject are commonly used to determine ROM in studies of human flexibility, there is no explicit agreement on which specific feeling relates best to muscle stiffness. This is due to the variety of sensations people can experience, such as resistance, discomfort, and pain. Even though we measure joint flexibility by looking at both the force applied (torque) and the angle of the joint, the stiffness of the joint at its maximum range, which varies from person to person, is shaped by how sensation impacts it. Therefore, it would be better to measure joint stiffness at a specific joint angle that is the same for everyone being tested and, ideally, stretch the muscle group to a tense but pain-free state.

Generally, it is thought that flexibility is a common feature that is the same across the whole body. However, studies suggest that this isn't the case. Flexibility is specific to each joint and the movements it can make [1]. For instance, just because someone has good ROM in their shoulder doesn't mean they will have the same in their hip. Likewise, having good ROM in one hip doesn't guarantee the same range in the other hip. This means measuring the ROM in one joint doesn't work as a reliable way to guess the ROM in different parts of the body. Even though these differences might be caused by the unique physical stresses placed on the connective tissues of a particular joint, we still don't fully understand how this happens.

Tendon extensibility or stiffness might be thought to play a significant role in determining joint flexibility. Yet, the change in tendon length when stretched passively is less than when muscles contract maximally. Tendons are notably stiffer than muscles [6]. Therefore, the overall stiffness of the muscle-tendon unit largely depends on muscle stiffness. Given this, tendon tissues don't significantly affect joint flexibility, including ROM and stiffness. From a mechanical standpoint, it makes sense to think that the stiffest muscle mainly affects ROM and joint stiffness, among other factors like sensation.

Animal studies have shown that type I muscle fibres are generally stiffer than type II fibres [7,8]. This stiffness might be due to variations in the type of titin, a protein in the fibres, or differences in the amount of collagen. Collagen is a key component in intramuscular connective tissues, including the epimysium, perimysium, and endomysium [9]. Notably, the perimysium is a major factor contributing to muscle stiffness [9,10]. These elements are crucial to consider during studies examining how genetics might influence physical traits like flexibility.

Genetic Influences and Flexibility

Flexibility and joint hypermobility differ from person to person and are strongly inherited traits, with genetic factors being about 50% responsible [11,12]. Supporting this, a study looked at 483 identical and 472 non-identical female twin pairs and found a significant genetic influence on self-reported joint hypermobility [12]. Additionally, another study involving 300 identical and non-identical male twins found that genetics accounted for 47% of the differences in how much their lower back could bend [13]. Specifically, the ability to bend forward was determined mainly by genetics (64%), whereas bending backward was more influenced by environmental and behavioural factors [13].

Moreover, three further studies involving identical and non-identical twins, children, and young adults showed that between 18% and 55% of the differences in how far people can reach in a sitting position could be explained by genetics [11,14,15]. Collectively, these studies confirm the impact of genetic factors on how flexible individuals are. To date, only five genetic markers linked to flexibility have been found, and further studies are needed to confirm these findings. The next section of this article gives an overview of the most researched genetic markers related to flexibility.

Collagen Mutations and Flexibility

Mutations in the collagen, type V, alpha-1 gene (COL5A1) are known to lead to classic Ehlers-Danlos syndrome (EDS). This syndrome is noted for joint hypermobility and other symptoms related to connective tissue [16]. Research has also explored how a specific area within the COL5A1 gene, known as the COL5A10 untranslated region (rs12722), affects ROM. This area has been the subject of multiple studies. It is currently considered the most thoroughly researched example of how genetic differences influence flexibility traits in genetics research [17,18].

The gene COL5A1 is responsible for producing the α1 (V) chain of type V, an essential structural protein found in the extracellular matrix (ECM) [19]. This protein is necessary for regulating the size and shape of other major fibrillar collagens that support tendons, ligaments, and muscles [20]. Research has shown that a common change from a C to a T in a single nucleotide within the rs12722 polymorphism might affect the stability of COL5A1 mRNA. This can lead to a decrease in the production of type V collagen [21]. Specifically, it was suggested that people with the TT genotype of the COL5A1 gene are likely to produce more type V collagen [19]. This increase in collagen could result in less flexible tendons, reducing the ROM in joints and raising the risk of developing soft-tissue injuries [22-26].

The first research linking the COL5A1 rs12722 (T to C) gene change with flexibility in muscles and tendons was carried out in 2009 [22]. They discovered that individuals with the CT version of this gene in their lower limbs were not as flexible as those with either the TT or CC versions. This led them to conclude that the COL5A1 rs12722 gene variation directly relates to how flexible people are when measured. Another study later on, involving a larger group of healthy, active Caucasians, indicated that having the CC version of the COL5A1 rs12722 gene helps guard against the flexibility loss that often comes with ageing, specifically in the sit-and-reach flexibility test [17]. This protective effect of the CC genotype was notably significant in older participants, unlike in younger ones.

Furthermore, recent research found no link between the COL5A1 rs12722 gene variation and the sit-and-reach flexibility in young Brazilians [27]. However, the same study observed that the CC version of the gene was linked to better performance in the straight-leg-raise flexibility test among young Koreans and Japanese [28]. This study also suggested that those with the CC gene variant tend to have a more flexible body overall. Additionally, the CT version of the COL5A1 rs12722 gene is associated with greater joint flexibility, more instances of knee hyperextension, and a higher rate of injuries among elite Italian rhythmic gymnasts, which further indicates this gene's role in joint flexibility [29].

Research has further explored how the COL5A1 rs12722 gene variation affects overall joint flexibility, ROM, and the likelihood of injuries to the anterior cruciate ligament (ACL) [43]. They found significant links in women, particularly showing that those with the CT version of the gene had more knee hyperextension and overall greater joint flexibility than those with the CC or TT versions.

Compared to earlier studies, later research showed no link between the COL5A1 rs12722 polymorphism and flexibility when measured using the sit-and-reach test and passive straight-leg-raise methods in Korean participants [31]. Overall, the studies reviewed support the influence of the COL5A1 rs12722 polymorphism on the variations in ROM among individuals.

Lastly, additional gene polymorphisms related to collagen (COL1A1 rs1800012, COL3A1 rs1800255, COL6A1 rs35796750, COL12A1 rs970547, and rs240736) have been examined in two separate studies [32,43]. The studies found that the COL1A1 rs1800012 and COL12A1 rs970547 polymorphisms were linked to joint laxity in 124 people who engage in recreational activities [43]. However, the COL3A1 rs1800255, the COL6A1 rs35796750, and the COL12A1 rs970547 gene variants showed no significant connection with flexibility in sit-and-reach, straight-leg-raise, or total shoulder rotation ROM in a group of 350 healthy, physically active Caucasians [32].

Genetics and Muscle Stiffness

Muscle stiffness significantly affects how flexible our joints are, and there are differences between the sexes; women generally have less muscle stiffness than men [33-35]. Research has shown that the amount of oestrogen in the bloodstream is inversely related to muscle stiffness, which is because oestrogen reduces collagen production [30,36,37]. The influence of oestrogen on muscles is carried out through oestrogen receptors [38,39]. A recent study suggested that two specific genetic variations (rs2234693 C/T and rs9340799 G/A) in the oestrogen receptor 1 gene (ESR1) may explain why muscle stiffness varies among different people [40].

Researchers have examined how two specific genetic variations in the ESR1 gene relate to muscle injury history in 1311 top-level Japanese athletes. Furthermore, they explored how these genetic differences — specifically at rs2234693 and rs9340799 in the ESR1 gene — affect muscle stiffness in physically active people. These genetic differences are found in a part of the ESR1 gene known as the first intron, located 397 and 351 base pairs before exon 2, respectively. They are identified by the special enzymes PvuII and XbaI, which recognise the sites rs2234693 and rs9340799. These variations influence how the ESR1 gene works and how its product is activated, affecting how estrogen functions in the body [41].

The ESR1 rs2234693 genetic variation is linked with muscle injuries and stiffness. This connection was identified using elastographic techniques to evaluate flexibility. In particular, it was found that individuals with the C allele are less likely to suffer muscle injuries and tend to have less muscle stiffness than those with the T allele. Furthermore, the ESR1 rs9340799 variation shows a significant relationship with muscle stiffness in the semitendinosus and semimembranosus muscles, but only in female subjects. This method could greatly affect research exploring the relationships between genetic variations, muscle stiffness, and sports-related injuries.

Training is Still the Most Important Influence

Research demonstrates that genetics does play a role in determining a person's baseline flexibility due to factors like the composition of collagen in the connective tissues and the structure of the joints. However, these genetic factors set the stage, not the outcome.

The more significant influence on flexibility comes from consistent and targeted training. When we regularly engage in flexibility exercises, such as stretching or yoga, we cause changes in both the musculoskeletal tissues and the nervous system. These alterations include the lengthening of muscles, increased tolerance in stretch perception, and an enhanced ability to relax muscles actively. Such adaptations are underpinned by the body's natural response to stress and adaptation.

Repeated stretching, for instance, increases the production of proteins within the muscle fibres that add new sarcomeres - the basic units of a muscle - in series at the ends of muscles, effectively increasing their length. Even when this process does not occur, an increase in joint ROM signifies enhancements in how far the associated muscles can stretch, i.e., it gets longer. Consistent practice also alters the nervous system's response, reducing the reflexive tension in muscles during stretching, thereby increasing ROM.

Therefore, while genetic predispositions set certain parameters, the extent to which we can develop flexibility is profoundly influenced by regular and disciplined training. The body adapts remarkably to the demands placed upon it, and flexibility is no exception. Practising does not just make perfect; it makes it possible.

Summary

It is widely understood that the environment influences our physical flexibility. Yet, recent research has illuminated a fascinating genetic component, attributing approximately half of the variance in flexibility to genetic differences [42]. Genes such as COL5A1, ACTN3, and ESR1 play significant roles. Despite these insights, the volume of studies investigating the impact of these genetic variations remains scant. Flexibility is not just a function of our genetic endowment; it is also shaped by the anatomical design of our joints, ligaments, tendons, muscles, and even factors such as skin texture, body fat percentage, historical injuries to these tissues, body temperature, activity levels, age, and sex. Understanding how genetic differences translate into varying levels of flexibility within controlled environments cannot be overstated. Recent research has begun to unravel the intricate genetic architecture that underpins flexibility. Yet, there is a clear imperative for further research to decode the genetic contributions to this physical trait thoroughly.

The field of genetics sheds light on why some individuals exhibit a natural propensity towards greater flexibility than others. It also equips us with a refined set of expectations regarding the outcomes we can anticipate from our physical training. Consider, for instance, a middle-aged man who has led a sedentary lifestyle and experiences a natural stiffness. It is highly improbable that he will achieve the anatomy-defying backbends performed by a twenty-something female contortionist showcased on Instagram, even if he commits to her "backbends in 30 days" programme. The stark reality is that the typical anatomy of the male hip and pelvis precludes most adult men from ever accomplishing a full 180-degree side split. While they might approach this feat, a shortfall of approximately 5-10 degrees usually remains. This discrepancy often goes unnoticed in photographs, obscured by clothing.

Indeed, it is incumbent upon trainers, therapists, and others engaged in promoting flexibility to spend time understanding the role of genetics in determining the ROM an individual can achieve. This knowledge is critical in offering precise guidance and necessary reassurance to clients, patients, and athletes. It is particularly relevant when one’s genetic makeup may prevent them from reaching their aspirational levels of flexibility or, conversely, when it bestows upon them an excessive degree of flexibility - as is the case with hypermobility - which can be equally problematic. Nonetheless, it is arguably even more essential that such professionals keep abreast of the most current and effective training methods. Mastery of these techniques will enable them to circumvent any genetic limitations adeptly, thus maximising the potential for physical development.

Think about sailing: a ship on the vast ocean is like our bodies embarking on the journey of flexibility training. Genetics, much like the ship's design, dictates the potential reach of our voyage - the types of waters it can navigate and the farthest points it can reach. However, it is the skill and perseverance in training, analogous to the act of sailing itself, that propels the ship forward. Through consistent effort, a sailor harnesses the wind, navigates through calm and stormy waters, and ultimately determines how far and how effectively the ship travels. Similarly, with dedicated flexibility training, we can push beyond the natural limits set by genetics, exploring the full potential of our physical capabilities, reaching distances that might seem unreachable at the outset. Just as a skilled sailor can make an ordinary vessel achieve extraordinary feats, so too can a dedicated person significantly enhance their flexibility, proving that while our genetic makeup sets the stage, our persistent training commands the journey.

Dive deeper into this subject with an exclusive webinar .

On June 9, I will be releasing an in-depth webinar that expands upon the points raised in this article. It will dive deeper into the genetic impact of flexibility in both everyday life and athletic performance. It will also teach you how to identify the best type of training for your flexibility based on your genetics, so you can work with your DNA to maximise range of motion, rather than fight against it. The webinar will be pre-recorded and accessible for life. You must be a Forum subscriber or Master Flexibility Trainer course member to get it.

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