I find this question interesting because we often, myself included, assume a person is less flexible, if they happen to have a bigger build – however, it’s not always the case. This is even though several studied have found a correlation between increased muscle mass and increased stiffness or reduced ROM (Neto et al. 2013; Ryan et al. 2009; Suga et al. 2021).

One of the reasons why you can still be very flexible, even with a large amount of muscle, is that muscle morphology isn’t the only factors that affect flexibility – another important factor is stretch tolerance, which I won’t be getting more into here (Check out this post if you want to read about the what happens when we get more flexible). It should also be noted that strength training can be an effective way of increasing flexibility (Afonso et al. 2021), even though you might increase muscle mass at the same time.

In this post I’ll be going over why an increase in muscle mass, might not always be bad for flexibility as well as give you some very basic pointers about how to approach strength training if you want to maintain or even increase flexibility through strength training.

To understand how changes in muscle mass might affect flexibility, we’ll first need to go over some basic muscle anatomy.

Skeletal muscles are made up of bundles of muscle fibers called fascicles, fascicles are made of bundles of myofibrils or muscle fibers (Jorgenson et al. 2020). Around the muscle fibers, there are layer of connective tissue – together with the muscle fibers, this makes the muscle work kind of like an elastic band when pulled, even when the muscle is relaxed.

The fascicles can increase size in 2 ways; they can get thicker (this is called radial hypertrophy) – this would equate to getting a thicker elastic band – you’ll need more force to stretch it out. This is why an increase in muscle mass, can affect flexibility negatively.

Fascicles can also increase size by growing in length (this is called longitudinal hypertrophy). This would be the same as getting a longer elastic band. It’s still as stiff, but it can be stretched further if we pull with the same amount of force. If the muscle mass is increased this way, it could have a positive effect on flexibility – Several studies have found that flexible individuals have longer fascicles compared with normal individuals (Blazevich et al. 2012; Moltubakk et al. 2018), while other have found a correlation between fascicle length and reduced passive resistance to stretch (Blazevich et al. 2007) and ROM in young males (Hirata et al. 2020).

Figure 1: The whole muscle is made up of bundles of fascicles. Each fascicle is made up of myofibers, which we also call muscle fibers.

Muscle size is often determined by measuring the Cross-Sectional area (CSA) of the muscle. It makes sense, that if the muscle fibers grows radially (become thicker), the CSA of the whole muscle also increases. However, depending on the structure of the muscle, an increase in fascicle length can also increase the CSA of the muscle (Jorgenson et al. 2020).

Skeletal muscles can be structures in different way, in regards to how the muscle fibers are oriented in relation to the tendon. Overall, we can differ between muscles parallel muscles where the fibers are going more less straight from tendon to tendon (example A in figure 2) and pennate muscles, where the fibers are attached obliquely to the tendon (exambe B, C, and D in figure 2).

Figure 2, from (Jorgenson et al. 2020) and (Orlandi D. 2015). (A) fusiform, (B) unipennate, (C) bipennante, (D) multipennate. Blue lines indicate the plane for the anatomical cross-sectional area (CSA) (i.e., the CSA that runs perpendicular to the longitudinal axis of the muscle), and green lines indicate the plane for physiological CSA (i.e., the CSA that runs perpendicular to the longitudinal axis of the fascicles/myofibers).

Muscle size is often determined by measuring the Cross-Sectional area (CSA) of the muscle via ultrasound. In figure 1, the blue lines indicate the plane for the anatomical cross-sectional area (aCSA) (i.e., the CSA that runs perpendicular to the longitudinal axis of the muscle), and green lines indicate the plane for physiological CSA (pCSA) (i.e., the CSA that runs perpendicular to the longitudinal axis of the fascicles/myofibers). If the muscle fibers grow radially (become thicker), the CSA of the muscle will increase – but in pennate muscles, an increase in fascicle length will also increase the aCSA of the muscle (Jorgenson et al. 2020).

This is reflected in numerous studies that have found an association between whole muscle aCSA and fascicle length, among others a systematic review by Ryoichi Ema et al. (2016) that included data from 38 independent studies.
It seems, that an increase in aCSA can be caused by both radial and longitudinal hypertrophy in pennate muscles.
An increase in muscle aCSA caused solely by radial hypertrophy would have a negative effect on flexibility while an increase in muscle aCSA due to longitudinal hypertrophy, could have a positive effect on flexibility.

So in parallel muscles, an increase in muscle size will most likely have a negative effect on flexibility. In pennate muscles, the effects of increasing muscle size on flexibility, will depend on the mechanism behind the hypertrophy, whether it’s caused mostly by longitudinal or radial hypertrophy.

If you want to strength train and grow bigger muscles without limiting your flexibility, it could be possible by maximizing hypertrophy through longitudinal hypertrophy. If you want to read more about how to increase flexibility through strength training, I wrote a blog post about this subject, that you can find here – but the main takeaway, is that you’ll want to make sure to train your muscles at their longest range.

Referencer:

Afonso, J., R. Ramirez-Campillo, J. Moscão, T. Rocha, R. Zacca, A. Martins, A. A. Milheiro, J. Ferreira, H. Sarmento, and F. M. Clemente. 2021. “Strength Training versus Stretching for Improving Range of Motion: A Systematic Review and Meta-Analysis.” Healthcare (Basel) 9 (4). https://doi.org/10.3390/healthcare9040427.

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Suga, T., M. Terada, K. Tomoo, Y. Miyake, T. Tanaka, H. Ueno, A. Nagano, and T. Isaka. 2021. “Association between plantar flexor muscle volume and dorsiflexion flexibility in healthy young males: ultrasonography and magnetic resonance imaging studies.” BMC Sports Sci Med Rehabil 13 (1): 8. https://doi.org/10.1186/s13102-021-00233-z.