Ask A Trainer: What’s the Difference Between Flexibility, Stability, and Mobility?

Understanding the relationship will give you more purpose in the gym.

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Remember P.E. in junior high?

We wore nondescript solid color gym shorts and a basic tee. And if you were like me, you also had braces, bangs, and some sweet sport goggles. Based on my education today, I realize now that my gym teacher—who also happened to coach the football and girls’ basketball teams—knew very little about strength training and conditioning.

What I learned back then: 

  1. Flexibility is touching your toes.
  2. Mobility is avoiding a tackle on the football field. 
  3. Stability is not having to wear ankle braces on the basketball court.

I know now that these three concepts are critical to both performance and injury prevention—especially for us endurance athletes who are doing activities that include high repetition.

My P.E. teacher taught us the three primary pillars of fitness—strength, flexibility, and cardiovascular endurance. We now know, however, that human movement and behavior are far more complex, as these pillars cannot work in isolation without injury, pain, and/or kinetic disorder.

Flexibility = Length

Flexibility can also be a bucket term for the movement of a system of joints. It directly correlates with range of motion and mobility, but has very little to do with general strength, balance, and coordination. If we elevate “traditional fitness” to the standards of today’s endurance athlete, we’re actually talking about “performance.” An athlete focused on performance outcomes must also be aware of two additional pillars—mobility and stability—which, like the previous three (strength, flexibility, and endurance), cannot work in isolation.

Mobility = Movement

Mobility is about how freely a joint can move throughout a complete range of motion without restriction from surrounding tissue (tendons, muscle, and ligaments, for example). Muscle length (flexibility), muscle tension (strength), and neuromuscular control (coordination) are all components of mobility.

Stability = Control

Stability is the ability to maintain control of a joint movement or position by coordinating the actions of the surrounding tissues. Remember that our skeletal system is designed to bear load/stress—not our connective tissues. Having great joint stability equates to proper joint alignment, strong surrounding muscular tissue, and healthy ligaments and tendons. While mobility is the ability to produce a desired movement, stability allows you to resist an undesired one.

So what should you be focusing on now as you prepare for the 2021 season?

The off-season is traditionally the time of year where we focus on foundational strength. But before you head to the gym in the hopes of powering over any weaknesses in your swim, bike, or run, I’d like for you to carefully revisit two concepts from above, stability and mobility, to help you prioritize your time in the weight room.

When athletes run into weakness or injury, it’s usually because there’s incongruence between areas that should be mobile versus the ones that actually are. The same could be said for inconsistency across areas of stability.

How does this correlate to strength? You are only as strong as your weakest link.

For example, I’ve had many athletes who complain about chronically tight hamstrings. They spend time working to increase flexibility in that area through stretching. But in reality, the hamstrings aren’t really the problem. Typically, there’s a lack of overall mobility at the hips that’s causing the hamstrings to tighten in order to create additional stability across areas more prone to weakness—like the knees and the lumbar spine.

Remember your body actually wants to protect you from injury. Recognize that tightness and stiffness can be symptoms of compensation (in addition to over recruitment), as your body seeks to remain in harmony between mobile and stable joint systems. Finding that harmony is the key to proper body mechanics.

Here is what that harmony should look like:


An athlete who is stable and mobile in the appropriate places will leverage maximal muscular tension across a given movement or chorus of movements. That efficiency is the true marker for a “strong” endurance athlete. Developing this balance and leveraging this efficiency should be the basis for all reps and sets completed in the gym.

So let’s get strong this “off-season.” Let’s get super durable. Let’s chase some new PR’s in the weightroom. But first, make sure we move well (mobility) with great control (stability). This is the foundation that will help us truly uplevel our 2021 season.

Kate Ligler has specialized in endurance training in both functional strength and conditioning, as well as technical program creation for cyclists, runners, triathletes, and multi-sport endurance athletes for well over a decade. She is a NASM cPT in addition to a NASM CES (corrective) and PES (performance) specialist. 

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