Tolerance vs. Capacity

What’s the difference?!

Terms like “tolerance” and “capacity” get thrown around a lot these days in the worlds of reconditioning and performance. Oftentimes, there can be quite a bit of confusion as they are often cited incorrectly or interchangeably. We pose the question: in this particular instance, do semantics matter? Providing clean definitions and examples of these concepts allows us to decide in which circumstances one term may be most pertinent and how that determination may influence decision making.

Let’s begin with this idea of tolerance, which is often also referred to as “tissue tolerance.”

When considering the term tolerance, I start with a very simple question: “Can they do it?”

In other words, can the individual perform a desired activity or intervention? Additionally, if they can do it, are there any undesirable consequences or pain associated with the activity during or immediately following?

These questions become extremely important as they become a gauge of whether or not the individual has the capability to perform the desired activity at the predetermined intensity. If they cannot perform, or if they have a major increase in symptom onset or have to utilize a compensatory offloading strategy, they can be said to be lacking tolerance to that specific activity.

Limitations in tolerance may be due to a multitude of factors such as heightened acuity of tissue sensitivity, muscular inhibition, localized swelling, tissue damage, and force generation deficits.

Let’s use a return to run scenario as an example:

If we are working with a runner who has been dealing with some long-standing Achilles tendinopathy, we may ask ourselves if the individual can tolerate specific loading strategies and positions of the foot and ankle.

We probably want to know if they can maintain a sustained isometric single leg heel raise hold to assess the tolerance of the heel chord and gastroc-soleus complex to a low level isometric loading strategy.

If they are able to perform a prolonged heel raise hold with no exacerbation of symptoms, we may then move on to a single leg heel raise test to assess the tolerance of the Achilles tendon and associated musculature’s tolerance to controlled isotonic loading through a full range of motion.

Still no issues? Perfect! Perhaps next we will assess the individual’s tolerance to more elastic demands such as those seen with skipping or pogo hop variations! The questions remain the same, but the assessment strategy merely becomes more demanding and multifactorial as the process proceeds (from single joint, isometric all the way through multi-joint, multi-directional, and compound movements as seen with sprinting and change of direction).

What about the concept of capacity?

Capacity is the successor to tolerance. It does not become overly relevant until an individual demonstrates adequate degrees of tolerance to the desired activity. Quite simply, if I cannot do any of the things that I need to do, the amount that I cannot do is irrelevant!

Capacity begs the question: “how much ______ can this person handle?”

This may take the form of local capacity as seen with isolated tissue loading (how many heel raises can they perform prior to fatigue or loss of heel height?) or more systematically as a product of how efficiently the individual produces energy to sustain a desired intensity, duration, and frequency of a task (how long can they run prior to reduction of a set pace?).

Both local and systemic capacity take into consideration the repeatability of a specific task as a measure of endurance and the degree of decline in performance with the onset of fatigue. Capacity involves potent volume considerations.

Why do we care about the onset of fatigue?

As we have seen from literature, fatigue changes everything. We adopt different movement strategies under fatigue. We load structures differently. We become less fluid, we may lose the ability to dissipate forces across multiple joints and tissues, we may utilize increased and unfamiliar degrees of joint excursion in efforts to do whatever it takes to keep going.

Envision the last mile of a marathon. How fluid do the runners look as they approach the finish line as compared to when they started? When fatigue sets in, complex movement strategies have to be simplified, regardless of the potential ramifications.

Returning to our return to run example:

Say the individual shows excellent tolerance to isometric, isotonic, and short duration lower limb plyometric interventions. Now they have initiated a return to run program only to realize that after 1 mile, they start to notice increased ache and pulling in the same place they had pain before! Frustrated, they come back to you and ask if all their hard work was for nothing?!

Of course it wasn’t. This is simply a result of the tissues not yet being adequately prepared to endure sustained and repetitive bouts of loading at the intensity necessary for running.

In this event, we can likely surmise that they have a local capacity deficit of the gastroc-soleus and the Achilles tendon which would benefit from progressive increases in volume of elastic loading. Necessary interventions may include an interval running program to provide intermittent rest periods between bouts of loading, higher volume of skipping and pogo iterations, and education regarding what degree of symptoms are acceptable versus undesirable.

Taking one step further, let’s say the individual can run up to 5 miles before the heel begins to really bother them. But now they realize it only seems to hurt when they find themselves out of breath and really having to dig deep to keep going. Perhaps now the limiting factor is the onset of fatigue due to a lack of overall systemic capacity and impaired conditioning! In this scenario, as the fatigue accumulates, their running mechanics and loading strategies across the foot and ankle may change drastically in such a way that now stresses the Achilles in an unfamiliar fashion. Perhaps this could take the form of increased ankle joint excursion, increased eccentric gastroc muscular action, or even due to reductions in the elasticity and impaired efficiency of the stretch shortening cycle.

While tolerance and capacity possess many commonalities, posing the question of which appears to be the rate limiting factor can help drive your treatment process and future implementation strategies. Additionally, if an individual appears to be deficient in a particular capacity, the same thought process can be applied to decipher if local capacities or systemic capacity are the likely causation.

-Mike Reinhardt, PT, DPT


Article: The 45 second anterior knee pain provocation test: A quick test of knee pain and sporting function in 10-14 year-old adolescents with patellofemoral pain

  • While very simple to execute, the AKPP test showed relevant associations with improvements in self-reported knee pain and improved sporting functions in an adolescent population. This test can be easily performed with no equipment and may offer clinically relevant insights into quadriceps and patellofemoral tolerance to loading and return to sport potential.

Listen: The E3Rehab Podcast – Episode 55 Biomechanics, Exercise Prescription, & Programming w/ Dr. Scot Morrison

  • Scot Morrison and Sam Spinelli discuss the differences and when/why behind using constrained versus open environment exercises. They differentiate what instances you may want to bias a particular tissue through an assortment of loading variations (open environment) versus utilizing a particular pattern (constrained environment). The conversation also delves into the significance of intention and impulse considerations with trained and untrained populations.

Book: Think Again – Adam Grant

  • This is an exceptional read that challenges the reader to question their belief systems, their biases, and how they respond to contradictory information. Adam Grant uses a plethora of examples that drive home the point that individuals who are willing to take a scientist’s mindset with their belief systems are often more adaptable and have a higher probability of predicting future outcomes!

Social Media:

  • Zach is a PT, ATC working at Elite Sports Physical Therapy in Tinton Falls, NJ. He works with athletes of all levels recovering from a variety of different injuries. Through his Instagram page, it is Zach’s hope to help other healthcare professionals, as well as athletes going through their own recoveries, to understand what sports rehab can and should look like.
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