Forces and Tissues: The Real Cause of Running Injury

Joe Uhan writes about the tissue and force abnormalities which can cause running injuries.

By on July 9, 2019 | Comments

Stay the CourseWhen an injured client walks into my office, I have two goals:

  • To do something that day that helps them feel better, and
  • To figure out how the injury happened and how to prevent it (and something like it) from happening again.

The first goal is the easiest. We do manual therapy, mobility, strength, rest, ice, and heat. It’s so easy, in fact, that I half-jokingly share with my clients my favorite Voltaire quote, The art of medicine consists of amusing the patient while nature cures the disease.” While getting better is usually easy, it can be trickier to stay healthy.

In order to prevent injury, we need to better understand the factors causing it. To begin, let’s define an injury as the breakdown of the body’s tissues due to excessive physical load. This breaking down can either be due to mechanical or chemical factors. The physical load is the force imparted on tissue, either volitionally by physical activity–running–or by involuntary factors such as falls or accidents. Since the vast majority of injuries I treat do not involve trauma, we will focus on the voluntary forces of running.

The million-dollar question of injury treatment and prevention is, what constitutes an excessive load? How do we know how much is too much? Moreover, how can load X be safe and effective running at time Y, but the same load causes severe injury at time Z? Perhaps the more important question is, what constitutes healthy versus unhealthy tissue? Successfully completing a run depends on our body’s ability to absorb load, so any deficit in tissue integrity decreases capacity to absorb force. No integrity, no run.

Thus, injuries are born at the intersection of tissue and force. Abnormalities of either or both dictate whether we run pain-free or create injury. In my first-ever iRunFar article back in 2012, I briefly outlined the tissue-force relationship. In this article, I expand on how the interaction of force and tissue dictates either healthy or injurious running.

The Definition of Forces

Let’s start by talking about the simpler and more recognized factor in injury, force. In physics, force is defined as the product of mass and acceleration. For our purposes, we can define force as the amount of energy generated by impacting the ground during running.

Force has characteristics of quantity and quality. Quantity is straightforward. The more we run, the more force is imparted. It is also readily acknowledged as a primary factor in injury. Non-runners are quick to jump onto the quantity hypothesis for injury causation, “So-and-so got injured because they ran too much!” But did they actually run too much? And what does that even mean? Force goes deeper than mere quantity.

Force has acute and chronic characteristics. Because of the adaptive nature of tissues, which we will address later, some forces are absorbed easier than others. On one end, acute forces can be extremely stressful and injurious. Many runners fear sprinting, for example, because it is a sharply increased force load that makes them sore. This soreness is a result of a larger force than tissue is accustomed to. Likewise, an inversion ankle sprain represents an acute force, wherein that force is applied in both high quantity and at a unique angle.

However, there is a relevant flip side. Chronicity is also an injury factor. Running the same surface at the same speed every day can cause tissue breakdown over time. While we’d like to think that tissue always adapts to consistent force, a simple analogy explains this concept well. Just as constant friction on the same spot of skin may cause a callus, it can also cause a blister.

Lastly but most importantly, we return to quality. How forces are applied to the body plays an enormous role in injury creation. How we run–our stride mechanics–is the most powerful factor in how forces are applied and absorbed by the body. The characteristics of posture, hip mobility, foot contact, and even arm swing heavily dictate how much and in what ways forces are imparted into tissues.

A good example is how bone tissue is extremely strong but also extremely specific in its strength. The long bones of the legs–the femur and tibia–can absorb incredible forces, but only if applied straight through the length of the bone. Yet at different angles, such as oblique angles, bending, or torsion, much smaller forces can cause fracture. This specificity, due to its complex internal microstructure, is likely dictated by the balance between the need for strength as well as lightweight efficiency. So, if we apply the same force at a different angle, the outcome may be widely different. This is why running efficiency is so important and needs to be examined constantly for every runner trying to reach his or her potential while avoiding injury.

Normal force is defined as:

  • A volume of running that has previously shown to be sustainable;
  • A quality of efficiency, where the force is imparted biomechanically, as dictated by our anatomy and physiology, to be most easily attenuated and impart the least amount of stress; and
  • A consistency that allows for optimal adaptation (thus avoiding sharply acute changes).

The Definition of Tissues

While quantity of force is easy to measure and quality is easily observed, even with an untrained eye, tissue integrity is both ill-defined and extremely difficult to measure. For this reason, it is often overlooked and looms enormously in the injury equation. Tissue integrity has several characteristics:

  • Strength – Does the tissue possess normal passive and/or active strength to capacitate loads?
  • Mobility – Does the tissue have anatomically normal extensibility or the ability to fold and lengthen in an efficient way?
  • Physiological balance – As living cells, does the tissue possess optimal nutrition, hydration, and stress-rest (both immune and hormonal) balance?

Strength has both extrinsic and intrinsic qualities. Is the tissue big or strong enough to support the load? Thankfully, our physiological adaptation creates strength quickly. We do an activity, the acute stress causes micro-trauma to tissues, we allow the tissues to repair, and hyper-compensation results. Because of this, it is my clinical experience that tissue strength is not a dominant primary factor in injury. However, if a runner lacks the strength to move their body in an efficient way, a strength deficit can secondarily cause injury. We’ll talk more about this shortly.

Adaptive shortening, or the gradual loss of soft-tissue extensibility, is a common occurrence with heavy running loads over many months and years. Mobility may be the easiest to measure, though deficits can be difficult to perceive. The Performance Mobility Series is intended to help runners identify specific mobility metrics necessary to achieve and maintain normal tissue and joint mobility.

The loss of a tissue’s functional mobility then creates abnormality in two ways. First, the tissue may lose its ability to absorb forces. Secondly and far more importantly, mobility loss can alter running mechanics and the quality of force loading. For example, range-of-motion loss in hip flexors or hamstrings can significantly alter the running stride and change how forces are applied to the body. Again, inefficient force loading–applying forces outside the tissue’s specific design–is a potent stressor and injury producer. All else equal, a runner with Jim Walmsley’s posture, hip mobility, and foot fall might at the same given speed be able to absorb, say, 70 miles per week of training, while more inferior stride mechanics might cause injury at 35 miles per week.

Lastly but perhaps most importantly is the tissue’s physiological balance. This is by far the most overlooked aspect of injury because it is so insidious and difficult to measure at the tissue level. Yet at the same time, physiological balance is representative of whole-body health. As such, pervasive, big-picture physiological states of health and wellness or a lack thereof directly impact tissue integrity.

I have written on this concept with multiple angles, ranging from the impact of running during and after major illness and the impact of chemical stressors–both running intensity and non-running factors–on tissue integrity. In short, chemical stressors and physiological deficits can radically decrease the mechanical capacity of tissue. We cannot ignore acute illness, chronic inflammatory conditions, or unremitting physical or psychological stress. They all impact tissue integrity.

Abnormal Tissues and Forces

If the definition of injury is a breakdown of the body’s tissues due to excessive physical load, then I contend that every injury is the result of abnormality of either force and/or tissue. Specifically, various factors can either multiply force and/or diminish tissue capacity. This results in a force-tissue imbalance and an injury.

Moreover, an injury’s severity also depends on the amount of distortion between force and tissue. For example, slightly modified forces such as from overstriding due to stiff hips on otherwise normal shins may result in shin splints. Likewise, persisting through your typical training week while under heavy work stress might result in increased muscle soreness.

However, almost every serious injury involves both abnormal forces and abnormal tissue. Last month, Tracy Høeg MD, wrote a fabulous article on iRunFar about the science of bony stress fractures, which included several insightful ultrarunner case studies. In each case, potential force and tissue abnormalities are evident and include:

  • Systemic illness,
  • Acute/novel force application,
  • Potential stride alterations due to previous injuries, and/or
  • Potential work and lifestyle stress.

Again, I contend that serious injuries such as stress fractures result from abnormality in both force application and tissue integrity. It takes a lot to break bone, but this is the one-two punch to do it.

In Review

If you’ve made it this far, then your reward is in knowing that the key to injury prevention is not:

  • Running exactly X miles at Y pace on Z days;
  • Drinking or not drinking milk or taking the entire vitamin alphabet; and/or
  • Doing several hours of rehab or prehab exercise daily.

Instead, the solution is maximizing tissue integrity and minimizing force.

For force mitigation:

For tissue integrity:

  • Prioritize optimal health throughout your body, and respect the limits of–and any deficits in–your overall health.
  • Maintain optimal hydration throughout the day, not just during activity.
  • Maintain optimal nutrition.
  • Acknowledge the importance of the stress-rest balance.

How we move is how we feel, but how we are is also how we feel. If we move with ease and efficiency in a healthy system, seldom will we have problems. Thus, healthy, sustainable, joyful, and injury-free running may lie in these crossroads.

Call for Comments (from Meghan)

  • If you look back at your last injury, can you pinpoint the force or tissue abnormalities which might have contributed to it?
  • Do you think you are running right now with a force or tissue abnormality? Can you describe it?
Joe Uhan

Joe Uhan is a physical therapist, coach, and ultrarunner in Auburn, California. He is a Minnesota native and has been a competitive runner for over 20 years. He has a Master’s Degree in Kinesiology, a Doctorate in Physical Therapy, and is a USATF Level II Certified Coach. Joe ran his first ultra at Autumn Leaves 50 Mile in October 2010, was 4th place at the 2015 USATF 100k Trail Championships (and 3rd in 2012), second at the 2014 Waldo 100k, and finished M9 at the 2012 Western States 100. Joe owns and operates Uhan Performance Physiotherapy in Eugene, Oregon, and offers online coaching and running analysis at