Understanding Repetitive Strain Injuries

Musculoskeletal injuries occur through one simple principle, the load (i.e. the physical stress or strain) placed on an anatomical structure exceeds that structures capacity to tolerate that load.  This seems logical to most people when thinking about traditional sports injuries like ACL tears or ankle sprains.  But what many people don’t realize is that the damaging and excessive loads do not have to happen all at once.  Instead, many injuries are categorized as overuse or repetitive strain injuries, meaning pain and tissue damage are not the result of strain or overload from a single event, but instead occur as the musculoskeletal system is exposed to a large number of repetitive forces over weeks or months. 

Perhaps the most obvious example is distance running injuries (although we could easily use golf, swimming or a number of workplace examples....typing and prolonged computer use are other good examples).  Tissue damage resulting from repetitive overuse is how the vast majority of running injuries occur.   Assuming an average stride rate of 180 steps per minute with a pace of 6 minutes per kilometre, a runner’s feet will strike the ground 1080 times over the course of a single kilometre.  For a 10K run this equates to 10,800 foot strikes.  With typical training programs involving 10K to full marathon distances the number of foot strikes can quickly rise into the millions in just a single month. 

Keep in mind that the musculoskeletal system is stressed with each of these strides, as the body must first absorb the impact force associated with the foot striking the ground, and milliseconds later must generate a propulsive force to propel the body forward.  Although the loads associated with each individual stride are well within the tolerance limits of the musculosleletal system, the repetitive application of these loads can produce a combined fatigue effect over time, resulting in a reduction of the body’s capacity to endure the forces generated with each stride.  If this pattern continues tissue damage can occur, resulting in injury.

Stress, Adaptation, and Over-training

Although repetitive strain injuries are a consequence of repeated musculoskeletal stress, this does not necessarily mean that all stresses are bad.  In fact, some degree of stress is necessary if we want to build strength, speed, and greater endurance.  This is the focus of all strength or athletic training programs, including endurance training for sports such as running. Essentially, the athlete performs an activity which stresses the body.  Although the nature of the specific activity may focus this stress at a different system or anatomical structure (i.e., running will focus stress on different areas compared with swimming or weight training), the intention is the same.  Stimulating a training response will allow the body to more easily meet the demands of that stress the next time it is encountered.
 

Figure 1. Click image to enlarge

This process can be demonstrated by a basic stress-response curve, illustrated in Figure 1.  The vertical axis represents our body’s performance capacity.  For our purposes we can define this simply as our body’s ability to meet a certain demand or perform a certain activity.  In this case to run a certain distance or at a certain speed. The horizontal axis represents time.  As our body is stressed during exercise, tissues breakdown and our energy stores are depleted, resulting in a temporary reduction in our performance capacity.   It is more difficult to meet the demands of the given task.  Then, as the training session ends the body shifts to an anabolic state as it works to rebuild the damaged tissues.  Assuming the exercise session has provided an appropriate stimulus, tissue remodeling processes will result in a temporary increase in exercise capacity, often referred to as a stare of super compensation. 

If another training session is performed while in this state of super compensation, a workout that is slightly more challenging since our performance capacity is elevated, our body will once again be broken down, and then can once again be built back up to another super compensated state.  The repetition of this process over weeks and months leads to a steady increase in our fitness and performance capacity, and forms the basis for all progressive exercise and athletic training programs. It is how we become stronger, faster, and gain endurance.  

Figure2. Click image to enlarge.

Although on the surface this seems like a simple, straightforward process, there are several variables that must be considered and carefully manipulated within the training program to ensure the desired training effects occur.  First is the training program itself, particularly the timing and the nature of each workout.  To produce the desired training effect it is important that the stress associated with each training session (to stick with our running example, this is usually thought of in terms of volume and intensity), is just beyond what the body is accustomed to experiencing.  If the training session is too easy it will be insufficient to stimulate the body to adapt to a new level of fitness. If it is too hard the body will need an extended time to recover, thus compromising subsequent workouts. 

The timing of each workout is also important.  To create a carry-over from previous training sessions, workouts with the goal of building speed or increased endurance should be timed to coincide with a state of super compensation.  However, it is important to realize that this super compensated state is associated with structural, chemical, and hormonal changes.  These changes have a metabolic cost as the body must expend energy to maintain them.  If the body does not experience another situation which taxes this new level of fitness, the body assumes these adaptive changes are an unnecessary use of energy and will slowly drift back to its previous fitness levels.  Therefore, if too much time elapses between key exercise sessions the ability of the subsequent workout to build on the previous one will be lost.  

                 

On the other hand, performing a strenuous workout too soon may be even more detrimental as the body may not have fully recovered from the previous training session.  This is associated with a training stress being applied to the body while it remains in a state of reduced fitness and exercise capacity.   The training session will further breakdown already damaged tissue.  Unfortunately, just as positive training effects can have a cumulative effect over time so can these negative adaptations.  In this case, it is the tissue damage and diminished capacity to withstand the forces associated with running that build over time.   Eventually the damage will accumulate to a the point where the affected muscle, tendon, or bone can no longer tolerate the demands associate with the activity, culminating in pain and injury.  I often refer to this as passing a symptomatic threshold.  This is the hallmark presentation of overuse injuries.  Pain seemingly occurs out of nowhere, however, as you can see the tissue damage and reduction in exercise capacity has actually been accumulating for some time.  

 

Why It’s Not Just About a Proper Training Plan

               

It is often believed that repetitive strain injuries occurring in athletes are a result of training errors (i.e. over-training).  The volume and intensity of training simply exceeds the body’s capacity to recover.  Based on the stress-response curve outline above this may seem like a logical assumption, however, the fact is that training errors alone cannot account for the majority of these injuries.  To be clear, this is not to say that following a proper training program with carefully planned workouts and recovery periods is not critical.  In fact, failure to do this will almost certainly lead to over-training and injury.  However, the reverse is not necessarily true.  For example, many runners will carefully follow a proven training plan, a plan that has been successfully utilized by countless other runners, yet still become injured.  Why is this?  What is it that would prevent one runner from training as hard, as long, or as often as another runner of similar stature and ability before developing an overuse injury?  This is where biomechanics come in.

               

Figure 3. Click image to enlarge.

Simply stated, we can think of biomechanics as your quality of motion, i.e., how your body moves during complex activities.  How the body moves is a critical factor with respect to repetitive strain injuries because it determines how forces are absorbed, distributed, and generated throughout the body.  To better understand the importance of proper biomechanics let’s keep in mind that the basic stress-response curve could theoretically represent any number of elements, such as the aerobic or anaerobic energy system, or a given muscle, tendon, joint, or bone.  In fact, with each workout or training cycle, each system or structure would have its own stress-response curve (Figure 3).  This has profound consequences with respect to injury, as it is possible that multiple areas of the body could be experiencing positive adaptation and improved performance capacity over time, yet other areas may be undergoing an over-training effect.  For example, classic symptoms of over-training include symptoms of fatigue and diminished performance as signs that the body is failing to properly adapt the training program.   These symptoms tend to be related primarily to poor adaption of your energy system and endocrine system, but not necessarily your musculoskeletal system. 

                

When the body moves in a less than optimal - in other words, there are faulty biomechanics and poor quality of movement -  it alters how forces act on the body.  For example,  recall that with each running stride an impact force is generated between the foot and the ground, and that this force will travel up the leg though the foot and ankle, knee, hip, and finally into the spine.  Under normal circumstances, the dissipation of this force is shared between the various body segments.  However, faulty biomechanics driven by problems such as tight or weak muscles can alter this normal force dissipation, causing stress to be concentrated at a particular anatomical structure or region.  Due to the repetitive nature of running and other athletic activities, even a minor biomechanical problem can had profound effects as the altered loading pattern is repeated millions of times in the course of training, eventually culminating in injury to the overloaded area.  All of this is often going on behind the scenes even as notable improvements in strength and endurance are occurring. 

Preventing Repetitive Strain injuries

                 

One of the real problems with repetitive strain injuries is that although they slowly develop over time, this development is often not associated with any obvious early symptoms or warning signs.  But this does not mean there is nothing that can be done to help prevent these injuries.  Keep in mind that prevention of this type of injury really calls for a load management strategy.  So first and foremost, as suggested above, it is critical that you follow a proper training program that includes proper rest intervals (and yes, for some of you Type-A exercise addicts... you need to actually abide to these recovery days, they are there for a reason).  Second, work on improving your biomechanics.  Uncovering biomechanical problems does not have to be difficult (see the previous post on Movement Pattern Screening), but it does need to be done... you can’t correct a problem until you know that it’s there.  Often this can be just a matter of adding some key stretches or exercises to your routine.  We commonly do this type of thing in our clinic (click here for more info), but if you are not in our area look up a good trainer or movement based therapist.  If you need some help with this www.ActiveRelease.com is a good place to start.  ART® is a great hands on soft tissue technique that is geared towards the type of soft tissue pathologies seen with repetitive strain injuries.  Practitioners certified in this technique also tend to be well versed in biomechanical assessment.