The Envelope of Function

Many of us experience injuries from time to time, particularly as we strive for a personal best.  We might be wanting to run further, lift more weight at the gym or play more holes of golf.  But for some reason... when we increase the workload, we end up with a niggle of some sort.

Why is that?

The concept of the 'Envelope of Function'¹ is not a new one but it is commonly not well understood by patients.  In a nutshell it is a representation of a tissues ability to deal with loading.


The above diagram is a basic representation of the Envelope of Function.  The y-axis represents how much force/load is required to produce a repetition, where as the x-axis represents the number of repetitions required to move outside the envelope.


Lifting a heavy weight

  • Lifting something heavy like a log is a high load.  This means that we may only be able to lift it 1-2 times without pushing beyond the envelope.

Running for a moderate distance

  • Able to run at a moderate intensity for a moderate length of time (repetitions).  

Walking for a prolonged period

  • Walking is a relatively low load activity which means we can carry out many repetitions (steps) while staying within the envelope of function.

Lets not get fooled into thinking that only exercise related activities are of concern.  Indeed all physical activities are influenced by the envelope, with each joint and muscle of our body having its own unique capacity for loading.  Have you sat in a chair before while craning your neck to the right to watch the action?  No problems if it is for a minute or two, but over an extended period the neck can become quite stiff and sore.  Maybe you've been unfortunate enough to have had a back pain.  We commonly have people in the clinic after 'throwing their back out' while getting up from the couch.  With a little probing I often hear that there has been a huge day of gardening, followed by 3hrs of recovering on the couch.

The question is... was it the poor slumped posture on the couch or was it hours of physical labour that isn't part of your daily life which pushed you outside your envelope of function?

The Impact Of Injury

Following an injury, our body's ability to cope with loading may be diminished.  Not only have the injured tissue been affected but the precise control of the tissue from the neurological system has also been altered¹.

After rolling your ankle... your normal 10km might be reduced to only being able to manage 5km.  Anything past the 5km mark is pushing outside of the new 'reduced' envelope of function and potentially exacerbating symptoms such as pain and swelling. 

How Does Surgery Impact The Envelope?

Lets imagine that when we rolled our ankle last month, we were unlucky enough to have ruptured the Anterior Talo-Fibular Ligament (ATFL).  Now this ligament plays an important roll in providing stability for our ankle and prevents us from rolling our ankle repeatedly.  We could choose not to have a surgical repair the ligament and embark on a conservative rehabilitation program.  Such a program aims to strengthen the muscles that support the ankle and improve the proprioception of the ankle.  Even with the very best of rehab, it may not be possible to get things back to 100%. 

Alternatively the surgery can repair the damaged ligament and restore the stability back to the ankle.  Easy, problem solved!

There may be more to the story.

Though ligamentous stability has been regained, the ankle (and any joint) relies is a complex combination of motor control from the nervous system, muscular strength in the muscles and information gathered from receptors within the joint, muscles and skin².


So What Is The Point Of This Post?

We often think the human body in the same way we do our cars.  If something breaks, we can get it fixed and it should be good as new.  

  • Tyres worn out?  Get some new ones and an alignment and you're good to go
  • Broken windscreen?  Easily fixed

But we aren't simple mechanical creatures.  We consist of a complex integration of thousands of biological processes which can be easily affected by external influences.  Even with the power of modern medicine, it is often impractical to believe that by influencing one structure (or process) wouldn't have a potential cascading effect on other processes.  A torn muscle, even with surgical intervention, results in some scarring of the tissue which alters its previous function.  In turn, this slight alteration of tissue length/strength/elasticity has the potential influence the joints and limbs that rely on that repaired muscle.

How To Maximise Your Function

As a general rule, it is always good to seek a professional opinion about an injury as soon as you can.  Early intervention often leads to improved outcomes, and in this case that means maximising your capacity for activity.  Your physiotherapist is your expert on load management and can help guide you with which exercises are appropriate for each stage of your rehab.  If your exercise demands are high and you're injured area is struggling with the load, a supportive device might be prescribed to help reduce your risks of further injury.  Always listen to your body while exercising, especially after an injury.  If it is telling you that an area is getting consistently sore with loading it is recommended you speak with your local physio or GP.

  1. Dye, S. F. (1996). The knee as a biologic transmission with an envelope of function: a theory. Clinical orthopaedics and related research325, 10-18.
  2. Lephart, S. M., Pincivero, D. M., & Rozzi, S. L. (1998). Proprioception of the ankle and knee. Sports medicine25(3), 149-155.