Category Archives: For Gym Teachers

“How can anyone be so out of shape?” 

“Why don’t they  just try harder?” 

“Don’t they realize they’ll have to run more laps?”

Anyone who has ever ran laps in grade school gym class can probably recall stragglers.  Some were children with asthma or other health issues, but some consistently finished for no obvious reason.  For such children, it may very well have been the case that trying harder simply wasn’t an option.

When a child with no obvious physical infirmity is unable to meet basic physical fitness criteria, effort should be made to rule out McArdle’s disease, a rare and often undiagnosed muscle disorder marked by extreme exercise intolerance.

How McArdle’s Disease Makes it Difficult for Children to Run Laps

Also known as glycogen storage disease Type V or phosphorylase deficiency, McArdle’s disease is an inherited condition which impairs energy metabolism in skeletal muscle. During moderate to intense physical activity an enzyme called glycogen phosphorylase provides contracting muscles with the energy required to do work. In McArdle’s disease this enzyme either is either non-functioning or missing, and the result is premature exhaustion and failure which may be accompanied by cramp-like injuries to muscle tissue.

Screening Children for McArdle’s Disease: Simple Signs to Look For

Even with the wealth of information now available on the web, screening McArdle’s disease in children remains challenging. Undiagnosed individuals may have a healthy appearance and symptoms can easily be mistaken for laziness or behavioral problems.

For this reason, it is critical that the right people know exactly what to look for. Physical education instructors, teachers, school nurses and even other students can screen McArdle’s disease on the basis of one or more of the following criteria:

  • They seem abnormally out of shape despite a healthy appearance.  People with McArdle’s disease have trouble sustaining low- to moderate-impact activities such as climbing stairs or walking steep inclines, and consistently perform below average on standardized fitness evaluations.
  • They have trouble getting in shape.  Because their muscles do not metabolize energy normally people with McArdle’s fail to respond as expected to endurance exercise regimens.
  • They may experience cramping injuries accompanied by dark-colored urine.  As children reach adolescence, increased body weight, musculature and opportunity for competitive sports training may precipitate episodes of muscle failure and injury during activity. Telltale symptoms include painful cramp injuries called rhabdomyolysis, sometimes accompanied by myoglobinuria, a potentially life-threatening condition indicated by urine darkened by proteins entering the bloodstream from damaged muscle tissue.
  • They may avoid fitness evaluations or other physical activities. The conspicuous symptoms of McArdle’s disease can make children self-conscious and embarrassed, and the stigma of feeling different may lead them to avoid participating in physical activities altogether.


If you know of anyone who fits any of these criteria, please reach out to them and learn more about the challenges they face.  Children with undiagnosed McArdle’s disease may suffer from the perception that they are simply out of shape or lazy, so approach them with discretion but be patient and persist in asking the right questions.

Most importantly, always remember that diagnosis and treatment of McArdle’s disease should be left to qualified medical professionals.  Recent advances in medicine make conclusive diagnosis a matter of one or two clinic visits and a minor blood test.

Children and adolescents with McArdle’s disease can live active lives and be physically fit, provided challenges are undertaken safely and within the scope of the limitations of this disease.  Catching symptoms early gives children with this rare disease a better chance to feel normal.

For additional information about McArdle’s disease visit any of our homepage resources or the Muscular Dystrophy Assocation website.


The medical condition known as rhabdomyolysis recently entered the national news when a number of college football players from the University of Iowa were hospitalized with symptoms that included dark urine (myoglobinuria) and impaired kidney function. University officials were quick to defend the strength and conditioning program which has played such a critical part in Iowa football’s success under head coach Kirk Ferentz.  Meanwhile, the local press scrambled to inform the public with medical information, with varying degrees of accuracy.

What Is Rhabdomyolysis?

In the simplest terms, rhabdomyolysis is damage to skeletal muscle tissue in which muscle fibers are ruptured and release their contents into the bloodstream.  Blood labs can quickly identify  elevated levels of muscle proteins in the blood, but the “smoking gun” before a stricken individual even goes to the hospital is urine tinted dark or even brown from iron-containing myoglobin.

Doctors first described this condition in the mid 20th century in association with crush injuries, such as sustained during falls or from automobile accidents.  The blood work associated with rhabdomyolysis shows the presence of muscle proteins.

In the case of the Hawkeye football players, it was learned that the hospitalized players had undertaken extreme workouts which included squats of 200+ pounds repeated for as many as one-hundred repetitions, within a 20-minute window.  While the average college athlete or former athlete may be no stranger to difficult workouts, that these football players wound up hospitalized should come as no surprise to people who have or are familiar with McArdle’s disease.

Muscles obtain massive amounts of energy on short notice by rapidly breaking down starch (called glycogen) into glucose molecules.  During strenuous physical activity such as  squats, the process of glycogen metabolism floods the working muscle cell with a surplus of ingredients for chemical energy, precisely at the time when it needs it most.

McArdle’s disease illustrates how critical this metabolic process is for meeting the demands of even brief anaerobic exercise.  Due to a missing or non-functioning enzyme, people with McArdle’s disease cannot metabolize muscle glycogen into glucose.  As a result, during strenuous activity they experience premature and dramatic fatigue and often may also experience painful muscle cramps and rhabdomyolysis.  It is very common for individuals with McArdle’s disease to be hospitalized following episodes of rhabdomyolysis, due to the complications such as a liver problems.

People with McArdle’s disease get injured during exercise because their muscles effectively run out of energy without warning.  Skeletal muscle requires energy both to contract and relax in sequence; when someone with McArdle’s disease gets out of a car and attempts to walk up three flights of stairs, they make demands of their legs which the muscle cells cannot reasonably meet. The quadriceps, the body’s largest muscle group, are abruptly robbed of even the energy necessary to bear the body’s weight against gravity.  The predictable result is failure, as the muscle stops responding altogether, and rhabdomyolysis, or often both.

In the case of the University of Iowa football program’s  incident, rhabdomyolysis occurred as the result of extraordinarily demanding workouts.  Too much weight, lifted too many times and in too short a time period, using the body’s largest muscle group (the quadriceps.)  Even for powerful college athletes, the body has limits which no amount of perseverance can safely overcome.

Who is At Risk for Rhabdomyolysis?

There are two primary groups of people at risk to experience rhabdomyolysis: athletes, and people with McArdle’s disease.  However, because people with McArdle’s often otherwise appear completely healthy, symptoms have historically been dismissed as laziness or poor physical conditioning.  Additionally, many people with the disease develop personal methods of coping specifically to avoid such traumatic episodes of muscle failure.

Athletes, ironically, are at perhaps greater risk due to the lack of general public knowledge about rhabdomyolysis and muscle injury, combined with stereotypical expectations for varsity athletes. Muscle failure, dark urine and days of soreness may simply be considered normal for extreme competitors, but the rhabdomyolysis-related hospitalization of more than a dozen college football players at the University of Iowa proves that the concept of simply “pushing through” fatigue is a myth, and a hazardous one.

This was forwarded to me today:

They are attempting to commercialize the software. I’m mainly interested to see if computational modeling like they are doing could be applied to McArdle’s Research. I know they use Santos to see how much gear they can load onto a soldier, walk him around and see what he can physically handle. I imagine whatever numerical models drive that could certainly be modified to reflect our energy bottleneck, right?

Simulation capabilities are getting better every day and a high fidelity, biomechanically accurate human model could be really relevant for McArdle’s issues

This link discusses strength and fatigue modeling using a computerized simulation. ?Such a model might one day help doctors visualize the physical limitations imposed by the disease.

One of the risks associated with McArdle’s disease is the painful cramping that can occur during moderate to intense physical activity. The exact nature of this cramping injury has not been investigated, but clinically it shares symptoms with crushing-type injuries such as those sustained during falls or auto accidents.

People who have McArdle’s disease may experience injury without even consciously over-exerting themselves, for example while carrying a heavy box for a few moments. Since muscles need chemical energy to relax as well as contract, an abrupt and precipitous drop in the cell’s immediately-available energy can subject cramped muscle fibers to forces that exceed their tensile strength. This is probably what causes the injury.

Anyone who has experienced McArdle’s “cramping” knows that the cramp itself is just an indicator that the muscle has actually been injured. In addition to immediate inflexibility, swelling, and pain, other symptoms such as myoglobinuria and fever may follow after one tries to extend the cramped limb and “walk off” the injury. To date, there is no treatment for McArdle’s disease, and the typical E.R. treatment for these cramping injuries usually involves intravenous saline and pain relievers. The more unlucky individuals wind up being treated for impaired renal function.

Another problem with these injuries is the difficulty regaining strength and flexibility in the injured areas. ?To date, the medical world cannot successfully regenerate muscle cells in a therapeutic application for humans, but there is promising research on the horizon. Until then, people who have sustained repeated muscle injuries have to make do with what they have.

What People with McArdle’s Disease Can Do About It

  • Be Fit
    Most clinical research on McArdle’s disease suggests that a regimen of disciplined, regular physical activity of an appropriate intensity and duration can not only improve symptoms but also make muscles less susceptible to injury.
  • Be Flexible
    Flexibility decreases risk of additional injury in the affected area as well as other areas which may be secondarily weakened by that inflexibility. For some people with McArdle’s disease, stretching following walking offers immediate and dramatic pain relief.
  • Be Aware, Be Patient and WARM UP
    It’s very easy to over-do it and get an injury quickly without even realizing it. ?Be conscious of your energy levels, your blood sugar, your heart rate, and your general feeling of wellness. Don’t avoid physical activity altogether, but when undertaking anything remotely strenuous understand that you do not have the luxury of being able to rush things. Take your time and get properly warmed up.Once you’re warmed up, stretch periodically.

McArdle’s disease doesn’t have to leave you sedentary and in pain.The more you know and do, the better off you are.

A quick Google search of “adult-onset McArdle’s disease” will yield volumes of medical abstracts on patient studies. I’ve seen plenty of other abstracts that point out that the disease is not often diagnosed until at least the second or third decade of life. Perhaps this is because this is around the time the body’s metabolism starts to slow down a little, and the “weekend warrior” injuries become more common (and considerably more complicated, without glycogen in the picture.)

There may in fact be an adult-onset form of McArdle’s disease. I also suspect that many newly-diagnosed adults have lived with the disease their entire lives and simply gone without diagnosis, “toughing it out”. (In another article, I talk about how children with McArdle’s disease in particular are particularly vulnerable.)

A concerted campaign to educate grade school gym teachers, pediatricians and health care professionals about McArdle’s disease could spare many people the punishment of trying to force their bodies to do things they simply weren’t equipped to do. This may enable them to participate in alternate activities tailored to their unique condition, and improve their health later in life as a result through earlier proactive management of the disease.

Ultimately the question of whether or not there is an adult-onset form of McArdle’s is probably not as important as is education and information. The more people know about McArdle’s disease, the less likely it is that children and adults who remain undiagnosed will suffer and live with unanswered questions.  Your feedback on this topic is welcome.

Muscle fatigue and weakness experienced during even moderately intense physical activity can in fact simply be caused by a lack of physical conditioning, but the human body is a robust machine that, when working properly, responds rapidly and efficiently to changing demands.  When muscle weakness is acute and to the point of failure (non-response), however, it is time to seek medical attention.

People with McArdle’s Disease may experience this type of muscle failure.  It is described in medical journals as “rapid onset muscle weakness and cramping,” but this doesn’t give a very clear description.  More accurately, it can be described as a diminished response from the active muscle group as a function of time.  For example, when someone with McArdle’s Disease tries jogging, their experience is more or less normal for anywhere between 10-30 seconds, depending on factors like incline, recent diet, physical condition, etc.  It is after this initial period that the individual with McArdle’s experiences an abrupt and rapid decrease in energy that is not so much about being “out of breath” as it is about muscles simply no longer responding.

I’ve put together a graphic representation of what this feels like.  Anyone with McArdle’s Disease knows that intense exercise elicits an exponential decrease in energy in a matter of seconds, usually well under one minute:

This is a rough, non-scientific graph that is rather subjective but nonetheless probably an accurate graphical description of the sensation of McArdle’s Disease symptoms.  The exponential curve representing energy capacity as a function of time during intense exercise corresponds to what I know about reaction rates; i.e., there is a fixed concentration of phosphocreatine as well as creatine kinase in the cell atany given moment, so as ATP generated from glucose metabolism is the limiting factor in the overall picture, as the concentration of phosphocreatine (which is used up creating ATP for immediate use) drops dramatically, so too does the “reaction rate” of physical energy available from the active muscle group.

Muscle fatigue and weakness experienced as a result of McArdle’s Disease seems to vary according to the intensity of the activity.  Walking on flat ground, for example, apparently has an energy demand per unit time that is low enough to allow the cell to overcome the obstacle in the traditional energy pathway and create enough energy per unit time to permit the continued activity.  Some physicians speculate that individuals with McArdle’s Disease can experience a “2nd wind” phenomenon following warm-up as a result of adaptive measures the body takes in response to physical activity.  In any event, between walking and jogging there seems to be a rather pronounced drop-off in energy per unit time.  Speaking for myself, I can tell you that no amount of conditioning to date has allowed me to completely overcome the dramatic drop in muscle energy which results from taxing activities such as jogging or climbing stairs.  Walking, however, is a different story.  The graph below compares the sensation of performing these activities with McArdle’s Disease:

As the graphic indicates, walking may elicit some pain and weakness initially but with calculated persistence, this is overcome and walking is easily sustainable for long periods of time.  I am even able to ascend moderately large hills, following such a warm up and using efficient “management” of the terrain and my energy.

The purpose of this page is to give physicians or other health professionals as clear a picture as is possible of what the symptoms of this disease are like.  The more is known about the disease, the better the quality of life for those with it.

Children with McArdle’s Disease are at significant disadvantage.  Children aren’t able to articulate themselves as well as adults and their symptoms are easily confused with being lazy or stubborn or simply uncooperative.  Children who are lagging behind because they are experiencing painful muscle failure from McArdle’s Disease may find their discomfort compounded by scolding or teasing from classmates or teachers. At some point, all children endure some form of teasing from friends or siblings or classmates.  However, those children more vulnerable to teasing for whatever reason – small stature, weight issues, appearance, or anything else – may be singled out, and excessive attention of this kind is unhealthy.

It is incumbent upon grade school physical education teachers, nurses, pediatricians and anyone else in a position of relevant authority to educate themselves on McArdle’s Disease and other conditions whose symptoms are easily mistaken for behavior issues or lack of cooperation.  Even the most observant teacher cannot tell simply by looking at a child whether or not their muscles work properly, and the way the symptoms of McArdle’s Disease manifest is deceptively similar to a simple lack of physical condition or laziness.

How to Know if a Child Has McArdle’s Disease

Here are some things you may observe:

  • Does the child complain of weariness on even moderately long walks?
  • Do smaller children complain of weariness and ask to be carried?
  • Do hills, stair cases, and other inclines elicit the responses above?
  • Does the child complain of “pain” in the legs in such incidents?
  • Does the child have difficulty performing even an average number of sit-ups or push-ups, even in a modified form?
  • Does the child have difficulty or otherwise avoid running laps in gym class?
  • Does the child demonstrate, in general, a lack of physical fitness that contradicts an otherwise healthy appearance, assuming they are not overweight?
  • Has the child ever complained of or demonstrated unusual muscle stiffness and cramping?
Any of these observations are worth investigating.  Again – children have only responsible adults as their advocates, and cannot reasonably be expected to know that something is wrong with them that they cannot help.  
The symptoms of McArdle’s Disease are subtle but specific.  Your attention and action can mean the difference between years of physical and emotional discomfort, and a more normal childhood.  If you are a physical education teacher who would like to know more about the disease, please contact us for information.