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.