Precision Medicine: The Art of Exercise Dosing

By: Domenic Fraboni

A core principle in pharmacology is the dosage/response curve. Pharmacologists endlessly test drugs to discover the effective dose, toxic dose, therapeutic index, and other important dosages that they can use with patients. The difficulty is, not everyone responds to these drugs in the same way. What may be an effective dose for one individual could cause more severe adverse reactions in another individual and possibly have no effect at all in a third patient. Furthermore, some patients may have other unrelated conditions that could affect drug absorption, distribution, metabolism or elimination. These four areas could also be affected by other drug-drug interactions. One drug the patient is already taking could slow down the metabolism of a new drug, resulting in a lower toxic dose of the new drug for that patient. This reflection will be focused on how exercise dosing is very similar to drug dosing because of the fact that there are so many factors that can impact an individuals response to exercise.

First, there are a few responses that our tissues can primarily have to exercise. If we stress the tissues past their normal level, we can damage the tissue, which then may undergo hypertrophy (or be rebuilt to be able to handle that load in the future). If the stress placed on the tissue is too large and held for too long a duration, this can cause tissue destruction. This is the equivalent of a drugs toxic dose for exercise. We can also dose exercise for maintenance. If we do activity and exercise consistent with what the patient normally does on their average day, this would cause no change in the tissue but help the patient maintain their fitness level. The final dosing of exercise is too low a dose. With this dose the patient will respond with tissue atrophy (muscle, ligaments, bone) and reduction in overall cardiovascular health. This would be similar to taking too low a dose of medication for an infection so that the medication would not be able to completely eliminate the pathology.

Similar to the need to be aware of drug-drug, drug-comorbidity, and drug-exercise interactions in pharmacology, we must always be aware of drug-exercise, exercise-comorbidity, and exercise-activity interactions in exercise dosing. As soon as a patient enters our clinic, we must be aware of what drugs they are consuming. Certain drugs may reduce tolerance to resistance exercise while others will reduce tolerance to aerobic or cardio exercise. For example, a patient who has hyperthyroidism, and is taking medication for their condition, may have some heat intolerance and muscle fatigue if they received their medication recently. It would be important to dose their exercise according to where they are in their cycle of supplemental thyroid administration. Similarly, someone who has been taking corticosteroids for an extended amount of time may be undergoing muscle wasting. It will be important to dose their exercise based on their initial response. As physical therapists it will be important to also be aware of any comorbidities, or other health conditions, our patients have. If you are trying to improve cardiac health in and individual with severe osteoarthritis in their knees, you probably will not put them on the treadmill and have them run five miles. It would be more appropriate to find different, more comfortable ways to work on cardiorespiratory health with that patient. Finally, we always need to be dosing exercise for our patients with the end in mind. This is where I’ll talk about the “exercise-activity” interaction. We need to be able to gauge the response that our patients will have to a certain dosing of exercise. If they are very active during the day and need to be on their feet, we won’t do heavy eccentric leg exercise (as to prevent delayed onset muscle soreness). Depending on what stage of tissue healing and what their physical daily requirements are, we may adjust the dosing of exercise to optimize their function.

As I have illustrated, the dosing principles in exercise can be seen as very similar to the principles of drug dosing. There can be toxic and therapeutic doses. You must be aware of the comorbidity interaction with exercise. Drug dosing can also cause some serious side effects that physical therapists much be aware of when dosing exercise. The unfortunate truth is that there is no exact dosing principle for administering exercise. It must be done on a case-to-case and patient-to-patient basis. As physical therapists, we are the pharmacologists of exercise.

Acknowledgements: I want to give a quick thank you to our pathopharmacology professor, Dr. Nathan Hellyer, for teaching us both about the principals of drug dosing and how that applies to principals of  exercise dosing and application.