As physical therapists, exercise is a vital part of our practice. We are the movement experts! With that in mind, dosing the intensity of our exercise interventions effectively should be a priority.
Why We Prescribe Isometrics
Isometrics are a widely prescribed type of muscle contraction, with several benefits described in the literature:
- Safely loading tissue, such as in post-operative patients
- Preserving muscle tissue and avoiding atrophy
- A proposed analgesic effect, leading to less pain while still loading tissue
One downside of isometric contractions can be patient specific. Explaining isometric contractions and having patients perform them effectively can be a challenge. I’ve often seen patients use compensatory motions, or push so hard I fear they’ll put a hole in the wall.
What the Research Found
A recent article in Orthopaedic Physical Therapy Practice (OPTP) explored how accurately people can estimate their maximal voluntary isometric contraction percentage (%MVIC). Sixty-seven subjects performed isometric shoulder abduction at different %MVIC levels, and the actual force produced was measured with a handheld dynamometer.
The findings generally showed that subjects undershot or overshot the target %MVIC. Speaking from experience, this is no surprise.
Image spot (optional): isometric shoulder exercise against resistance
What This Looks Like in the Clinic
In my experience, isometric contractions performed by patients tend to be “failed” concentric contractions. In other words, patients try to perform a motion and fail, creating an isometric contraction. This often leads them to overshoot the target %MVIC.
Patients can also undershoot. One example I see often is shoulder external rotation after a rotator cuff repair. Patients often use compensatory motions such as supination or abduction to complete the movement, producing less than the desired %MVIC.
So What Do We Do?
With this information, what should we do as PTs? Stop prescribing isometrics? Change our cues? Use handheld dynamometers to be accurate?
Reference
Smith J, Pointer O, Summerlin C, et al. Can Healthy Adults Accurately Estimate Various Percentages of Their Maximal Voluntary Isometric Contraction? Part Two. Orthopaedic Physical Therapy Practice. 2026;38(3).
