One of the most fundamental principles in fitness is progressive overload: gradually increasing workout stress over time, so that your body adapts positively to that increase. Perhaps that’s adding five pounds to your squat each time you lift to build strength, or lengthening successive runs to go from a mile to a marathon.

But while overload is easy on paper, it’s far more complicated in real life. Human bodies don’t adapt linearly in even the best of conditions, and progress is even more unpredictable once you factor in life stress, travel, lack of sleep, or a night of heavy drinking and too much dessert. Continuing to overload beyond what your body can keep up with leads to overtraining, which in turn causes illness and injuries, setting progress back.

So as you move forward in training, it’s useful to be able to monitor how well your body is adapting. While there are a number of approaches that work, one of the simplest and most empirically validated is tracking heart-rate variability (or HRV).

We tend to think of our heart as beating in a steady tick-tock. In reality, each beat varies a bit from the last. In fact, a healthy heart has a great deal of variability, whereas increasing regularity (as data from the Framingham study and others have consistently shown) drives increasing risk of heart disease.

Heart-rate variability results from the balance between the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is like the gas in a car, revving our bodies up for increased output, whereas the parasympathetic is like the brakes, bringing us down into rest and relaxation.

When the sympathetic nervous system overwhelms the parasympathetic, your heart-rate variability decreases. And, similarly, when your sympathetic nervous system overwhelms the parasympathetic, you’re on the road to overtraining.

As a result, monitoring heart-rate variability is a great way to simultaneously monitor overtraining.

While, previously, measuring HRV required specialized equipment (whether an EKG or a chest-strapped heart-rate monitor), the brilliant folks behind the app HRV4Training recently developed and clinically validated an approach to measurement using just your smartphone.

The way it works is simple: each morning, right after you wake up, you hold your finger over the phone’s camera lens for one minute. From that, the app determines your HRV for the day, compares the number to your moving averages over the past seven days and two months, and kicks out a simple recommendation: something like “go ahead and train, but limit intensity,” or “if you planned intense training, go for it.”

HRV provides a great window into how you're adapting to the progressive overload of workouts, and it’s a powerful tool in helping to keep you healthy and injury-free, moving forward over the longer haul.

So download HRV4Training, and overload yourself, just the right amount.