A lot of people who run with heart rate monitors often see their heart rate spike at the beginning of a run, only to subside after a mile or two. This kind of spike only happens if you didn’t warm up long enough.

• If the body isn’t warmed up, there is little blood flow to the muscles (and therefore little oxygen).
• For a short period of time, the muscles have to work anaerobically, increasing the heart rate.
• The body rushes to shunt blood away from the organs and towards the muscles. This is a major stressor.
• The spike in heart rate subsides when increased blood flow (and the oxygen that comes with it) allows the muscles to work aerobically.
• Therefore, the spike itself is an indicator that your heart rate was inadequate.
• It takes 12-15 minutes for the body to warm up properly.

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Before starting a bout of exercise, our body’s internal machinery is largely inactive. The metabolism is working at a very low level, the big muscles are relatively quiet, and blood is moving largely within the core—cycling through the various organs, and back to the heart and lungs.

Muscles are fed by vast networks of capillaries—tiny blood vessels existing within the muscles themselves—which ensure that blood goes to and from every muscle cell. During rest, the majority of these capillaries are constricted. Very little blood goes in or out of the muscles.

This eases the demand on the heart during rest: constriction of the capillaries and peripheral blood vessels means that the overall volume of the cardiovascular system is greatly reduced. The heart doesn’t need to pump very hard to maintain blood pressure, which keeps the heart rate relatively low.

During exercise, muscles demand a huge volume of blood flow, and so the capillaries dilate to accommodate it. But the body isn’t designed in such a way that the capillaries can expand pre-emptively. They expand due to exercise itself. Asking the body is asked to exert itself from a cold start can be a major stressor: it has to drain blood from major organs abruptly and it has to shove them into muscles whose capillaries haven’t dilated yet—a process that can send the body into shock.

Because of this, a proper warm-up—a period of very low-intensity activity—is important for all exercise, but is critical for running: Every step we run, our legs have to break our fall. It takes a big use of the muscles to make this happen.

Without proper blood flow, the muscles are on their own. If the capillary networks haven’t yet expanded, very little blood is getting to the muscles for those first few minutes. This is a problem because blood carries oxygen. No blood, no oxygen. But even without oxygen, the muscles still need to find a way to perform the required activity. In this situation, the only way to accomplish this is by working anaerobically.

I’ve written before how anaerobic work is intricately tied to the stress response: when the body is under stress, it raises the heart rate and kicks up the functioning of the anaerobic system—which is able to provide energy at a massive rate—in order to deal with a presumed threat to its existence. The connection between stress, anaerobic activity, and a high heart rate runs deep: if any of the 3 increases, the other two will follow.

The observed spike in heart rate is a direct indicator of increased anaerobic activity.

It subsides after a mile or two is because it typically takes the aerobic system 12-15 minutes to activate completely. Blood finally pervades the muscles, bringing oxygen and allowing the aerobic muscle fibers to do their thing.

Heart rate drops to a manageable level once the aerobic system is in play—and to the degree that it comes into play.

Here’s the important part: A spike in heart rate doesn’t just tell us that our aerobic system wasn’t fully on yet. It also tells us that our warm-up was inadequate. The spike in anaerobic activity means that blood and oxygen was largely absent from the muscles. The body was forced to rush to bring blood to the muscles. Blood was hastily drained from the organs, and unceremoniously shoved through capillaries that hadn’t yet expanded to accommodate it.

That amounts to a lot of unnecessary stress.

The solution: warm-up for longer.