Last year, I tried my hardest to give zone two training a go. Working out at around 65-75% of your maximum heart rate has been proven to be an incredibly effective way of building cardio fitness, and in the middle of half-marathon training, I was happy to have an excuse to slow things down a little and focus on my form.

But no matter how slow I ran, my heart rate would not stay in zone two. It was like keeping track of a disobedient toddler: one moment it would sit comfortably in range, and the next it would be up to 180bpm. 

Slowing down enough to make it sit steadily around 140bpm – which was just above my zone two threshold – meant I was practically walking. Staying in zone two would have required me to go for a leisurely stroll around the block, which didn’t feel particularly effective for half-marathon training. It was frustrating.  

I know I’m not the only one who has felt this way. A friend who runs regularly has complained countless times about how fast his heart rate shoots up when he starts running, and Strong Women editor Miranda Larbi has also found it hard to nail zone two training.

It seems as if, even among people who exercise regularly, some people’s heart rates seem to be more ‘sensitive’ to working out than others. But why does this happen? And when does it become a cause for concern? We asked a cardiologist to explain what’s going on. 

How high your heart rate goes when you’re exercising is influenced by a few factors. Fitness level is one, but it’s far from the only thing that plays a part.

Dr Oliver Guttmann, consultant cardiologist at HCA Healthcare’s The Wellington Hospital, explains that factors such as age and temperature can also play a role. “The younger you are, the higher your maximum heart rate,” he explains. “Your heart rate may also be increased when exercising in warmer or more humid conditions.” This is because hot temperatures cause more blood flow to the skin, so your heart must work harder to push the blood further.

Stress can also make a difference, due to the impact the stress hormone cortisol has on the body. When you’re in ‘fight or flight’ mode, the hormone triggers a heart rate increase to prepare your body to handle an attack. And if your heart rate is already high before you start exercising, it’ll reach a higher maximum.  

Sex should also be a consideration, Dr Guttmann points out: “Women typically have a slightly higher heart rate than men as men’s hearts tend to be larger than women’s, meaning women’s hearts have to work harder to pump the same amount of blood.”

Research has also found that genetics can also play a role in maximum heart rate, as can your height, amount of muscle mass and any recent caffeine or alcohol consumption. Health conditions such as hyperthyroidism and anaemia will also lead your heart rate to go faster; the latter makes it difficult for red blood cells to carry oxygen around the body, making the heart beat faster to support the flow.

The technology used in most fitness trackers also tends to be less accurate for those with darker skin tones, so it’s worth keeping that in mind.

It’s for all of these reasons that comparing your heart rate to others during exercise isn’t an accurate predictor of fitness – there are so many factors at play. 

If you’re worried about your heart rate being too high when exercising, there’s a simple way to check. “Your maximum heart rate can be calculated by subtracting your age from 220 – however, it may vary 15-20 beats per minute in either direction,” Dr Guttmann says.

Vigorous exercise should be carried out at around 70-85% of your maximum heart rate – so if you’re 35 years old, you should be sitting between 130 and 157bpm.

You don’t need to obsess over your heart rate, but it’s a good idea to be aware of how high it’s going, Dr Guttmann suggests.

“If you go higher than your maximum heart rate for long periods of time it can be detrimental to your health, increasing your risk of experiencing chest pain or arrhythmias,” he says.

“If you’re new to exercise, it’s especially important to keep within your boundaries of your maximum heart rate and stop exercising if you’re experiencing dizzy spells or light headedness.” 

It’s clear that having a high heart rate during exercise isn’t always something to worry about, but when does it become something you should talk to a doctor about?

Some key warning signs are when your heart rate exceeds your maximum (that number found by taking your age away from 220) or when exercise leaves you feeling unwell, dizzy or lightheaded. But anything you’re concerned about is worthy of professional help.

“If you are concerned about your heart rate level when exercising, consult your GP,” Dr Guttmann says. “They may want to refer you to a cardiologist to run tests to ensure it’s not a symptom of a more serious health condition.” 

Images: Getty