Exercise is known to alleviate anxiety. But what happens when working out makes you stress?

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Earlier this year, Lena Dunham struck a chord when she spoke about the positive impact exercise had on her mental health.

“To those struggling with anxiety, OCD, depression: I know it’s mad annoying when people tell you to exercise, and it took me about 16 medicated years to listen … it has helped with my anxiety in ways I never dreamed possible,” the author wrote on Instagram.

RELATED: The first thing you should do if you’re depressed or anxious.

Dunham’s not alone here; research has established the value of regular physical activity for people living with mental illnesses.

People who exercise have been found to experience less depression and anxiety symptoms than those who don't, while a study earlier this year found a link between a sedentary lifestyle and an increased risk of anxiety. Physical activity can improve sleep, increase energy levels, elevate mood and balance mood swings, decrease stress, and distract from worrying thoughts.

For some people, however, exercise can actually be a source of anxiety because its physiological processes — increased heart rate, spike in adrenalin, quicker breathing, sweating — can be disconcertingly similar to those of a panic attack.

RELATED: Emma Stone explains how she manages her panic attacks.

Blogger Summer Beretsky, who lives with anxiety, feels these affects so acutely she's come to actually fear exercise.

"The rapid heart rate reminds me of my worst ohmygod-I-swear-this-is-a-heart-attack breed of panic attacks. I am always afraid that the quick breathing will make me pass out — even though I know the extra oxygen I’m inhaling is 100 per cent necessary, normal, and natural," Beretsky writes on PsychCentral. (Post continues after gallery.)

Although anecdotally this phenomenon isn't unheard of, research into it is fairly sparse. However, a 2010 study in the journal Hippocampus examined the effect of exercise on the emotional behaviour of adult mice, and found the mice exhibited some signs of stress and anxiety-like behaviours.

Joe Bonington, strength and conditioning trainer and founder of Sydney adventure gym Joe's Basecamp, has witnessed this reaction in people who are predisposed to anxiety.

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One of his clients who took up running found her breathing would get "all over the place", which in turn would make her anxious and prompt an attack. Another has an extreme fear of lifting objects above her head, including weights. Bonington says even a workout with a highly competitive atmosphere, like Crossfit, can be triggering for someone who has anxiety.

Why does it happen?

Dr Mandy Deeks, psychologist at Jean Hailes for Women's Health, believes anxious feelings resulting from exercise are likely related to an individual's level of fitness, and their sensitivity to their body and its reactions.

"People with anxiety are quick to think in a negative way, that something really bad might be about to happen or that they are somehow in danger. If intense activity restricts breathing or makes someone feel like their body is out of control, then yes — this may set off thoughts that something bad is happening," Dr Deeks says, adding that she hasn't seen this exercise-induced reaction in a patient before.

RELATED: This is what it actually feels like to have a panic attack.

"They also tend to have a heightened awareness of their body and are super tuned in to changes to breathing rates, feeling stirred up and sweating."

Bonington, who experienced debilitating anxiety attacks in his twenties, says people develop this awareness after their first attack. "It becomes a self-fulfilling prophecy: every time your heart starts getting elevated, you then worry you're having an anxiety attack. Once that starts your heart starts racing even further then you think, 'Oh my God, now I'm having a heart attack'."

How to reduce exercise-induced anxiety

If exercise causes you distress, it might seem logical to eliminate it entirely. Yet regular exercise is so beneficial for the management of anxiety in both the short- and long-term; even a brisk walk is enough to improve your thinking, mood and memory.

So, depending on how severe your anxiety is, sticking with exercise is generally a good idea. There might be discomfort at first, but choosing the right exercise, easing yourself in, and finding strategies to build yourself up slowly will help.

RELATED: How to ease yourself back into exercise when you've seriously dropped the ball.

Dr Mandy Deeks advises taking your level of fitness and general health, along with any injuries, into account when deciding on the type, amount, and intensity of exercise you feel is achievable for you. Pick something you enjoy and that motivates you and makes you feel good when you've finished, and don't base that decision entirely on anyone else's opinion.

"Telling someone they should walk or run or play tennis or swim for a set number of days per week can set someone up for failure, and that is the last thing you would want to do... If you have never liked running then that may increase anxiety even more. No one needs to put added pressure on themselves," Dr Deeks says.

If you have your heart set on a particular activity, there are ways to ease yourself in without stressing yourself out. For instance, if you love to run but find the elevated heartbeat panics you, Joe Bonington suggests paring it right back to a walk — from there, you can very gradually build yourself up to power-walking, and eventually jogging.

RELATED: What not to do when someone is having a panic attack.

Dopamine, produced by the body during exercise, is tied to feelings of reward and achievement. So by ensuring you do some form of workout, big or small, you'll build up your confidence and make exercise in general feel more achievable.

To ward off any potential panic attacks, Dr Deeks recommends learning some relaxation techniques. Practising the feeling of being 'calm and relaxed' as a skill — and understanding how it feels compared to feeling anxious — means you can call on it when panic starts showing its head.

"This calms breathing, slows the heart rate down and the body’s response to fear so that you are able to think more clearly and challenge some of the unhelpful thoughts that can come with anxiety," Dr Deeks explains. (Post continues after gallery.)

How to deal if you have a panic attack during a workout

If you find yourself in a heightened state of anxiety during a workout, stop what you're doing and take a step back. It's important to focus on regaining control over your breathing.

“Often when we experience a sensation as if we are going to suffocate, we try to breathe faster in order to get more air, and it results in letting off too much carbon dioxide, which might result in you feeling agitated. Try abdominal breathing for a few minutes," Yuliya Richard, a psychologist at Blue Horizon Counselling, tells The Glow.

More generally, learning about what happens to your body during a panic attack can help you rationalise and process thoughts you might have during one.

“Imagine that you are just observing the sensations come and go, they are not dangerous, they are simply uncomfortable and they will pass,” Richard says.

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Also, don't hesitate to speak to your personal trainer or any gym staff you work with about your anxiety. "They can help you and let you know it's going to be all right. People are very understanding of mental illnesses and anxiety," Bonington says.

Does exercise make you feel anxious? How have you coped with this?

If you're living with anxiety, there are some great online resources to help you manage and understand it — Jean Hailes for women's health's anxiety online resource and Beyond Blue's anxiety facts page are both great places to start. If you need to talk to someone about your anxiety, or any other mental health issues, speak to your doctor, call Beyond Blue's helpline on 1300 22 4636 or use their web chat for immediate support between 12pm and 3am.