You breathe in. You breathe out. You rarely think about it, except when it comes to exercising. You know breathing is important; it supports your effort and can help make you more efficient. Here are a few simple tips to help you with the simple act of breathing [1], broken down by exercise type.

• Cardio: Avoid shallow breaths since they are often an indicator that you are working too hard. Shallow breathing also indicates that you haven't established a suitable breath pattern for your activity. You want to take stronger and deeper breaths when doing cardio, so take the time to find your rhythm.
• Strength training: Generally you want to breathe out on the difficult part of the lift or move to help stabilize your body during exertion. So you exhale when you are lifting a hand weight to your shoulder during a bicep curl. This helps you engage your core to help prevent your from swaying into your heels, which is cheating and could set you up for a lower back injury. In Pilates classes, we often start exercises with the phrase, "Inhale to prepare" to remind people to breathe in before they "work."
• Stretching: Slow steady breathing is preferable when stretching and will help you relax. Try to focus on breathing from your diaphragm, which will make your belly move out on the inhale and not your chest and shoulders.

Ready. Set. Breathe.

Source [2]