Beta blockers slow your heart rate, which can prevent the increase in heart rate that typically occurs with exercise. This means that it might not be possible for you to reach your target heart rate — the number of heartbeats per minute you aim for to ensure you're exercising hard enough.
No matter how hard you exercise when taking a beta blocker, you may never reach your target heart rate. Keep in mind that being unable to reach your previous target heart rate doesn't mean you're not getting cardiovascular benefits from exercise.
There's no precise way to predict the effect of beta blockers on your heart rate. An exercise stress test, which checks blood flow through your heart while you exercise, can measure how hard your heart pumps while you're taking beta blockers. Your doctor can use this information to adjust the target heart rate you should work to.
If you don't have an exercise stress test, you can use a perceived exertion scale, such as the Borg Rating of Perceived Exertion Scale, which relies on your judgment of how hard you're working based on effort, breathlessness and fatigue. Ask your doctor for help finding and using an exertion scale.
For most workouts, your best bet is to aim for physical activity that feels somewhat hard — it takes effort but you can continue. If you can't talk while you're exercising, you're probably overdoing it.