On the flipside of underperforming, is pushing yourself too hard. You always need to understand and be aware of your limitations. Injuring yourself can put you out of the game for weeks! Make sure to take the time to use proper form. If it’s a really hard one, try a modification of the exercise.
If you injure a part of your body, you can still work around the injury (after consulting a doctor!), but there are some key points to remember. Generally speaking, if you have sustained an injury in your upper body, such as a rotator-cuff injury or torn biceps, for example, you can most likely do high-intensity, lower-body cardio like running, step, jump rope, elliptical, spinning, and so on. You can also continue to do exercises for your lower body, such as lunges, squats, leg presses, or dead lifts.
If your injury is in your lower extremities — knees, ankles, feet, shins, or calves — try low-impact forms of cardio such as swimming, recumbent biking, rowing, and shadow boxing with light weights in each hand. Again, generally speaking, you can also continue to do resistance-training moves for your upper body, such as back, chest, abs, and arm exercises.