1. Lifting too much weight too soon.
This usually applies for newbies. You are increasing the risk of torn ligaments, tendonitis, arthritis at the joints, and stress fractures to name a few. The more weight you lift, the greater the risk is. If your muscles are adequately stretched/warmed up, you are not going to hurt yourself. Lifting heavier is the only way to increase your strength and build some mass but you will do more damage than good if you lift much heavier than your muscle can handle.
How heavy is too heavy?
If you can’t finish the set with a good form, reduce the weight.
2. Not stretching before and after a workout.
Don’t stretch your muscles the moment you arrive at the gym. Do it after warming up. Stretching your muscles improves the range of motion of your joints. When the muscle tissues are too tight, a little tear could happen if you force a joint to move through its full range of motion. Warming up and stretching can prevent this from happening.
3. Not warming up before and cooling down after a workout.
Stretching is not warming up. It is really important that you perform a warm-up before stretching. It is not a good idea to perform stretching before your muscles are warm enough. Rotate your joints (from toes to your finger) followed by at least five minutes of aerobic activity. For each weight exercise, perform one or two sets of exercise with lighter weight to warm-up the muscles.
4. Not working out within the target heart range.
To get the maximum benefits of cardio, you need to workout within the target heart zone three or four times a week. Exercising at this level promotes weight loss and burns fat. It reduces the risk of coronary heart disease, obesity, and other health related effects. Doing cardio at least 3 times a week also helps lighten up your mood. If you are a beginner, start at a lower target heart zone (perhaps 50% of your maximum heart rate). As your fitness improves, exercise at higher intensity from 60 to 90 percent of your maximum heart rate.
5. Running too far too soon.
Be moderate. Run too far and you can get injured and lose your motivation. To measure your fitness level, use X-minute mile instead of X miles per hour. A beginner will jog from 10 to 15 minute mile. A world-class-trained male athlete might run a 6-minute mile.
The process of building muscle is a trial and error. Most of the theories in bodybuilding are debatable and it can be overwhelming to digest all the information on how to eat and train the right way.