The cause is frequently unknown. Hypertrophic cardiomyopathy is often and dilated cardiomyopathy in a third of cases, is inherited from a person's parents. Dilated cardiomyopathy may also result from alcohol, heavy metals, coronary heart disease, cocaine use, and viral infections.
Consider the number of reps. You can also confuse your muscles by increasing or decreasing the number of repetitions you do during any given exercise. For example, if you normally do 12 repetitions, do 15 or 18 repetitions, but with a lower weight than you would normally use.
The causes of a fitness plateau are usually due to the body adapting to the exercise regimen. Muscle confusion is a key part in keeping those muscles from adapting to the same workout all the time. Try to mix up your workouts go to some different classes like kickboxing, kettlebell.