Warm up before each workout
Research has determined that soft tissue injuries are significantly reduced if a person warms-up for a period of 5 to 10 minutes prior to beginning a resistance training program.
Studies have also shown that while resistance training can infrequently cause an increase in the number of arrhythmia experienced during a workout, warming-up prior to resistance training can significantly reduce the occurrence of these arrhythmia.
Finally, it is well known that warm muscles can perform better, and neural tissue can react faster at the higher temperatures that are a result of warming-up.
Bottom line; make sure to warm-up before working out.