An exercise should be halted if marked or sudden pain is felt, to prevent further injury. However, not all discomfort indicates injury. Weight training exercises are brief but very intense, and many people are unaccustomed to this level of effort. The expression “no pain, no gain” refers to working through the discomfort expected from such vigorous effort, rather than to willfully ignore extreme pain, which may indicate serious soft tissue injuries. The focus must be proper form, not the amount of weight lifted.
Discomfort can arise from other factors. Individuals who perform large numbers of repetitions, sets, and exercises for each muscle group may experience a burning sensation in their muscles. These individuals may also experience a swelling sensation in their muscles from increased blood flow also known as edema (the “pump”). True muscle fatigue is experienced as loss of power in muscles due to a lack of ATP, the energy used by our body, or a marked and uncontrollable loss of strength in a muscle, arising from the nervous system (motor unit) rather than from the muscle fibers themselves. Extreme neural fatigue can be experienced as temporary muscle failure. Some weight training programs, such as Metabolic Resistance Training, actively seek temporary muscle failure; evidence to support this type of training is mixed at best. Irrespective of their program, however, most athletes engaged in high-intensity weight training will experience muscle failure during their regimens.
Beginners are advised to build up slowly to a weight training program. Untrained individuals may have some muscles that are comparatively stronger than others; nevertheless, an injury can result if (in a particular exercise) the primary muscle is stronger than its stabilizing muscles. Building up slowly allows muscles time to develop appropriate strengths relative to each other. This can also help to minimize delayed onset muscle soreness. A sudden start to an intense program can cause significant muscular soreness. Unexercised muscles contain cross-linkages that are torn during intense exercise. A regimen of flexibility exercises should be implemented before and after workouts. Since weight training puts great strain on the muscles, it is necessary to warm-up properly. Kinetic stretching before a workout and static stretching after are a key part of flexibility and injury prevention.