Weight training can result in an increase in joint flexibility or it can have the opposite effect – a decrease in flexibility. It depends not only on how the exercise is done, but on how much weight is used and the range of motion over which the weight is moved. Also, adaptations to weight training with beginners are different from the level or kind of adaptation by more experienced trainees.
If you use relatively light weights so that you go through a full ROM, you will be able to increase your flexibility. In exercises such as lateral arm raises when you go through a full ROM so that the arms end up directly overhead you can increase shoulder flexibility. You can do the same in front arm raises, lateral prone raises, back raises, etc.
Exercises such as reverse trunk twists are excellent for increasing rotational flexibility of the spine, as well as strengthening the internal and external obliques. To increase the ROM in supination and pronation, use a Strength Bar at full length. There are many other exercises -- if not most -- that can be done through a maximum ROM that will result in an increase in flexibility or maintenance, once an optimal range of motion is established.
As you increase repetitions, sets and/or use greater resistance, your weight training will result in a loss of flexibility. There are several reasons for this:
1. The greater the number of repetitions and/or sets, the greater is the tendency to shorten the ROM.
2. The greater the total number of repetitions the greater is the tendency for the muscle to tighten up and shorten after the work.
3. When you handle very heavy weights you rarely fully extend the limbs because of the loss in mechanical advantage of the muscles.
4. The use of heavy weights brings about residual tonus in the muscles, which, when sufficiently strong, keeps the muscles in a shortened state after the workout.
When you use a greater number of repetitions and/or sets, you will invariably find that as you approach the last repetitions or sets, ROM will be decreased. This typically occurs when fatigue begins to set in or when the muscles begin to tighten from the amount of work being done. The more work you do, the greater will be the likelihood of a decrease in flexibility in the joints affected.
Because of this, all heavy or intense weight training programs should be supplemented with stretching, preferably after the workout and that the stretching is active in nature. This is especially important when the spine is involved in weight bearing and may become compacted, as for example, when holding weights on the shoulders or overhead. Active stretching at this time can be done to regain the normal ROM in the involved joints.
For example, if you do multiple sets of the biceps curl, you can do a straight arm hang on a high bar to regain the straight arm position. For the lower back, hanging is also very beneficial, whether it be on a high bar to get a full stretch of the spinal vertebrae, or on the Yessis Glute-Ham-Back Machine in which you hang down from the hips with the trunk vertical (inversion).
Keep in mind that the stretching at this time is not for an increase in flexibility; it is merely to regain the normal ROM in the joints that you had prior to the exercise. Stretching after the workout is also effective for the prevention of muscle soreness or to decrease the severity of muscle soreness.