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When weight training I've been told that to avoid muscle inflexibility to always perform lifts through the full range of motion of the joint. Does this extend to isometric exercises, where there is no movement at all?

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I'll try to answer this another way, and state that full range of motion (ROM) exercises do indeed increase flexibility. This is primarily due to the viscoelastic response, where the muscle and connective tissue is pulled taught like a rubber band.

I discussed this a bit on an answer about why squats are harder than deadlifts, the viscoelastic component being a major factor.

Anecdotally, I can speak to your question by saying that a body generally only has the flexibility it needs. If you only did isometric exercises, I would imagine that you would lose flexibility. But provided you are stretching your body in other ways, I believe the range of motion will stay.

Apologies for needing to drop away from empirical and enter less peer-reviewed data, but that's the best I can come up with.

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Do isometric exercises decrease flexibility?

Assuming you mean ROM when you refer to “flexibility”, from the studies I’ve read, isometric exercises do not decrease flexibility. In the International Journal of Sports Physical Therapy (Int J Sports Phys Ther. 2012 Feb; 7(1): 109–119.), a recent article, Current Concepts in Muscle Stretching for Exercise and Rehabilitation, reviewed stretching as used in exercise and rehabilitation. The article discussed Static, Dynamic, and Pre-Contraction stretches. Isometric stretching is a form of Static stretching. As such, the review indicated that

Static stretching is effective at increasing ROM. The greatest change in ROM with a static stretch occurs between 15 and 30 seconds; most authors suggest that 10 to 30 seconds is sufficient for increasing flexibility. In addition, no increase in muscle elongation occurs after 2 to 4 repetitions.

The article goes on to say

Static, dynamic, and pre-contraction stretching are all effective methods of increasing flexibility and muscle extensibility; however, these modes may be more effective in specific populations. Several authors have noted an individualized response to stretching; therefore, stretching programs may need to be individualized.

More research is needed, however, because some studies indicated that

Static stretching often results in increases in joint ROM. Interestingly, the increase in ROM may not be caused by increased length (decreased tension) of the muscle; rather, the subject may simply have an increased tolerance to stretching.

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