Now, after studying how people move in my corrective-exercise practice for close to 20 years (yikes!), I think that a lack of flexibility closes down more lives than a lack of strength. And I think that more frequently than not, the lack of flexibility leads to the loss in strength, usually more so in the opposite muscle to the tight one. (Tight hip flexors lead to weak glutes for example).
Strength-declines follow flexibility-declines, because people tend to stop doing activities which become awkward due to a lack of flexibility, such as getting down onto the floor. Strength is then lost due to disuse. If one does not move regularly through the existing range-of-motion, often all the muscles around a joint become tight impeding function further. Which then decreases strength further. And of course, the less flexibility one has, the less available motion there is to strengthen.
If there is very little slack in the system, it no longer takes much to pull a joint out of its optimal axis of rotation. If the give in the system is no longer adequate, pain is more likely to occur. Simply restoring adequate range of motion can go a long way to decreasing pain, because it puts slack back in the system and makes it more forgiving.
Also too much tightness creates too much compression at the joint, potentially adding to a wear problem within the joint itself, particularly if that joint is not in its optimal alignment.
In most weight-training programs, the emphasis is placed on lifting heavier weights, as opposed to increasing the range of motion. The usual way of progressing the squat, for example, would be to increase the weight as strength improves. But usually people compromise their range of motion as the weight feels heavier and heavier.
The best way I have found thus far to increase flexibility without compromising on the strength-training component of an exercise program, is to work the exercise to the maximum range of motion without allowing any compromise in form, and set up the exercise in a way that forces maximum range of motion with every repetition.
I use box squats to force the full range-of-motion on every repetition. I use stackable stools as my “box”, and determine the least number of stools my client can sit on while leaning forward as if to get up, maintaining a neutral spine, feet flat on the floor etc. Usually this is lower than most people tend to squat to in a gym. The exercise is to stand up, then sit down for the appropriate number of reps using a weight that is challenging and yet does not compromise form.
The first priority goal for progressing is to lower the “box” by taking away stools as flexibility improves. Usually this makes the exercise significantly harder, so increasing the weight is not necessary. If the box cannot be lowered because doing so would cause the low back to round or the knees to roll in, the weight can be increased instead.
For most people that are not accustomed to exercise, the starting weight is their bodyweight. I find often on set 3 or 4, once the tissues are good and warm and the joints are well lubricated, I can take away a stool, thereby increasing the range of motion. The long-term goal for weighted squats is to get to 1 stool (about 9 inches from the floor) or slightly lower using a step, as long as the spine can be held in neutral throughout the range of motion for any weighted squat.
The long-term unweighted goal is to be able to squat to the floor and feel able to stay there for extended periods of time. The low back will round in this position, which is fine for healthy backs.
For those that regularly do squats in the gym, forcing the range-of-motion in this way will mean a huge decrease in weight. But in my opinion, gaining that range-of-motion is a far worthier goal than pushing a heavy weight, as it will translate into an ease in movement in daily life, and far less pain as the slack in the system is restored. And let's face it. How useful is it really, to be able to squat a gazillion pounds through a short range of motion anyway? Do you regularly carry your fridge on your back?