In part 1 we talked about how to use specificity for developing strength and in part 2 of this series energy systems were discussed in the same context. Part 3 is going to discuss flexibility.
Specificity and flexibility
Flexibility refers to a muscles elasticity or ability to lengthen. Inflexible muscles restrict movement around the joints they cross, while flexible muscles allow for freedom of movement. Training flexibility is important because, just like all other fitness attributes, if you don't use it - you lose it. As far as specificity goes, this simply means that flexibility training should be done for specific muscles that cross joints.
Passive and active ranges of motion
Most flexibility training I've seen done focuses on increasing passive ranges of motion, but neglects or glosses over active ranges of motion. Passive range of motion is the range of motion available across a specific joint with assistance, such as when you lie on your back and have someone bring your leg as far forward as possible to stretch out the hamstrings muscle group. Active range of motion refers to the range that is available when you are using only muscular power (no other assistance including gravity) to create the movement.
.Why does this matter?
Everyone has some discrepancy between how far they can actively move through a given range of motion and how far they can do so passively. A little difference is normal and fine. A large difference between the two ranges signals an increased risk of injury. This is due to the simple fact that there is a larger range of motion freely available that isn't under muscular control. I feel like a little more explanation will be useful to many readers at this point. Returning to the lying hamstring stretch example, imagine two individuals for comparison. The first individual can actively bring her leg up to 90 degrees, but if she hooks her foot with a skipping rope she can bring it another 30 degrees. The second individual can also actively bring her leg up 90 degrees, but she can only passively move it another 10 degrees. The first girl has a full 30 degrees of motion that she can't control, while the second only has 10 degrees of motion out of her control. Movements occurring in the range outside of muscular control are more likely to result in injury. In this case the first girl has the advantage of a greater total range of motion, but the disadvantage of more range of motion outside of her active control as well. She would do well to first work on improving her active range of motion as opposed to more passive stretching further exacerbating her imbalance. The second girl may have a perfectly good range of motion for her activities, or she may need to increase her flexibility (again it comes back to specificity - she has plenty of range to safely walk around, but insufficient for high-end ballet)
How to improve flexibility
As noted above, passive flexibility training is the kind most commonly seen where a stretch is relaxed into and held for a period of time often around 30 seconds. This is a great way to maintain range of motion and relax muscles after a workout. To really increase the range of motion in particularly tight muscles this same technique works over time, but the stretches may need to be held longer for optimal results. After about 60 seconds it is likely that the point of diminishing returns is reached and holding the stretch longer still will have little benefit. Also holding a stretch to short, such as bouncing into and out of it will actually reflexively tighten the muscles and have the opposite effect of that which is desired.
Training active ranges of motion involves doing exercises through full ranges of motion as the most basic starting point. Sticking with the hamstrings, Romanian deadlifts through a full range of motion will strengthen the hamstrings along their full length. when the nervous system recognizes that a muscle is capable of returning from a specific range of motion it will actively allow it to go into that range. If the nervous system is "unsure" of a particular muscles ability to get back out of a specific range it will inhibit that range for safety reasons.
For more advanced ranges of motion techniques such as positional isometrics may be used where you take your muscle to the limit of it's range of motion and statically contract it against an immovable object to increase strength at the terminal range.