4. Flexibility

4. Flexibility - the ability to maximize the range of motion at a given joint.

The last thing you'll catch me doing in pulling out my goniometer (no Brian, thats not a dirty word) and measuring the specific range of a muscle group. First off, I don't care.  Second, we're not readily concerned with muscle groups as individual entities anyway. The bigger, more overriding question is: are you able to easily and without pain move through a movements full range of motion. If not, then lets look at the factors that are limiting you. It may very well be that you have trouble keeping your knees out over your toes in a squat and one of the causes of that could be tight internal hip rotators.  Then lets address that. There's no reason to just be gumby for the sake of being gumby. 

Off my soapbox - heres the rundown from answefitness.com:

Factors Affecting Flexibility

There are a number of factors that can influence a person’s flexibility, including:

• Genetics
• Connective tissue elasticity
• Composition of tendons or skin surrounding the joint
• Joint structure
• Strength of opposing muscle groups
• Body composition/type
• Gender
• Age
• Activity level
• Previous injuries or existing medical issues

Why Flexibility Is Important

Improving flexibility allows you to perform nearly all activities more effectively, can prevent injury, improve balance and coordination and enhance athletic or recreational sports performance.  Improved flexibility can also make exercise more effective — especially weight or resistance training — because it allows you to perform exercises through a muscle’s full-range of motion, encouraging greater strength and growth.

Measuring Flexibility

Flexibility is measured using two primary methods:

1. Indirect Flexibility Measures: Indirect Flexibility Measures have you perform specific, prescribed manual movements and then measure your relative flexibility while performing those motions. Examples include the Sit & Reach Test, V-Sit Reach Test, Trunk Rotation Test, and Groin Flexibility Test. 
2. Direct Flexibility Measures: Direct methods of measuring flexibility use calibrated measuring instruments like the Goniometer or a Flexometer to measure the flexibility of a joint.

How to Improve Flexibility

The best method to increase flexibility is progressively stretching the muscles surrounding a joint.

There are four forms of stretching that you can perform yourself to improve flexibility:

Dynamic Stretching: Dynamic stretching involves repeated, fluid, gentle, movements that create mild tension, but are not painful or exhaustive. Dynamic stretching is typically used during warm up and cool down periods. Dynamic stretching can be specific, sports related (like swinging a golf club or bat), or a lower intensity activity like walking. Dynamic stretching can also be incorporated into a light warm-up set during weight or resistance training, using a small amount of resistance through the exercises full range-of-motion (for example, squats or chest presses.)

Static Stretching: Static stretching involves holding the muscle in a stretched position for a period of time. The actual stretching movement should be slow and controlled. It is usually held from 15-60 seconds. Static stretching is typically used to improve flexibility in tight muscles by gradually and progressively stretching and lengthening the muscle.

Contract-Relax Method: The Contract-Relax Method has you contracting against resistance from a partner then stretching. This works by inhibiting the stretch reflex and allows you to stretch further than normal.

Contract Antagonist/Relax Method:  This stretching method uses isometric contraction of the opposing muscle before stretching. This operates on a similar principle to working antagonistic muscles back-to-back during weight or resistance training.

Although regular stretching (especially static stretching) is one of the most effective ways to improve flexibility, there is very little evidence that stretching before or after sports or exercise prevents injury. In fact, some research suggests it may actually negatively effect performance.