According to SynerStretch, there are three factors to consider when determining the effectiveness of a particular stretching exercise:

1. isolation
2. leverage
3. risk

Isolation

Ideally, a particular stretch should work only the muscles you are trying to stretch. Isolating the muscles worked by a given stretch means that you do not have to worry about having to overcome the resistance offered by more than one group of muscles. In general, the fewer muscles you try to stretch at once, the better. For example, you are better off trying to stretch one hamstring at a time than both hamstrings at once. By isolating the muscle you are stretching, you experience resistance from fewer muscle groups, which gives you greater control over the stretch and allows you to more easily change its intensity. As it turns out, the splits is not one of the best stretching exercises. Not only does it stretch several different muscle groups all at once, it also stretches them in both legs at once.

Leverage

Having leverage during a stretch means having sufficient control over how intense the stretch becomes, and how fast. If you have good leverage, not only are you better able to achieve the desired intensity of the stretch, but you do not need to apply as much force to your outstretched limb in order to effectively increase the intensity of the stretch. This gives you greater control.

According to SynerStretch, the best stretches (those which are most effective) provide the greatest mechanical advantage over the stretched muscle. By using good leverage, it becomes easier to overcome the resistance of inflexible muscles (the same is true of isolation). Many stretching exercises (good and bad) can be made easier and more effective simply by adjusting them to provide greater leverage.

Risk

Although a stretch may be very effective in terms of providing the athlete with ample leverage and isolation, the potential risk of injury from performing the stretch must be taken into consideration. Once again, SynerStretch says it best: Even an exercise offering great leverage and great isolation may still be a poor choice to perform. Some exercises can simply cause too much stress to the joints (which may result in injury). They may involve rotations that strain tendons or ligaments, or put pressure on the disks of the back, or contain some other twist or turn that may cause injury to seemingly unrelated parts of the body.

Some Risky Stretches

The following stretches (many of which are commonly performed) are considered risky (M. Alter uses the term `X'-rated) due to the fact that they have a very high risk of injury for the athlete that performs them. This does not mean that these stretches should never be performed. However, great care should be used when attempting any of these stretches. Unless you are an advanced athlete or are being coached by a qualified instructor (such as a certified Yoga instructor, physical therapist, or professional trainer), you can probably do without them (or find alternative stretching exercises to perform). When performed correctly with the aid of an instructor however, some of these stretches can be quite beneficial. Each of these stretches is illustrated in detail in the section X-Rated Exercises of M. Alter:

the yoga plough

In this exercise, you lie down on your back and then try to sweep your legs up and over, trying to touch your knees to your ears. This position places excessive stress on the lower back, and on the discs of the spine. Not to mention the fact that it compresses the lungs and heart, and makes it very difficult to breathe. This particular exercise also stretches a region that is frequently flexed as a result of improper posture. This stretch is a prime example of an exercise that is very easy to do incorrectly. However, with proper instruction and attention to body position and alignment, this stretch can be performed successfully with a minimal amount of risk and can actually improve spinal health and mobility.

the traditional backbend

In this exercise, your back is maximally arched with the soles of your feet and the palms of your hands both flat on the floor, and your neck tilted back. This position squeezes (compresses) the spinal discs and pinches nerve fibers in your back.

the traditional hurdler's stretch

This exercise has you sit on the ground with one leg straight in front of you, and with the other leg fully flexed (bent) behind you, as you lean back and stretch the quadricep of the flexed leg. The two legged version of this stretch is even worse for you, and involves fully bending both legs behind you on either side. The reason this stretch is harmful is that it stretches the medial ligaments of the knee (remember, stretching ligaments and tendons is bad) and crushes the meniscus. It can also result in slipping of the knee cap from being twisted and compressed.

straight-legged toe touches

In this stretch, your legs are straight (either together or spread apart) and your back is bent over while you attempt to touch your toes or the floor. If you do not have the ability to support much of your weight with your hands when performing this exercise, your knees are likely to hyperextend. This position can also place a great deal of pressure on the vertebrae of the lower lumbar. Furthermore, if you choose to have your legs spread apart, it places more stress on the knees, which can sometimes result in permanent deformity.

torso twists

Performing sudden, intense twists of the torso, especially with weights, while in an upright (erect) position can tear tissue (by exceeding the momentum absorbing capacity of the stretched tissues) and can strain the ligaments of the knee.

inverted stretches

This is any stretch where you "hang upside down". Staying inverted for too long increases your blood pressure and may even rupture blood vessels (particularly in the eyes). Inverted positions are especially discouraged for anyone with spinal problems.