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.
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.
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.