Quoted from a book I own:
Edit: Read everything before you judge
1. Bodyweight Training Requires Very Little Equipment
There has never been a system of strength training more perfectly in harmony with the principles of independence and economy, and there never will be. Even the most ardent weightlifter will have to admit this fact.For the master of calisthenics, his or her body becomes a gymnasium. Most exercises require no equipment, although if you wish the exercises can be enhanced with a few items that can be found lying around in almost any home. The very most you'll need is a place to hang from, and every one of us can locate such a place if we look; stairs, a loft hatch, even the branches of a tree! No gym is required, and very little space-at most, the equivalent to the length of your own body, often even less.
Whereas other strength training systems use metal weights, cables, chains or machines to produce resistance, the vast majority of calisthenics exercises exploit a free form of resistance-gravity. With no gym or equipment required, there is nothing to store away; no clutter. Plus, it means that you can train anywhere you happen to find yourself-on holiday, in a different city, at work-anywhere. You aren't tied to specific locations. This factor is precisely why calisthenics has survived and thrived in prisons, where equipment is minimal and a prisoner could be moved anywhere- even solitary confinement-at a moment's notice. Another big plus is that calisthenics training is free. No equipment means no financial investment, and no gym means no membership fee. Ever.
2. Bodyweight Training Develops Useful, Functional Athletic Abilities
Calisthenics is the ultimate in functional training. This is another one of the reasons it's so popular with convicts-when trouble kicks off, you need to be able to really move in prison. "All show and no go" might be okay in a nightclub, but in the Pen you had better be able to handle yourself. In nature, the human body doesn't need to move barbells or dumbbells around. Before it can move anything external at all, it has to be able to move itself around!
The legs need the strength to be able to easily manage the weight of the torso in athletic motions, such as running or in combat; the back and arms require the power to be able to pull or push the body up or away. It's sad to see that so many modern bodybuilders don't understand this fact. They train, first and foremost, to be able to move external objects. They may become very good at it, but this approach neglects and eventually compromises the prime athletic directive of self-movement.
I have met hulking trainees who could squat five hundred pounds, but who waddled up a flight of stairs, wheezing like old men. I know one powerlifter who can bench press four hundred pounds, but who can hardly comb his hair due to his uneven, unnatural physical development. The practice of calisthenics will not cause any of these movement problems, because it is essentially a form of training in movement. Old school calisthenics will make you supremely strong, but no matter how advanced you become in this area, you will only ever become more agile and limber in your movements, never less, because you are training the muscles to move the body rather than something external.
3. Bodyweight Training Maximizes Strength
Calisthenics movements are the most efficient exercises possible, because they work the body as it evolved to work; not by using individual muscles, or the portions of a muscle, but as an integrated unit. This means developing the tendons, joints and nervous system as well as the muscles. This synergy in motion is what causes calisthenics to build such impressive strength. Many weight-trainers-no doubt influenced by bodybuilding philosophy-believe that rippling muscles are the source of strength. In fact, it's the nervous system that causes the muscle cells to fire, so your strength and power are largely determined by the efficiency of your nervous system. The nervous component of strength explains why one man can have muscles far, far smaller than another, yet be vastly stronger.
Very strong men will all tell you that tendon strength is probably more important for true power than muscle size. Calisthenics motions work the joints and tendons as they are meant to be worked, resulting in greater levels of power than weight-training movements can develop. (See reason 4.) Another reason why calisthenics are so efficient in developing raw strength is that they train the athlete to work multiple muscle groups at once. A bodyweight squat, for example, works not just the quadriceps at the front of the thighs, but the gluteus maxim us and minimus, the spine, the hips, abdomen and waist, and even the muscles of the toes. Proper bridging works over a hundred muscles! This fact overlaps perfectly with reason 2, given above, because the body has naturally evolved to move in a compound, holistic fashion.
Many bodybuilding motions-particularly those done on machines-artificially isolate muscles, causing uneven development and lopsided functioning. In bodybuilding and a lot of weight-training, you get locked into a simple groove when performing techniques. This means that relatively small areas of the physical system (sometimes only individual muscles) get targeted by an exercise. But when training in calisthenics, you are forced to move your entire body; this requires coordination, synergy, balance and even mental focus. All these things develop nervous power, as well as muscular strength.
4. Bodyweight Training Protects the Joints and Them Stronger-for Life
In prison, you need to be all-over strong-no matter how old you are. Being hindered by weak or painful joints would make you very vulnerable, however big your muscles might look. It may surprise you, but this is one important reason why a lot of convicts deliberately avoid weight training. One of the major problems with modern forms of strength and resistance training is the damage they do to the joints.
The joints of the body are supported by delicate soft tissues-tendons, fascia, ligaments and bursae-which are simply not evolved to take the pounding of heavy weight-training. Weak areas include the wrists, elbows, knees, lower back, hips, the rhomboid complex, spine, and neck. The shoulders are particularly susceptible to damage from bodybuilding motions. You'll be lucky to find anybody who has been lifting weights for a year or more who hasn't developed some kind of chronic joint pain in one of these areas.
Don't just take my word for it. Go into any hardcore gym and you'll see lifters wrapping their wrists and knees, strapping their backs up with high-tech belts, and applying stabilizing straps around their elbows. The locker room will stink of menthol heat rubs and analgesic liniments, all applied to keep the pain at bay. Joint problems are a bodybuilder's constant companion.
When the bodybuilder starts to abuse steroids, these problems become even worse; the muscles begin to develop at an incredibly fast rate-faster than the joints can keep up. By the time most bodybuilders are in their late thirties, the damage is done and pain IS a way of life, whether they stop training or not. This damage is done because bodybuilding motions are largely unnatural. In order to place a great deal of emphasis on the muscles, the body is forced to hoist heavy external loads in motions
and at angles not usually found in nature. One side-effect of this punishment is a vast amount of stress on vulnerable joints, joints which are forced to endure this horror repetitively over time. The result is soft tissue tears, tendonitis, arthritis and other maladies.
The joints become inflamed and scar tissue or even calcifications begin to build up, making the joints weaker and stiffer. Bodybuilding movements primarily target the muscles, which adapt much faster than the joints; this means that the more muscular and advanced a bodybuilder becomes, the worse the problem gets. When performed properly and in sequence, the calisthenics motions in this book will not cause joint problems-on the contrary, they progressively strengthen the joints over an athlete's lifetime, and actually heal old joint injuries. This beneficial effect occurs for two reasons. The first reason
is basic physics; the resistance used is never heavier than the lifter's own bodyweight. The ridiculous, excessively heavy loads so admired in bodybuilding do not occur. The second reason is down to kinesiology-which is the science of movement. Simply put, the body has evolved over millions of years to be able to move itself, first and foremost; it was never "designed" to lift progressively heavier external loads on a regular basis.
A kinesiologist might say that calisthenics movements are more authentic than weight-lifting techniques. When the body has to lift itself, in a pullup or squat, for example, the skeleto-muscular structure naturally aligns to the most efficient and natural output ratio. When lifting weights, this natural shift does not occur-in fact the bodybuilder has to learn to move as unnaturally as possible to force maximum emphasis onto the muscles. Pullups are a good example of the "authentic" nature of calisthenics; humans evolved, like our primate relatives, pulling ourselves up into trees by the branches.
This anatomical heritage still exists in the human body, which is why people adapt very quickly and safely to pullup training. A bodybuilding alternative to pullups is the bent-over row; humans did not evolve to execute this movement, and as a result many lifters quickly injure their spine, lower back and shoulders when performing this exercise. The authentic movements offered by calisthenics apply the power of the joints naturally, as they evolved to be used. The result is that they develop in proportion to the muscular system, becoming more powerful over time rather than weaker and worn down. As the joint tissue rebuilds itself, former aches and pains are worked out of the system, and future injuries are avoided.