Kettlebells are well known for their ability to develop unparalleled strength and endurance minus the muscle bulk that goes along with traditional strength training methods. Additionally, kettlebell athletes have highly responsive and pliable soft tissue. Their tissue quality more closely resembles that of a dancer or yogi than that of a powerlifter or strongman, and response rates to bodywork are much higher, yielding greater results with fewer sessions. This is due in part to the forced relaxation that kettlebell endurance sets engender; it's also related to kettlebells' ability to stimulate the fascial mechanoreceptors, providing increased neurological input.
The tendency when lifting any kind of weight, whether it's a bag of groceries, a toddler, or a kettlebell, is to tense the body in preparation for load. Humans have mastered the ability to tense their muscles. On any given day in your gym, you can find someone pounding out sets like there's no tomorrow. Ask this guy to lie on the floor and relax completely and I guarantee you'll find that he holds various body parts off the floor, unable to let go. We've become accustomed to tension and even value high levels of muscular tonus. Americans, in particular, constantly seek the "hard body look."
Unfortunately, chronically contracted muscle fibers are detrimental to both physical strength and long term structural health. In terms of strength, muscle fibers that are already contracted have no potential energy; they cannot contract further, and thus, they cannot assist you in lifting a weight. This equals reduced strength. In terms of tissue health, healthy soft tissue is defined by its pliability and hydration. Unhealthy tissue is dehydrated, causing the ground substance (the fluid matrix in which cells are bathed) to become more viscous and loaded with toxins and metabolites. Contracted muscle fibers are not only constantly emitting metabolic waste, they are also unable to pump fluid into and around the cells. How easily nutrients are able to enter a cell is determined by the density of the fibrous matrix and the viscosity of the ground substance.
During a long kettlebell set, any unnecessary tension is eradicated through the sheer necessity of efficiency. It's simply impossible to simultaneously hold onto chronic muscular bracing and perform a high number of technically correct lifts. As the athlete relaxes and learns to contract only the muscle fibers required to lift the weight, his tissue becomes more soft, supple, and fluid. Also, excess muscle tissue is atrophied, leaving only strong, functional fibers in place. In a kettlebell athlete, every single fiber of a muscle is functional - able to both contract and relax. This explains the lack of excessive muscle bulk.
You are probably familiar with the proprioceptors in muscle tissue called muscle spindle fibers. These smart cells communicate the length of a muscle to the neurological system and prevent injury due to over stretching. Recent research has uncovered four additional kinds of mechanoreceptors in fascia (muscle spindle fibers are the only proprioceptor not located in fascia), and I postulate that the dynamic, fluid movements of kettlebell training stimulate these. Because kettlebells are an integrated exercise, they provide sensory feedback from all areas of the body. Golgi tendon organs, located in joint capsules and ligaments of peripheral joints, decrease muscular tonus, preventing injury due to hyper contraction. Pacini receptors are sensitive to rapid pressure changes and provide proprioceptive feedback for movement coordination. Ruffini receptors inhibit overall sympathetic activity (the body's response to stress, as in the fight or flight response), and interstitial receptors, when stimulated, increase fluid supply to the tissue region. Stimulation of all four of these mechanoreceptors would explain the unique tissue quality found in kettlebell athletes.
Kettlebells are the ideal tool for development of healthy, strong tissue. Using kettlebells for strength-endurance training creates tissue relaxation in chronically contracted muscle fibers, allowing greater fluid flow and enhanced cellular nutrition. It also pares down excessive muscle density that may be a block to strength and the local flow of fluid. Healthy tissue is imperative in maintaining longevity. Rigid, unresponsive connective tissue has been connected to chronic pain conditions. Additionally, kettlebells may stimulate the mechanoreceptors located in fascia. A 1993 article published in The Journal of Spine, the leading journal for the field of orthopedics, found that patients with chronic low back pain had almost a complete absence of sensory nerve endings in their low back. This was correlated to a plastic deformation of the area (twists and adhesions in the fascial tissue). The brain, with a lack of sensory input stemming from the low back, created an enlarged mental map of the area and the patients experienced pain. This is exactly the same process that happens with "phantom pain" in amputees. The lack of sensory input from the missing limb causes the brain to mentally map the area, creating a pain associated story. When amputees receive real stimulation through a prosthesis, however, the pain significantly decreases. Thus, including activities in your fitness regime that stimulate your sensory receptors is critical in maintaining a healthy body.