Darren - Good post!
However, proprioception and kinesthesia are seen as interrelated and there is considerable disagreement regarding the definition of these terms. You've explained proprioception very well, however kinesthesia is a key component in hand-eye coordination, and training can improve this sense. The ability to swing a golf club, to catch a ball or shoot a longbow requires a finely tuned sense of the position of the joints. This sense needs to become automatic through training to enable a person to concentrate on other aspects of performance, such as maintaining motivation, focused attention or in hunting, seeing where animals are located.
Perhaps motor learning and the retention of motor learned skills is what folks want to define under the "muscle memory" category. Movement (kinesthesia) is a critical part of our life, and it is a major component of our evolutionary development; without it, we could not survive. It has been suggested that our developed cognitive capacities evolved so we could make movements essential to our survival. For example, cognitive abilities evolved so we could use tools, build shelter, and hunt for animals.
When first learning a motor task, movement is often slow, stiff and easily disrupted and we loose our focus or attention. With practice, execution of a motor task becomes smoother, there is a decrease in limb stiffness, and muscle activity necessary to the task is performed without conscious effort. As a motor skill is learned, it is encoded into a certain area of our brain. The exact location of this motor skill storage is not known.
My entire point of this thread is the LEARNED MOTOR SKILL is stored in the brain and not in the muscle as the term "muscle memory" would suggest. A prime example is when a person suffers a stroke, the brain is impaired and muscles atrophy. The motor skill ceases to exist. If the muscles had this stored memory, they could still function. The muscles don't have memory and can only function with direction from the brain.
Danny