This paper came to my attention yesterday: The kinesthetic senses by Uwe Proske and Simon Gandevia, in Australia. By some refreshing turn of events, it is open access. It provides an historical backdrop to nearly everything we as physical therapists are about. We are all about restoring this function, kinesthesia, to people in whom it would seem to have gone missing. We always have been. Without good kinesthesia, motor control goes offline. When motor control goes off line, so does maximal function.
The paper begins with a brief intro to kinesthesia. It states at the outset that muscle and skin receptors account for most of the receptor input that helps the brain make motor choices. It specifically states,
"Peripheral receptors which contribute to kinaesthesia are muscle spindles and skin stretch receptors. Joint receptors do not appear to play a major role at most joints."
Wow. Right there, we can see a huge erosion under the sea shore of one of the defining organizational principles of orthopaedic manual therapy, which is that getting to and wiggling or popping the right joint in the right way will jumpstart a better motor output. Orthopaedic manaual therapy (and chiro) have used this idea to build themselves and have perpetuated it for decades, for a century. It's just not valid. It was a false hypothesis, and finally research has begun to trickle out that suggests this is the case.
After a brief description of the contribution of muscle spindle receptors to the kinesthetic senses, Proske and Gandevia begin to discuss receptors found in skin:
"Concerning the possible contribution to kinaesthesia from other receptor types, the summary view is that while a good case has been made for some cutaneous receptors, the evidence is less convincing for joint receptors. The cutaneous receptor most likely to subserve a kinaesthetic role is the skin stretch receptor, the slowly adapting Type II receptor served by Ruffini endings (Chambers et al. 1972; Edin, 1992). For kinaesthesia at the forearm, stretch of skin over the elbow during elbow flexion can provide information about both position and movement. Movement illusions generated by stretch of skin of the hand and over more proximal joints, when combined with muscle vibration were greater than when either stimulus was applied on its own (Collins et al. 2005). The authors made the point that this was not just a matter of skin input facilitating the muscle input and that cutaneous input generated by skin stretch contributed to kinaesthesia in its own right. More recent observations have shown that skin input can also have an occluding action. Signals from local, rapidly adapting receptors evoked by low-amplitude, high frequency vibration can impede movement detection (Weerakkody et al. 2007).(....)
While joint receptors were first thought to be all-important in kinaesthesia, the present-day view is that their contribution at most joints is likely to be minor. Typically they respond to joint movement, but often with response peaks at both limits of the range of joint motion (Burgess & Clark, 1969). They are now thought of as limit detectors. However, there are examples in the literature of responses across the full range of joint movement (Burke et al. 1988) and here joint receptors may play a role under circumstances in which input from muscle and skin is not available (Ferrell et al. 1987). "
This paper is an important one for manual therapists who seek to understand how it is that "light" manual techniques seem to do as well to help patients' brains connect up in terms of improved, observable motor output, as heavy joint-based, manipulative or mobilizing ones.
Proske, U., & Gandevia, S. (2009). The kinaesthetic senses The Journal of Physiology, 587 (17), 4139-4146 DOI: 10.1113/jphysiol.2009.175372