Yesterday it was made apparent that the feet are rife with afferent neurons that detect and report magnitudes and rates of loading, joint kinematics, and pressure distribution on the plantar surface of the foot. Also, I said that they are in such a high concentration for a reason. That reason will be expounded upon today.
Recall that these afferent cells are important for the maintenance of posture, detect aberrant movement during the stride, and also help us modulate leg stiffness and joint kinematics in preparation for ground contact (by way of feedback and feed-forward mechanisms, respectively), but that does not necessarily provide any significance for their placement in the feet. Enter the concept of concurrent activation potentiation.
Concurrent Activation Potentiation
Concurrent Activation Potentiation (CAP) has been well documented within the literature and states that a contraction of a remote muscle (RVC) can lead to heightened levels of strength or activation in the prime movers of an exercise. In research these exercises are commonly knee flexion and extension, squatting, and vertical jump testing, and time and time again, the implementation of an RVC can facilitate performance in strength, rate of force development, and power. In many instances, the gains in percentage performance while utilizing concurrent activation potentiation can exceed 30% compared to those efforts not utilizing RVC. Most frequently the RVCs performed that show gains in performance are contraction of muscles surrounding the neck, jaw, and hands. Additionally, most frequently, these methods result in more powerful jumps, knee extension, flexion, and upper body strength measures.
To some this idea may sound familiar, but the words concurrent activation potentiation doesn’t ring the bell. Another idea that has been resurrected and tossed around a bit as of late is the word “irradiation.” Irradiation in this sense is not the way of ensuring pathogens are removed from food, but rather is the spread of a response due to stimulation. This can either be seen as facilitation or inhibition of the synergistic movement patterns or muscles.
But just how does hand gripping and teeth clenching have anything to do with the feet? Did you catch the similarities?
What do the feet have in common with the other areas that, when activated, result in greater peaks in performance?
THEY ALL HAVE HIGH CONCENTRATIONS OF AFFERENT NEURONS PRESENT.
If this is the case, then why would stimulation of the feet not produce a similar effect of CAP?
While at this point there is little evidence available to support the claims, common sense says that the biologically similar mechanisms may produce similar responses to loading. The explanation for as to why this whole process ouccurs is somewhat debated, but it seems that muscle spindle activity, cutaneous sensation, and even joint loading are not solely responsible for the response. Instead, it is suggested that it may be the result of a motor overflow in the cortex of the brain and H reflex facilitation that likely occurs at a point proximal to the muscle spindle that leads to greater potential performance capacity. In the brains of monkeys there seems to be an integration of cortical areas that are somatosensory with those that are motor points, so the potential for sensory spillover in the brain that activates motor areas and neurons is a distinct possibility.
While not framed as “irradiation” or “CAP” there are actually volumes of evidence showing that stimulation of the plantar surface of the foot and joint kinematics do play a role in muscle activation patterns during gait and that irradiation definitely plays a role in locomotion (and it has to be though stimulation of the foot as it is the only interface with the ground). Stimulation of the plantar surface of the foot alters gait kinematics in a variety of animals, including humans. Alterations in plantar sensation also beget altered activation patterns and gait kinematics in healthy controls.
Soon, we’ll go into detail concerning sensation and sensation alterations and how it impacts gait, muscle activation, and injury potential.