Afferent Signal Loading (ASL, Dean 2016) describes the prioritization of a set of sensory signals passing through the ascending peripheral nerves and spinal cord to the brain for interpretation, aka ‘listening.’ ASL is important when interpreting LBP signaling, and more importantly, techniques to ‘hi-jack’ the LBP signal to manage ‘pain.’
The brain ‘listens’ to how hard the muscles are working; the amount of tension (high/low) on the muscle, how fast/slow the muscles are contracting, and the various positions of the joints. For example: if a person attempts to lift a bucket of something (water, or mixed concrete, or popcorn) without knowing the contents nor the weight, the person would likely prepare with some pre-tension in the arms, legs and lower back before lifting the bucket. When the load is popcorn, the brain senses less weight, and releases body tension and lets the elbow bend to raise the bucket. When the load is concrete, the brain senses unexpected extra weight, adds body tension, adds work in the arms, legs and lower back but not to the elbow since the brain senses that bending the elbow is not safe (or possible), and re-directs the work back to the legs and lower back.
The brain is ‘listening’ when a person performs work that involves the lower back, and the rest of the body simultaneously, but it listens to these, and other sense-signals simultaneously; however, these sense signals are prioritized… the brain is ‘listening’ to some sense-signals before others.
There are 3 factors to ASL: (1) the diameter (thickness) of the nerve fiber associated with the sensory signal, (2) the number of motor units associated with the sensory signal, (3) the muscle tension associated with the sensory signal.
Nerve diameter: largest diameter = greater signal speed; largest diameter loaded first
Motor units: largest (aggregated) = contraction priority; anaerobic units loaded first
Tension: greatest = contraction priority; these muscles loaded first
ASL is sequenced according to [diameter + units recruited + tension], activating (firing) which ever combination provides the most of all 3, then this becomes prioritized: (action = muscle & joint sensing.)
Afferent Signal Loading priority:
- First Priority: joint position changes + action loading + action speed + natural gravity sensing
- Second Priority: vibration sensing (vibration occurs in the joint capsule and the muscle belly*)
- Third Priority: broad tactile contact + pressure tactile contact + GTO (muscle tension) sensing
- Fourth Priority: temperature sensing + spicy (capsicum) + vapocoolant sensing (evaporation)
- Fifth Priority: fast ‘pain’ + pinch + sharp + cutting + electric ‘shock’ sensing (‘high danger’)
- Last Priority: slow ‘pain’ + dull/achy + burning + annoying + raw sensing (‘low danger’)
Low back motor-use feedback is the sensing of muscle action and joint use. The brain ‘listens’ to the demands of motor-use when it receives afferent signals, and makes cognitive choices about how to proceed with that muscle action and joint use (after mixing in emotional signals, memory signals, safety signals, and feasibility estimation.)
The nerve diameter that provides the brain with First priority senses are 100 times the diameter of Last priority sense. This means that the slow pain fibers are 100 times less activated and ‘listened to’ than muscle movement. Third and Fourth Priority sense signals are ‘listened to’ 3 to 10 times more than Last priority sense… this is why stretching, massage and heat/cold therapies are used for LBP… simply because the brain is ‘listening’ to those higher priority sense signals, NOT because stretching, massage and hot/cold therapies actually reduce ‘pain’, the therapies are only temporarily distracting the brain.
Afferent signal loading describes the intrinsic prioritization of sense-signaling. Motor units are the highest priority since they report the three-dimensional experience of the person; the most valuable information that brain should ‘listen to’ is the feedback that sustains survival. Muscle work is required to perform the various actions of basic survival: sheltering, acquiring and preparing food, and self-defense. Contemporary lifestyles are made for convenience, yet the prioritization of afferent signaling remains and is useful in ADLs, sports, hobbies, and the like.
*especially during eccentric loading, and novel concentric loading (includes open and closed-chain)
LBP Blog General information
These articles intend to (1) re-evaluate the prevailing clinical practices thought to manage low back ‘pain’, (2) submit and debate novel low back ‘pain’ contributors and mechanisms, (3) meet patient expectations & satisfaction and clinically meaningful results, (4) recommend a conservative non-surgical course of care to over-ride ‘pain’ instantly, and (5) restore ADLs and patient confidence on the first visit at low cost. This article has a companion podcast.
Dr. Dean Bio
Forester Dean is a chiropractic and physiotherapy sports medicine doctor practicing in Los Angeles, California. Dr. Dean is a lifetime athlete, and currently teaches tennis, track, boxing, yoga. The Core X System™ Campus flagship location was opened by Dr. Dean in 2020. www.preformancecxs.org
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