The Biomechanical Model was first introduced by Root et al to explain the link between foot pronation and resulting symptoms in the foot. Later this model was expanded to explain musculoskeletal symptoms occuring in the knee, hip, and back.
Sometimes the link between foot pronation (and related internal tibia/femur rotation) was consistent with the symptoms observed in the weight bearing joints proximal to the subtalar joint. At other times this link was not apparent and, in some cases, what the biomechanical model suggested was contrary to what was actually seen clinically.
To solve this paradox, an alternative model (Rothbart, 2011), was introduced linking gravity driven pronation to the development of musculoskeletal pain symptoms, the Neurophysiological Model:
- The plantar surfaces of the feet are embedded with millions of fast acting touch receptors (Meissners corpuscles). In a closed kinetic chain, at any given moment, certain areas of the foot are touching the ground. At each point of contact, the Meissner corpuscles are being stimulated. When a group of Meissner corpuscles are simultaneously stimulated, it is referred to as a pattern of stimulation. Rothbart has found that in PMS (a twisting or excessively pronating foot), the patterns ofstimulation are distorted compared to the patterns of stimulation in a non-twisting foot.
That is:
In a non-twisting foot, the mechanical receptors (e.g. Meissners corpuscles) are stimulated linearly across the heel and ball of the foot (e.g., a non-distorted pattern of stimulation).
In certain abnormal foot structures (e.g.,Primus Metatarsus Supinatus, AKA Rothbarts Foot - Rothbart 2014), the mechanical receptors are stimulated in a non linear pattern across the heel and of the ball of the foot (e.g., a distorted patterns of stimulation).
These Patterns of stimulation send signals to the cerebellum. These signals provide the cerebellum with a picture of where the body is in space (e.g., the bodys posture). Based on these signals, the cerebellum is constantly readjusting the bodys position in order to maintain an upright and stable posture.
Patterns of stimulation formed by hip driven pronation send non-distorted signals to the cerebellum. From these non-distorted signals, the cerebellum automatically maintains a non-distorted posture.
Patterns of stimulation formed by gravity driven pronation send distorted signals to the cerebellum. From these faulty signals, the cerebellum automatically maintains a distorted posture.
It is suggested that it is this distorted posture that results in the development of chronic muscle and joint pain, foot to head.
References:
Rothbart BA 2011. Primus Metatarsus Supinatus (Rothbarts Foot): A common cause of musculoskeletal pain - Biomechanical vs Neurophysiological Model. Podiatry Review, Vol.68, No.4, pp 16-18 July/August.
Rothbart BA 2014. How Rothbarts Foot Leads to Poor Posture and Chronic Pain. Positive Health Online. Issue 211 (January) Bodyworks.