MASS Theory, Postural Foot Biomechanics Model
PERSPECTIVE
Edward S. Glaser
David C. Fleming
Maximal arch supination stabilization (MASS) posture is the all-axis model of foot biomechanics. The foot is a machine with a tented structure. The foot experiences intermittent compression between the downward force of the body and the ground, which, in our society, is often a rigid surface like concrete or steel. Clinical observation confirms that, over a lifetime, most individuals are genetically predisposed to postural collapse. Postural collapse loosens the foot’s structure and postural elevation tightens the foot. As Root [1] proposed, loosening allows for shock absorption and adaptation to the terrain, and tightening prepares the foot for propulsion by creating a more rigid lever. This range of postures, or postural zones, more accurately describes the foot of any one patient rather than the single axis theories of the past.
Postural Zones
What the Sub Talar Joint (STJ) axis lacks in rotation, it more than makes up in translation. If we observe the foot at heel strike through midstance, we see a huge forward and plantar grade translation of the STJ axis. Southerland begins the “Seven Theorems of Compensation hi the Distal Human Lower Extremity” with the words, “The foot hits the ground in a forward rolling motion”[2]. Jacqueline Perry describes the axis of translation as the heel rocker mechanism [3]. One of the more brilliant aspects of foot design is the round heel. Like a ball, it has an infinite number of axes all passing through the center. This allows us to hit the ground from any angle—forward or backward—and apply the appropriate axis based on the direction of heel rotation with every step. Likewise, the STJ axis can translate through all of the following postural zones with each step.
^Clinical observation confirms that, over a lifetime, most individuals are genetically predisposed to postural collapse. J J
Pathological Zone
Tom McPoil’s tissue stress theory states that when microtrauma occurs faster than a person’s ability to heal, they experience a symptom [4]. During the last few degrees of postural collapse, tissue stresses are highest. Microtrauma occurring in this zone of foot posture causes symptoms.
Dysfunctional Zone
As the foot goes mto further elevation of its posture, there is a zone where, according to Hammel, there is no significant rotation around the STJ axis in any plane [5]. Foot orthotics that attempt to elevate posture into this zone often cause medial longitudinal arch pain as the foot repeatedly drops down to impact the orthotic. Hammel showed that from 25 to 90% of the stance phase of gait, no rotation in any plane occurs between the talus and the calcaneus. The forefoot hits the floor at 27% of the way through the stance phase. Ground reaction force applied to the forefoot displaces it superiorly in relation to the rearfoot. The
most significant postural collapse occurs at this time. Subtalar rotation in the transverse and sagittal planes occurs only from heel strike to 24% of stance. Therefore, subtalar rotation and postural collapse are independent events occurring at different times in the gait cycle. Early and excessive STJ rotation does, however, move the head of the talus off the anterior facet loosening the foot’s structure, and preparing the foot for postural collapse. Subtalar pronation is not synonymous with postural collapse, but it is a predicating factor. Subtalar supination is not synonymous with postural elevation, but it is highly beneficial for efficient propulsion. Pienynowski and Trotter showed that elevation of the foot’s posture made a significant improvement in the biomechanical efficiency quotient[6].
Functional Zone
As foot posture elevates beyond the dysfunctional zone, the anterior facet of the STJ approaches level in the transverse plane. This allows subtalar rotation to occur. This is where the talar head slides posterior and rotates its six degrees around the STJ axis. The closer the anterior facet is to level, the easier the subtalar rotation occurs and the rearfoot locks in the sagittal plane facilitating efficient propulsion.
Supination Instability Zone
Beyond the functional zone, there is a zone that is not always present, where the foot can be put into so much supination that
it becomes laterally unstable. As the downward force of the human body moves lateral to the foot, the propensity of inversion ankle sprain will increase due to a rotational moment created in that direction.
MASS Posture
MASS posture has several elements. First, it is the highest posture that the foot can attain at midstance, placing the foot in adequate supination to reach or approximate a level anterior facet of the STJ, putting it squarely within the functional zone. The idea is simple. If you want to control a motion, start at the beginning of that motion.
The foot poses a special problem. The soft tissues between the orthotic and the bones compress unevenly. Therefore, an essential element of capturing the foot in this elevated posture is that the soft tissues must be evenly compressed as they will be during use. There are many ways to achieve this.
A MASS posture composite leaf spring applies an even distribution of force per unit of area by remaining in full contact with the foot throughout the gait cycle. The foot never has to drop down to hit the orthotic because it is already touching it, which minimizes impact and thus tissue stresses. The combination of full contact (redistribution of force per unit area) eliminating hot spots and the lack of repetitive impact allows such a spring to apply a rather large corrective force
while remaining comfortable to most patients. Once you have the correct geometry of the spring, it is time to adjust the spring constant.
Calibration
How much vertical force should this leaf spring apply to the foot in an evenly distributed manner? Isaac Newton supplied the answer with his third law of motion: for every action there is an equal and opposite reaction. Applying that law to this problem: the amount of force the orthotic should apply to the body is directly related to how much force the body is applying to the orthotic. Three things influence the value of this force: body weight, foot flexibility, and activities of daily living (ADL). ADL is the range of forces applied to the foot due to the activities performed by it. All MASS orthotics are calibrated to each individual patient, something no other orthotic does.
Root found that in ideal gait, 60% of the force applied to the ground at toe-off should be under the first ray [7]. Higby measured the force distribution on the metatarsal heads at toeoff for MASS orthotics [8]. Initially, MASS posture orthotics transferred 44% more force to the first metatarsal head at toe-off than STJ neutral position orthotics with posts. At six weeks, this difference grew to 61% (p=.006)[8]. This means that when the arch is raised, the first ray not only comes down and lateral, but additionally increases its purchase of the ground.
Conclusion
Posture controls function. Postural collapse is the cause of functional impairment in the majority of foot patients and often leads to pain, pathology, and deformity. MASS posture is an aggressive approach to foot biomechanics. It attempts to restore as close to an ideal posture to the foot as each foot can tolerate with its individual anatomy.
References
1. Root, M. L., Orien, W. P., & Weed, J. H. (1977). Normal and abnormal function of the foot (Vol. 2). Los Angeles: Clinical Biomechanics Corporation.
2. Southerland, C.C., & Orien, W. P. (1995). Seven theorems of compensation in the distal human lower extremity. The Lower Extremity 2, (3), 1.
3. Perry, J., & Burnfield, J. M. (1993). Gait analysis: normal and pathological function. Slack.
4. McPoil, T.G.,& Hunt, G. C. (1995). Evaluation and management of foot and ankle disorders: present problems and future directions. Journal of Orthopaedic & Sports Physical Therapy, 21(6), 381-388.
5. Hamel, A. J., Sharkey, N. A., Buczek, F.L.,&Michelson, J. (2004). Relative motions of the tibia, talus, and calcaneus during the stance phase of gait: A cadaver study. Gait & posture, 20(2), 147-153.
6. Trotter, L. C., & Pierrynowski, M. R. (2008). Changes in gait economy between full-contact custom-made foot orthoses and prefabricated inserts in patients with musculoskeletal pain: A randomized clinical trial. Journal of the American Podiatric Medical Association, 98(6), 429-435.
7. Root, M. L., Orien, W. R, & Weed, J. H. (1977). Functions of the muscles of the foot. Normal and Abnormal Function of the Foot, pp250-252, edited by ML Root, WP Orien, JH Weed, Clinical Biomechanics Corporation, Los Angeles.
8. Hodgson, B., Tis, L., Cobb, S., McCarthy, S., & Higbie, E. (2006). The effect of 2 different custom-molded corrective orthotics on plantar pressure. Journal of Sport Rehabilitation, 15(1), 33.
Dr. Edward S. Glaser DPM studied mechanical engineering at SUNY Stonybrook, and went on to receive his doctorate in podiatric medicine at the New York College of Podiatric Medicine. With the founding of Sole Supports in 1992, Dr Glaser made his life’s mission a reality: ‘We make people better ’. MASS Posture Theory is a unique look at managing foot biomechanics from an engineering perspective, an all axis model of foot biomechanics, based on the individuals ’ idealized gait. Dr Glaser has earned a substantial following at home and abroad for re-visioning and re-engineering the way practitioners can significantly correct poor biomechanics in the foot and lower extremity. Dr Glaser can be reached at edasote supports.com
David Fleming was born in Miami, FL. He received the B.S. degree in Biomedical sciences from the University of South Florida. He joined the research service at the James A. Haley Veterans Medical Center concentrating in endocrine and cardiovascular systems. David Fleming can be reached at dflemingfrsolesupports. com
