When the Foot Hits the Ground, Everything Changes
BIOMECHANICS
Mark N. Charrette
From the moment the heel strikes a surface until the time the toes push off, a very predictable and complex series of motions occurs. The gait cycle, as it is known, has two distinct portions: the stance phase, which is weight bearing, and the swing phase, which is not.
Stance Phase
The stance phase, which comprises 60% of the gait cycle, begins when the heel strikes the ground. The force upon heel strike is equal to 2.5 times the body weight in a walking gait and 3.5 times the body weight in a running gait. This is the equivalent of a 160 lb individual being hit in the head by 80 pounds with each step. In addition, the typical adult in today’s modern society takes at least 5,000 to 10,000 steps per day. Many individuals take as many as 14,000 steps per day. These repetitive shockwaves, which start at heel strike and can be measured at the cranium, occur on each step taken. The cumulative effect of these forces can cause the ligaments supporting the arches in the feet to stretch, in a permanent way, and cause various musculoskeletal symptoms in the lower extremity and spine to develop. Symptoms involving the joints of the feet, knees, hips, and low back aie common.
Heel Strike
The heels strikes lateral to the midline of the foot on the posterior and lateral aspect of calcaneus with the foot in a slightly supinated position. The leg is externally rotated upon heel strike. As weight is transferred anteriorly onto the foot, a pronation motion occurs where weight is transferred toward the medial aspect of the foot. This motion is accomplished mostly at the subtalar joint by combining eversion, dorsiflexion, and abduction. As this motion occurs, there is a collapse of the medial and lateral longitudinal arches, and the navicular drops. As the navicular prominence drops in an inferior and medial direction, the metatarsals and toes move laterally. This will cause the foot to abduct or “toe-out.” Then the anterior and posterior tibialis muscles contract eccentrically to slow down the plantar flexion of the foot. When these muscles become overloaded and painful, it is known as “shin splints.”
Mid Stance
This is the portion of the stance phase where the heel and
“The cumulative effect of these forces can cause the ligaments supporting the arches in the feet to stretch, in a permanent way, and cause various musculoskeletal symptoms in the lower extremity and spine to develop. ï Ï
toes of the foot are both in contact with a surface. At mid stance or “foot flat,” all three arches of the foot decrease their height creating a longer, wider, and flatter foot. One of the most common conditions of the foot is excessive pronation or
hyperpronation. In this condition, the constant forces generated at heel strike will create permanent stretching of the soft-tissue retaining mechanism of the arches called plastic deformation. Structural support for the arches is primarily ligamentous from the plantai' fascia and the spring ligament. The greatest degree of excessive pronation can be seen and measured in the functional foot (weight bearing) at mid stance.
At mid stance, the tibia and femur are internally rotated maximally. These increased rotational forces are transmitted superior up the leg to the pelvis, specifically to the sacroiliac joint. Various pelvic subluxation complexes may develop, such as pelvic tilting (usually anterior or to one side) and innominate rotations (usually posterior and inferior).
Neurologically, this will also cause an increase in nociceptive firing (type-4 mechanoreceptors), potentially creating a pain response in various tissues and adding to the reflexive activation of the sympathetic nervous system, which creates a hyperexcitable central state in the body.
In cases of excessive pronation or hyperpronation, more stress is put on the knee, especially the medial collateral ligament, the medial meniscus, and the anterior cruciate ligament at mid stance. When this occurs, symptomatology involving the medial aspect of the knee is common. Patellar tracking problems, such as chondromalacia patellae, can also occur.
Propulsion
This portion of tlie gate cycle is also known as “toe-off” In this phase, the foot effectively becomes a lever. The posterior structures of the foot provide force as the ball of the foot serves as a fulcrum. This propulsion brings the foot away from the ground and launches it into the swing phase where no weight is borne until the next heel strike.
Swing Phase
In the swing phase of the gate, the pelvis rotates forward and the hip flexes, which accelerates the leg forward. Muscles contract concentrically pulling the body
“This will affect muscular tension and the amount of weight borne by various joints. ??
forward. The knee and ankle complex flex to clear the ground, and then they extend in order for the heel to strike on its posterior lateral aspect.
Leg Length Inequality
Excessive pronation or hyperpronation will create a functional inequality of leg lengths. This will affect muscular tension and the amount of weight borne by various joints. Strain on the body increases and endurance may be reduced so that routine movements will require greater muscular effort and cause fatigue. Pain and discomfort are common responses. Since the nature of pronation is usually bilateral and asymmetrical, this type of pedal foundation can create pelvic tilting and/or rotation.
So, it appears that bilateral and asymmetrical excessive pronation is developed slowly. In order to have a balanced lower extremity and spine, the three arches of the feet and the subtalar joint must be stabilized to allow for normal ranges of motion and block excessive motion.
References
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Dr. Mark Charrette is a 1980 summa cum laude graduate of Palmer College of Chiropractic. He is a frequent guest speaker at Chiropractic colleges worldwide and has taught over fourteen hundred seminars worldwide on extremity adjusting, biomechanics, and spinal adjusting techniques.
Contact info: [email protected]
