The Split Second Effect: The Mechanism of How Equinus Can Damage the Human Foot and Ankle

Abstract

We are currently in the process of discovering that many, if not the majority, of the non-traumatic acquired adult foot and ankle problems are caused by a singular etiology: non-neuromuscular equinus or the isolated gastrocnemius contracture. There is no question that this biomechanical association exists and in time much more will be uncovered. There are three basic questions that must be answered: why would our calves tighten as we normally age, how does a tight calf, or equinus, actually cause problems remotely in the foot and ankle, and how do the forces produced by equinus cause so many seemingly unrelated pathologies in the foot and ankle? The purpose of this paper is to address the second question: how does a tight calf mechanically cause problems remotely in the foot and ankle? There has been little evidence in the literature addressing the biomechanical mechanisms by which equinus creates damaging forces upon the foot and ankle, and as a result, a precise, convincing mechanism is still lacking. Thus, the mere concept that equinus has anything to do with foot pathology is generally unknown or disregarded. The split second effect, described here, defines exactly how the silent equinus contracture creates incremental and significant damage and injury to the human foot and ankle resulting in a wide variety of pathological conditions. The split second effect is a dissenting theory based on 30 years of clinical and academic orthopedic foot and ankle experience, keen clinical observation along the way, and review of the developing literature, culminating in examination of many hours of slow motion video of normal and abnormal human gait. To my knowledge, no one has ever described the mechanism in detail this precise.

Foot Kinetic and Kinematic Profile in Type 2 Diabetes Mellitus with Peripheral Neuropathy

“…Just before the heel-off phase, damaging forces are produced where both direct and indirect forces are created and magnified by the tightness of calf muscles restricting the tibial translation over the ankle joint. 57 The tightness of calf muscles could also result in altered kinematics at the ankle such as decreased ankle dorsiflexion at terminal midstance and toe-off (,108), as seen in the present study (Table 4).…”

“…56 The association of calf muscle (gastrocnemius and soleus) tightness with kinetics and kinematics of the foot could be well explained by the split second effect suggested by a study. 57 The study suggested that there were two types of forces (direct and indirect) created by the split second effect over a critical period of 120 msec during terminal midstance. Just before the heel-off phase, damaging forces are produced where both direct and indirect forces are created and magnified by the tightness of calf muscles restricting the tibial translation over the ankle joint.…”

Plantar fasciopathy: a current concepts review

“…Damaging forces appear when leveraged forces are transmitted to the foot and ankle as the tibia (and the body) passes over the planted foot. 24 Shortening of the gastrocnemius, congenital or acquired, is the most common cause of the lack of ankle joint dorsiflexion. During the late part of the second rocker of gait (just before heel lift) ankle dorsiflexion should be coupled with full knee extension.…”

American College of Foot and Ankle Surgeons Clinical Consensus Statement: Diagnosis and Treatment of Adult Acquired Infracalcaneal Heel Pain

“…An increased mechanical load due to a higher BMI seems a very plausible source of increased plantar fascial stress (46), and the association between the BMI and musculoskeletal symptoms in general is widely recognized (59). A tight or contracted Achilles tendon is also thought to produce greater tensile loads in the fascia through direct transmission of tension through the calcaneal trabecular system, as proposed by Arandes and Viladot (60) and/or by increasing its passive mechanical longitudinal tension as a method of counteracting the arch flattening effect of ankle dorsiflexion stiffness (61,62).…”