By Emily Splichal, DPM

Dynamic movement involves a relationship between gravity and the ground, creating impact forces that provide our bodies with the energy to move. Our relationship with gravity is the foundation of performance, and of pathology. A patient with a complaint of pain while walking often has had a breakdown in this relationship with impact forces, causing an insufficient loading response and inability to harness potential energy from gravity.

Our relationship to gravity begins with the human foot.1 The perception of the ground is instrumental in enabling us to move easily in diverse natural environments. Humans have adapted to stepping on to, off of, or moving over, soft, irregular, or slippery surfaces in a manner that minimizes metabolic cost, reduces impact forces, and stabilizes the vertical center of mass.1 As Nigg and colleagues have stated, the perception of these forces must be accurate to effectively harness the power of potential energy.2

And yet, advances in footwear, orthoses, and surfaces interfere with this natural process of perception, creating delayed, inaccurate, and insufficient loading responses. The result? Chronic overuse, impact-related injuries such as plantar fasciitis, Achilles tendinitis, stress fractures, and shin splints.

We practitioners must better understand the demands of dynamic movement as it relates to sensory stimulation. We begin with gravity and ground reaction forces, the stimuli that drive locomotion, which begins with foot contact.

Innervation of the plantar foot
The glabrous skin on the plantar foot houses unique haptic, or touch, receptors that are critical to how the brain perceives movement. Like the palm of the hand, the plantar foot contains a dense innervation of cutaneous receptors that convey a spatial image of stimuli, providing the ability to discriminate spatial details of surfaces and objects. High innervation densities translate to more afferent fibers and larger representations in the motor cortex.3 The foot is 1 of several areas of the body most sensitive to sensory stimulation.

Four distinct haptic receptors or mechanoreceptors innervate the glabrous skin of the human foot; slow-adapting type 1 (SA1), slow-adapting type 2 (SA2), fast-adapting type 1 (FA1), and fast-adapting type 2 (FA2).3

SA1 receptors perceive 2-point discrimination with a spatial acuity of 1 mm, and interpret whether a surface is smooth or rough, which translates into stabilization during walking.
SA2 receptors respond to skin stretch, which, when stimulated, downregulate the sympathetic nervous system.
FA1 receptors respond to low-frequency vibration, or flutter.
FA2 receptors respond to high-frequency vibration.
Kennedy and coworkers reported that 70% of the 104 mechanoceptors on the plantar foot are fast-adapting and sensitive to vibration, which conveys the amount of force with which the foot is contacting the ground, which helps maintain dynamic balance while walking.4 Robbins and colleagues reported that peak sensitivity of the mechanoceptors in the feet occurs at age 40 years; by age 70 years, the plantar receptors require twice the stimulation to create the same response. A decline in mechanoceptor density has been demonstrated at 69, 27, and 8 per mm at ages 3, 32, and 83 years, respectively.5

It is these plantar mechanooceptors that are critical to the perception of impact forces during dynamic movement. Although the sensory stimuli created by ground reaction forces is complex–including pain, skin stretch, heat and vibration–much focus has been on the vibrational stimuli created with each foot contact.2,5 FA1 and FA2 mechanoceptors of the plantar foot are used to perceive both low- and high-frequency vibration, which in turn creates an accurate loading response.3

Stimulation of these fast-adapting mechanoceptors creates a stiffening response in the soft tissue structures that reduces vibrational forces to 50% in 15-70 ms and to 5% within 60-300 ms. In the time taken for amplitude to reduce to 5%, typically no more than 2 oscillations occurred in the vibration.5

Muscle tuning theory
This stiffening response–the muscle tuning theory–is based on the concept that an accurate perception of impact forces creates an accurate stiffening response in myofascial tissue, thus allowing the storage of potential energy. The adult takes an average 5,000-8,000 steps each day; with each foot strike, 1-1.5 x body weight in ground reaction forces enter the foot at a rate of <50 ms.6 With regard to movement accuracy, not only must the degree of impact be accounted for, but also the rate. Rate of loading is defined as the peak vertical force divided by time to reach the peak vertical force.6

To put this 50-ms impact rate into perspective, fast twitch muscle fibers contract in 70 ms, meaning that impact forces enter the body faster than our muscles can react to them. Many patients who experience impact-related injuries demonstrate delayed reaction to impact, which often can be attributed to the inability of muscles to react fast enough to the stimuli of impact forces.

So, what is the solution?
Research has demonstrated that to effectively load impact forces, our loading response must be initiated before the foot contacts the ground; this is the pre-activation loading response.2 The limiting factor to pre-activation loading response is our perception of impact forces. Certain footwear, smooth insoles, injury, and disease can cause a delay in the perception of impact forces.

Robbins refers to this inaccurate perception of impact forces as a perceptual illusion, and has demonstrated that it can result in chronically increased impact forces that cannot be sufficiently damped by the neuromuscular system.5

The effect of footwear on movement accuracy
Common footwear sole materials, such as those used in many walking and running shoes, filter out mechanical stimuli that assist in precisely judging plantar load and orientation of the plantar surface with respect to the leg.5 It has also been demonstrated that as midsole thickness increases and hardness decreases, balance and stability during ambulation decrease.7 This instability may result from the altered perception of vibrational forces; we use impact forces not only to know how hard we strike the ground, but also to maintain dynamic balance.

Footwear innovation today trends toward a minimally cushioned shoe as a means to optimize movement accuracy. The enhanced plantar stimulation afforded by minimal footwear can translate to improved overall foot function. Chen and colleagues demonstrated that habitual shod runners who transitioned to minimalist shoes developed a significant increase in leg and foot-muscle volume.8 Safely transitioning to minimalist toe shoes was associated with a greater increase in intrinsic foot-muscle size compared with a control group wearing traditional running shoes.9

In addition to footwear outsole design influencing plantar sensory stimulation, insole design also has been shown to either positively or negatively influence the perception of sensory stimulation during dynamic movement. Waddington and coworkers demonstrated that movement discrimination scores in soccer players were significantly worse when wearing football cleats and socks, compared with barefoot data collected at the same time.10 The substitution of textured insoles for conventional smooth insoles in the football cleats was found to restore movement discrimination to those achieved barefoot. The addition of rigid irregularities to flat, fairly rigid insoles has been shown to reduce vertical ground reaction forces to mimic barefoot conditions.7 It was determined that the vertical deformations of textured insoles anchor the epidermis in the horizontal plane which causes shear distortion, providing stimulus to the plantar foot.

Clinical applications of movement accuracy
A deficit in movement accuracy must be considered in patients who complain of pain on ambulation.

This movement accuracy relies on the anticipation of sensory stimulation from gravity and ground reaction forces, followed by how quickly and efficiently those forces can be damped, stored, and elastically released. A delay—or illusion in perception by the plantar foot—puts the patient in a reactive state, which slows the patient’s ability to dissipate ground reaction forces. This reactive state can increase not only the risk of impact-related injury, but also forces and stress to joints of the feet, knees, and lower back.

Why might a patient be in a reactive state? As described earlier, footwear is one cause, but medically-associated alterations in movement accuracy must also be considered. A patient may develop decreased plantar sensitivity from a disease such as diabetes mellitus, Parkinson’s disease, and multiple sclerosis. These patients may benefit from the use of textured insoles or minimal footwear.

Our goal for our patients is better movement. Because movement is driven by sensory stimulation, the question for some of our patients may be how we can help the patient better perceive sensory stimulation of movement.

Dr Splichal is a podiatrist in private practice.