Z-CoiL Footwear has been extensively tested both in the Laboratory and in real life. With over 1 million pair sold, there is extensive data regarding the effectiveness of Z-CoiL footwear in the field.
Z-CoiL has three special parts that lead to its’ exceptional performance for those seeking relief from the pain and stress of walking on hard surfaces.
- A patented conical coil suspension system. This unique replaceable spring unit not only gives the user four times more compression distance that ordinary footwear but it can be adjusted to correct moderate or severe pronation by rotating the coil in 90 or 180 degrees. The primary cause of foot, leg and back pain is excessive impact.
- An orthotic. Z-CoiL features the world’s only built-in orthotic. This orthotic is critical in protecting the plantar ligaments in the foot. Flexible shoes are a primary contributor to plantar fasciitis and heel spurs.
- A rocker forefoot. Thick forefoot cushioning and rocker sole in the forefoot make rolling off the front of the foot soft and easy. Flat shoe require excessive bending in the forefoot and make pushing off each foot difficult.
Because Z-CoiL is so unique – it performs very different from conventional shoes. The primary difference shows up in the two independent studies below. Z-CoiL footwear dramatically reduces impact vs. conventional shoes. Impact is the primary cause of foot leg and back pain.
Z-CoiL footwear is foot gear for life. If you have any questions on these studies or how Z-CoiL might perform for your particular situation please call a Z-CoiL Technician at 800.268.6239.
Ergonomic Implementation to Reduce Plantar Pressure
Research study conducted by Wayne Scheler, New Mexico Highlands University
Evaluation of Z-Tech (DBA Z-CoiL) Running shoes
The pulse curves for the Recoil running shoes were typically of longer total duration than the curves for the elastomer cushioned running shoe. Pulse durations were observed to be as much as 50% longer for the Recoil shoes in some cases. This was particularly true for the lower energy impacts that would be typical of relatively light weight runners (100-150 lbs.) or heavier runners with relatively little vertical displacement in their running style (<2 inches).
The longer duration pulse curves for the Recoil shoes were due to the curves being generally more bell-shaped than the curves generated by the elastomer cushioned running shoe. The curves for the typical running shoe tended to have a steeper slope at the onset of the shock pulse and lacked the transitional slope that was observed in the curves for the Recoil running shoes. This was interpreted to represent a more gradual onset of shock forces in the Z-CoiL shoes as compared to the elastomer cushioned running shoe.
The energy return levels of the Z-CoiL running shoes were not specifically measured as part of the testing procedures. This was due to a lack of high speed video equipment that would be required to precisely quantify the energy return levels. However, based on the magnitude of the second impact pulse seen on several of the pulse plots, it can be stated with relative certainty that the energy return for the Z-CoiL running shoes falls somewhere in the range of 40% – 50%.
The date obtained in this project seems to indicate one area in which the design of the Recoil shoe may be an improvement over that of the typical elastomer cushioned running shoe. The initial impact forces appear to be less abrupt in the Z-CoiL shoes, resulting in a reduction of the jarring effect to the foot and the lower leg of a runner as the heel impacts the ground. The fact that the peak forces are equivalent to those of the traditional running shoe may be offset by the longer period of time required to reach these peaks after initial impact.
Sandia National Laboratories
Cybernetic Systems Integration
SAND No. 2005-5045 P
This report summarizes findings from a gait comparison study that was performed at Sandia National Laboratories in Albuquerque, NM in July of 2005. Test subjects were outfitted with two kinds of shoes, Z-Coil and a leading competitor. They walked and jogged while acceleration data was collected. The results showed significantly lower mean peak accelerations in the Z-Coils when walking (13.34%, P value <0.0001) and jogging (17.2%, P value <0.0001). It is proposed that these reduced accelerations may be a reason why wearers of the Z-Coil shoes report reduced joint pain compared to traditional elastomer-based footwear.
The accelerations in Z-Coil shoes were significantly lower for all individuals who did not bottom out the spring. Accelerations averaged 13% lower while walking and 17% lower while running.
Many wearers of Z-Coil footwear cite a pain reduction when wearing Z-Coil brand shoes vs. traditional footwear. While there are many testimonials expressing this viewpoint, there is little scientific evidence to explain it.
Al Gallegos, owner of Z-Coil, hoped to use the expertise of organization 6633, Cybernetic Systems Integration through a small business technology transfer project to quantify the advantage that his shoes offer over traditional footwear. Given a somewhat limited budget, it was clear that a traditional gait analysis was out of the question, as this typically involves force plates that are capable of withstanding high loads while having a very short time response as well as high-fidelity cameras and motion tracking hardware. Such equipment was prohibitively expensive for this project, but a suitable and more portable alternative was found in the form of a three-axis accelerometer.
An accelerometer works by having a tiny arm that flexes in response to accelerations; in much the same way that a person’s head will lean back as his/her car accelerates. When this arm flexes, it presses on a piezoelectric material that releases charge when compressed. The internal circuitry of the accelerometer converts the charge to a voltage proportional to the current acceleration. The accelerometer used in this study contains three of these arms all mounted perpendicularly, measuring accelerations in all three spatial dimensions.
In an effort to quantify the benefits of the Z-Coil footwear, we sought to validate our hypothesis that the reduced pain individuals experience is due to reduced foot impacts that occur while performing everyday tasks such as walking or jogging. As an addition to this project, the study hardware, the accelerometer and data logger will be provided to Mr. Gallegos for use in his store.
In a typical gait analysis, data is gathered from a force plate ( similar to a bathroom scale) which registers forces as a person walks across it. This results in a graph that is typified by that in Figure 2. The force increases as the person’s heel strikes the force plate. The largest force is a combination of two separable quantities. The first force quantity is the weight of the person, and the second is the force caused by the mass of the person being accelerated upward.
Figure 2: Force plate response of normal gait cycle.(Taken from Kohle, D. et al. “Clinical gait
analysis by neural networks: issues and experiences,” cbms, vol. 00, no. , p. 138, 10th 1997.)
The force plate gait analysis is useful for many reasons, but as noted above, it is quite costly, and actually does not show the data of interest to this study. While the impact of the heel striking the force plate does show up, the force associated is only a fraction of the person’s body weight. By measuring accelerations, we are measuring the speed with which the person’s foot comes to a halt, not what portion of their body weight is pushing down on the sole of the shoe. It is our hypothesis that the accelerations the foot undergoes are transmitted up through the body and could be a contributor to, or a cause of, joint pain.
After receiving Human Subject Testing Board approval, an ad was placed in the Sandia Daily News bulletin, and the first three females and males to reply were accepted to be study participants. One female participant dropped out of the study for health reasons, and an alternate took her place. After reading and signing documents of informed consent, each participant performed the test outlined in the Experimental Setup section.
As mentioned above, we did not use a force sensor to gather gait data, but instead an accelerometer. This approach, while not the traditional means of gait assessment, does give useful data. The similarity of the two approaches is intuitive when one considers Newton’s law of motion.
F = ma (1)
Where F represents force, m – mass, and a – acceleration. The relationship between force and acceleration is directly proportional and varies only by the multiplier of mass, which is constant for each individual. In the case of traditional force-plate assessment, the equation governing the vertical forces is:
F = m (a + g) (2)
The F, m, and a portions are the same as above, representing force, mass, and acceleration. The g here is the acceleration due to gravity, which when multiplied times mass is commonly referred to as weight. When these two are added, as when the person’s weight is fully over the force plate and their mass is accelerating upward, the result is the high peak in Figure 2. In contrast to the typical force plate analysis data, Figure 3 shows a typical plot of our vectorially summed acceleration data.
Figure 3: Typical plot of summed accelerations in normal gait.
It was hypothesized that the accelerations should be lower in the Z-Coil shoes because of the very definition of acceleration, shown below.
a = ▲v (2)
Where a is acceleration, Dv is change in velocity, and Dt is the change in time. The velocity with which an individual’s foot approaches the ground should be the same, regardless of shoe type. By landing on a spring, the time over which the foot comes to a stop is increased. Inspection of the above equation shows that as Dt increase, the resulting acceleration decreases. This is the advantage that the Z-Coils offer over traditional shoes.
Each participant was instrumented as shown below in figure 1. The accelerometer (a Crossbow Technologies 3-axis LP model) was attached via a steel clip to the outside of the right shoe. When the test participant was comfortable with the shoes, the attached data logger was triggered to gather data at 512 Hertz. Each participant walked on a flat surface for approximately 60 paces while data were collected for all three accelerometer axes.
After data were collected for both types of shoes with the participant walking at a self selected “normal walking pace”, the accelerometer was moved to an elastic band that was attached with Velcro just below the participants’ knee. Fitted like this, the participant jogged, again at a “normal jogging pace” for three recorded segments of approximately 20 paces each in each type of shoe. The accelerometer was moved to the knee for the running portion of the test to ensure accurate data recording. The accelerometer used in these tests is accurate to } 10 times the acceleration due to gravity, or 10 g, and because preliminary tests had shown that heel-based accelerations during jogging could exceed 10 g, the knee mounting was used for running to prevent saturation of the sensor.
By using a three-axis accelerometer, our results were three channels of data that represent vectors in the x, y, and z directions. With the participant’s foot flat on the floor, the positive x vector point forwards, the positive y is upwards, and the z vector points medially through the ankle. These were vectorially summed to produce a total acceleration vector at each moment in time using equation
Where |a| is the summed accelerations, ax, ay, and az are the component accelerations in their respective axes. The output of each axis is displayed on the initial plot that is generated by the Crossbow software as shown in Figure 4A. When the axes are summed as in equation 3, the same five steps appear as in Figure 4B.
For comparison, summed accelerations were used. The summed data were analyzed, and the highest peaks (one per gait cycle, or stride) were averaged to produce the statistic we used for further comparison, called here the mean peak acceleration (MPA). The significance of the difference between the statistics for Z-Coils and the other shoes was found using the Student t-test and associated P values are reported in the Appendix.
In general, the accelerations recorded while individuals were wearing Z-Coils were significantly smaller than those in the others. In the walking trial, all individuals recorded significantly lower accelerations while wearing Z-Coil shoes except participant number 3. In the jogging trial, participant number 3 again had higher accelerations in the Z-Coils, as did another participant, and one individual had the same mean peak acceleration in both pairs of shoes. During the jogging trial, participant number 6 noted that it felt like the shoe was bottoming out, but had not felt that during the walking trial. This is consistent with the data for number 6 who had a lower MPA in Z-Coils while walking and a lower MPA in the competitor’s shoes while jogging. Our hypothesis for these results was that the individuals who had higher accelerations in Z-Coils were bottoming out the spring at heel strike. Without the support of the spring, the participants’ heels were, in effect, hitting the running surface – in this case a concrete floor. This hypothesis seemed reasonable, especially when the weight of participant number 3 (250 lbs) was considered. This was verified by numerically integrating the y-axis acceleration data twice to get vertical heel displacement. This was done for the individual with higher accelerations in the Z-Coils while walking and onerepresentative individual who did not. The results are given below, and all participant’s heights and weights are presented in Table 3 of the appendix.
Table 2: Vertical heel displacement during heel strike and weight of participants.
In the case of the one individual who experienced higher accelerations in the Z-Coils while walking, the vertical heel displacement was found to be 1.1 inches. This very nearly matches the actual height of the bottom of the Z-Coil shoe above the floor. In this individuals’ case, it is clear that the spring is becoming fully compressed, and the bottom of the shoe is striking the floor. In every other case, the vertical displacement is within the useful travel range of the spring, resulting in reduced accelerations.
For most participants, the Z-Coil shoes had significantly lower accelerations while walking (5 out of 6), and jogging (4 out of 6). It appears that the use of a stiffer spring in shoes that will be worn by heavier individuals would be beneficial, and should result in reduced accelerations for all wearers of Z-Coil shoes. The analysis technique used in this study is somewhat limited in its utility, and the possibility of gait variation among subjects was not researched. While the conclusion that some individuals are bottoming out the spring is plausible, and mathematically verified, it is possible that some portion of the abnormal gait accelerations are due to another cause, including gait type or a lack of familiarity with the Z-Coil shoes. This work proved that individuals who are properly fitted in their Z-Coil shoes with the correct spring have reduced accelerations when walking and jogging. This finding allows the exclusion of the data from individuals who suffered from inadequate springs. With the exclusion of the data for those subjects who bottomed out the springs, the reduction in walking MPA is changed from 8.7% to 13.4%. The reduction in jogging MPA is changed from 5.1% to17.2%. These are illustrated by Figure7.
This study showed a statistically significant reduction in acceleration for most wearers of Z-Coil shoes. The link between this finding and joint pain is suggested, but not researched. This study could be used as a pilot to future work where matched cohorts of individuals with joint pain could be observed for several months or years to determine long-term benefits of reduced accelerations during everyday activities.