The Cybex Intelligent Suspension System

Based in Science, Designed to Safeguard Runners from Injury

Runners often complain that using a treadmill is hard on their back and can cause knee injury.
Is this true? Or are they using the wrong treadmill?


Many treadmill manufacturers have tried to reduce the impact caused by running by developing softer, more pliable tread decks. They reasoned “logically” that the softer surface would absorb some or all of the “pounding” effect. But when you dig deeper into the overall biomechanics of treadmill running, you find there is more to the question of how to reduce injury potential than by simply softening the running surface.

Does a softer surface reduce ground reaction forces on the body?

A study published in the Journal of Strength and Conditioning Research (2000)1 measured the effect of jumping and stepping on step benches with different amounts of stiffness. It found that despite the varying degrees of forgiveness, the magnitude of the "ground reaction forces" (the force absorbed by the body when it impacts a surface) was identical from one bench to the next. In running, an athlete must create a ground reaction force, equal to 200% of their body weight, in order to propel himself/herself through space. The research findings tell us that no matter how soft the surface is, the amount of force that the body is subjected to is always the same.

Another study from Biomechanics Vol. 7B (1981)2 tested the effects of soft-soled running shoes on ground reaction forces and found similar results. Again, the total ground reaction force was 200% regardless of the make up of the shoe - further establishing that a soft running surface does nothing to reduce the overall forces on the body (or the knee).

Soft tread decks may actually contribute to injury

Many think that ground reaction forces are always detrimental to runners, but this is not the case. From the moment the runner contacts the ground, the foot, ankle, knee, and hip are moving and rotating, and generally changing shape. The ground reaction forces are necessary in order to allow those structures to return to their normal configurations, especially when pushing off to launch the body onto the opposite leg. Another study published in Medicine and Science in Sports and Exercise (1992)3 found that soft, pliable surfaces actually create conditions that may contribute to injury, by failing to provide a stable and consistent surface that the hips, knees, and ankles need to synchronously return to their normal positions.

Ground force AND time of contact are key to injury reduction

During their hands-on phase of the study, Cybex scientists learned that the ground reaction force is not the only constant when it comes to running. The amount of time spent on the ground is about a quarter of a second (.234 seconds to be exact), and this is constant for all speeds between 5.0 and 9.0 miles per hour.

More than that, the time that the foot spends on the ground can be broken down into consistent phases, absorption and propulsion. Those consistent phases give us our first true window into how running stresses arise – and more importantly, how to design a treadmill to reduce the risk of those injuries.

The absorption phase is also when the greatest amount of impact stress occurs. Lasting just .083 seconds, the absorption phase begins at the moment the heel strikes the ground (or treadmill deck). During that short window of time, the load on the joints increases from 0 to 80% of body weight. That increase in force is six times greater than any other phase, creating the highest amount of shock on the body while running.
It is for this reason that the first tenth of a second is the most critical time during which to apply changes to the treadmill in order to help reduce the risks of injury.

The Solution

With all this information in mind, the solution to reducing the risks of injury through treadmill deck design is two-fold.

First, the running deck should flex in the front to absorb the initial shock and dramatic load increase.

Second, it must then transition to a firm, stable, and consistent surface that enables the runner’s joints to align properly as it moves into the propulsion phase. This allows the runner to accelerate into the next stride with all joints synchronized.

Using this scientific knowledge, Cybex has designed a treadmill that safeguards the runner from stress and strain by reducing initial impact and optimizing joint mechanics and stride.

The result is the Cybex treadmill based on the IS3 technology.

1 Salem, G.J., Ward, S.R., and Lee, T.Q. (2000). Regulation of plantar-foot kinetics during exercises on step benches with markedly different structural properties. J Str Cond Res. 14(1): 26-31.
2 Nigg, B.M., Denoth, J., and Neukomm, P.A. (1981). Quantifying the load on the human body: problems and some possible solutions. In Biomechanics, Vol VIIB, eds. Morecki, A., Fidelus, K., Kedzior, K., and Wit, I. University Park Press. Baltimore, MD.
3 Hamill, J., Bates, B.T., and Holt, K.G. (1992). Timing of lower extremity joint actions during treadmill running. Med Sci Sports Exer. 24(7): 807-813.