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BELT BASED TREADMILLS - Useful Information
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Many belt driven treadmills are unsuitable for accurate observational gait analysis, it is for the following reasons that 'normal' running gait behavior is difficult to replicate.
The Trampoline Effect
All belt-based treadmills pull a "running surface" over a board (deck), which is specifically designed to attenuate the impact of foot strike (sometimes referred to as an 'orthopedic running board'). This artificial impact attenuation masks the true biomechanics of the foot and its natural ability to absorb shock. In addition, any rebound from initial impact will have an adverse affect on foot function (often mistakenly observed as excessive rearfoot pronation).
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Contact Grip
During initial foot contact, friction causes the belt to instantaneously grip the board. This results in an abrupt breaking moment before foot flat is reached. While the belt is gripping it can be seen to bunch-up beneath the foot. When the load is sufficient the foot is 'jerked' towards an early 'foot flat'.
Early Heel Lift
The alterations to the stance phase do not stop here. It has been proven that the more powerful the treadmill motor is, the more likely it is that the foot will be "pulled backwards" beneath the body. Therefore the natural kinetics and kinematics of the propulsion phase are masked. Additionally these alterations to the natural running gait will be disconcerting to any runner, causing them decrease stride length and drop their centre-of-mass.
Shorter Swing Phase
If an early heel lift occurs, due to the unnatural accelerated backward movement of the weight-bearing limb, then non-weight bearing leg will naturally prematurely reach initial contact, shortening stride length.
Lateral Shift
Some clients with excessive toe-out angle and genu valgum frequently drive the belt laterally during the propulsive phase. As the belt is designed to slide it exaggerates the movement. You can identify this by monitoring the gap between the edge of the treadmill and belt.
Noise & Vibration
Most orthopedic treadmills 'bounce' slightly, this generates both unnecessary noise and vibrations. These unusual circumstances in addition to the aforementioned effects contribute to an unscientific environment for professional, accurate gait analysis.
'Propel' not 'pulled'
The running surface on a 'belt' driven treadmill is pulled over the running deck. The powerful motor that drives this belt also unbalances the propulsion phase of the runner by pulling the foot beneath the hip rather than allowing the foot to drive the hip over the foot. Therefore the running gait is altered as you are only replicating the propulsive movement not creating it.
Therefore in order to increase the validity and relevance of gait analysis, changes must be made! New 'slat technology' is addressing the aforementioned errors as they run on very low friction sealed bearings, therefore they reducing the need to 'pull' the slat around, thus creating a better environment for you to replicate natural gait behavior.
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