Abstract
Noticeable differences exist between treadmill and overground walking; kinematics, kinetics, and muscle activation patterns differ between the two. Many previous studies have attributed the differences to changes in visual information, air resistance, and psychological effects such as fear. In this study, we demonstrate that no treadmill serves as an inertial frame of reference. Considering the linear momentum principle, the finite sampling rate of the controller, and the limited power of the treadmill motor, we predict that 1) the error of the treadmill speed periodically varies depending on the locomotion phase and 2) this non-ideal behavior becomes more evident as the locomotion speed or the weight of the walker increases. Experimental observation confirmed our predictions by quantifying the variation of the actual treadmill belt speed and the ground reaction force in the anterior–posterior direction for different locomotion speeds and subject weights. These results emphasize a need for design criteria like the minimum sampling rate and the minimum motor power that treadmill locomotion studies should consider.
Highlights
Treadmills have been widely used for locomotion studies by virtue of their significant advantages: the speed can be controlled and the experiment can be done in a laboratory space, enabling simultaneous use of other stationary equipment
The model was obtained by multiple linear regression rather than mechanics; linear relations between the belt speed variation and all inputs are assumed and the corresponding coefficients were estimated by curve fittings
We demonstrate that the error of the treadmill speed depends on gait phase, the weight of a subject, and locomotion speed
Summary
Treadmills have been widely used for locomotion studies by virtue of their significant advantages: the speed can be controlled and the experiment can be done in a laboratory space, enabling simultaneous use of other stationary equipment. Savelberg et al reported that the speed of locomotion does not contribute to the belt speed variation This important study suggested that the energy exchange between the subject and the treadmill plausibly causes the kinematic differences between overground www.nature.com/scientificreports/. Sloot et al directly estimated the mechanical energy exchange between a subject and a treadmill by measuring the belt speed deviation and the ground reaction force using an instrumented treadmill[13] They reported that the total energy exchange is less than 1.6% of the work performed on the center of mass, the deviation of the belt speed is over 3%. The results verified the model prediction and showed that the speed variation clearly depended on the speed of locomotion as well as the weight of a subject and gait phase This finding differs from the results of the previous study by Savelberg et al.[12] but is consistent with the prediction based on mechanics
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