Full length articleEffects of belt speed on the body’s center of mass motion relative to the center of pressure during treadmill walking
Introduction
Treadmills enable task-specific, repetitive gait training over a wide range of walking speeds, which has been shown to improve the mobility and balance control of over-ground walking in patients with gait impairments, e.g., post-stroke and Parkinson’s disease [1], [2]. Walking speed affects joint kinematics, kinetics and muscle activation in the lower extremities [3], [4], as well as gait stability [5], [6], [7], [8]. Therefore, correct belt speed selection is critical for training in patients with gait or balance impairments.
Previous studies on the effects of belt speed on treadmill walking mechanics have been mostly on individual gait variables but few on the whole body balance control. Belt speeds were found to have different effects on different variables. For example, while the stance width [9] and ankle moment [3] are not affected by belt speed, an increase in belt speed leads to decreased stance and double-limb support phase [9], but increased stride frequency, stride length [9], peak angles, moments and powers at the hip and knee, as well as increased peak ground reaction forces [3], [10]. Relating these changes to whole body balance control and speed selection for balance training is not straightforward, although they directly affect the whole body’s center of mass (COM) motion, which in turn determines the ground reaction forces and the center of pressure (COP) [11]. From the limited literature on the effects of belt speed on COM motion during treadmill walking, decreased medial/lateral (M/L) COM excursion but increased vertical COM excursion were found with increasing belt speed [12], [13]. These observed opposite trends in different states/components of the COM lead one to question whether there is a speed at which a “best-compromise” between states of the COM and/or COP exists. Can it be the preferred walking speed (PWS)?
The body’s balance control depends not only on the position, but also on the velocity of the COM relative to the base of support or COP [14], [15], and is different between single- and double-limb support (SLS and DLS) [16]. Horizontal COM-COP separations have been used to quantify balance control during over-ground walking [17], but ignoring the vertical component of the COM is a limitation that cannot be dealt with by normalizing the separation by leg length or body height [18]. To overcome this limitation, Lee and Chou proposed the COM-COP inclination angle (IA), and reported the effects of walking speeds on the IAs during over-ground walking in the elderly [19]. They did not find evidence in IAs to indicate a unique balance control strategy at the PWS. However, it must be noted that they did not consider the velocity of the COM, and the SLS and DLS were not considered separately [16]. Up until now, no study has reported the effects of belt speed on the COM positions and velocities relative to the COP during SLS and DLS of treadmill walking. It remains unclear whether the control of COM motion relative to the COP would be optimized at PWS.
The purpose of the current study was to investigate the effects of the belt speed on the body’s COM motions relative to the COP during treadmill walking, in terms of the IA and its rate of change (RCIA). It was hypothesized that values in IA and RCIA would be significantly increased whenever subjects walked faster or slower than at the PWS.
Section snippets
Subjects and data collection
Twelve young, healthy male adults (age: 24.5 ± 2.3 years, height: 172.3 ± 6.3 cm, mass: 68.4 ± 8.3 kg) participated in the current study with written informed consent as approved by the Institutional Research Board. All participants were free from any neuromusculoskeletal dysfunction and had normal or corrected-to-normal vision.
In a gait laboratory, each subject wore 39 retroreflective markers [20] and walked at five different belt speeds, namely 2.5, 3.0, PWS, 5.0 and 5.5 km/h, in a random order on an
Results
The cadence, step length, duration of SLS, anteroposterior COP excursion and vertical COM excursion were significantly increased with increasing belt speed (Table 1). In contrast, the mediolateral COM excursion, duration of the DLS and the stance phase were significantly reduced with increasing belt speed (Table 1). No significant differences were found in step width, anteroposterior COM and mediolateral COP excursion between belt speeds (Table 1).
The changes of the IAs and RCIAs appeared to
Discussion
With increasing belt speed, the A/P excursions of the COP were increased with increased step length to accommodate for the moving belt surface, but the M/L excursions of the COP and the step width remained unaltered. In contrast to the COP, the A/P excursion of the COM was kept within a limited range while the M/L excursion of the COM was decreased linearly and the vertical excursion was increased linearly with increasing belt speed (Table 1). Decreased M/L excursion of the COM reduces the
Conclusions
The effects of the belt speed on the control of the body’s COM motions relative to the COP during treadmill walking were determined in terms of the IA and RCIA. The COM-COP controls were different between SLS and DLS but were inter-related to form an integrated whole. With increasing belt speed, a quadratic trend was found in the range of frontal IA during SLS, that of the PWS being the minimum. The PWS appeared to be the best compromise between frontal stability during SLS and smooth weight
Acknowledgements
The authors are grateful for the financial support from the Ministry of Science and Technology, ROC. Thanks also to Mr. Ting-Yi Chen for his assistance in data collection.
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