Full length articleEffect of motor-assisted elliptical training speed and body weight support on center of pressure movement variability
Introduction
Improving walking is a primary goal for many patients and their therapists following neurologic injuries and illnesses. Therapists employ a variety of approaches to address walking limitations including practicing gait skills in real or simulated environments [1], as well as engaging in intensive repetition of stepping on treadmills and with robotic assistance [2].
Clinical research increasingly emphasizes the importance of integrating movement variability into gait training so patients explore and learn different movement strategies beneficial for navigating real world environments [3,4]. While movement variability is relatively easy to integrate into overground gait activities in naturalistic environments, it can be more challenging to assimilate during treadmill and robotic stepping given the activities’ repetitive and constrained nature [5]. Purposeful modification of training parameters (e.g., speed, external assistance level) might provide a means for manipulating task complexity and promoting a wider repertoire of movement strategies so variability across repetitions is neither too constrained (i.e., lacking any variability across repetitions) nor too variable (i.e., overly disorganized and random) [6].
Various analytics characterize movement variability during walking. Linear variables (e.g., standard deviation of anterior-posterior or medial-lateral CoP displacement) provide insights into amplitude and dispersion of kinematic and kinetic data, yet fail to characterize a movement’s temporally evolving nature (i.e., impact of preceding cycle’s movements on subsequent cycles’ movements) [4]. In contrast, non-linear measures reveal temporal variations in movement patterns across a series of movement strides/cycles, but fail to describe amplitude (e.g., dispersion from mean). In the field of non-linear dynamics, entropy is indicative of the system’s regularity [6]. More regular systems display entropy values closer to zero [7]. Multiple non-linear tools exist for exploring data series regularity. For example, detrended fluctuation analysis and Lyapunov exponent require relatively long time series (i.e., many data points) to generate reliable estimates [8], while data length does not have as great an impact when calculating SampEn (SE) [9]. Combined, linear and non-linear measures offer a more comprehensive description of movement variability.
Neuromuscular control of gait has been characterized by analyzing center of pressure (CoP) progression across the plantar foot surface [10]. CoP advances sequentially from heel to forefoot across successive stance periods when non-pathological gait is performed at a comfortable speed [11]. During treadmill walking, linear and nonlinear measures of CoP progression have demonstrated sensitivity to both walking speed and amplitude of body weight support (BWS) [[12], [13], [14]], thus providing clinicians with two manipulatable training variables for altering movement variability during treadmill gait.
The ICARE, a motor-assisted elliptical (Fig. 1), is used to promote intensive locomotor practice in rehabilitation, medical fitness and home settings [15]. Patients train at speeds up to 65 cycles per minute (CPM) with a motor’s assistance for pedal advancement [16]. Each ICARE cycle mimics a gait cycle’s kinematic and muscle demands [17] and faster motor-assisted training speeds increase muscle demands in key muscle required for stability and shock absorption [18]. An integrated external BWS system helps individuals with weakness and/or balance deficits maintain upright posture while safely exploring stability limits. As strength and endurance improve, faster training speeds and lower levels of BWS are used to promote continued challenge. The ICARE uses an endpoint control strategy (i.e., user is only in physical contact with machine through pedals and handles) vs. other robotic devices which often use leg orthoses to provide more rigid limb movement control. ICARE’s motor provides a guiding force to cycle each pedal in an ovaloid path while the user activates muscles in each limb to stabilize the body in sagittal (e.g., vastus lateralis) and frontal planes (e.g., gluteus medius) over the pedals [18]. Because physical guidance is only provided under the foot’s plantar aspect, participants can vary kinematic and muscle activation patterns across strides. Plantar pressure recorded from a subject training at 40 CPM demonstrated posterior to anterior CoP progression with each cycle [19], however no other speed or BWS conditions were reported to help elucidate the influence of these parameters on movement variability. Understanding speed’s and BWS’s influence on anterior-posterior and medial-lateral COP motion could guide understanding of requisite muscle activation patterns and strategies the central nervous system uses to maintain stability over a dynamically shifting base of support.
This pilot study involved a secondary analysis of previously recorded data to help elucidate the influence of ICARE’s speed and BWS training parameters on movement variability and to facilitate design of future prospective studies. We hypothesized that ICARE pedals’ fixed ovaloid path would constrain linear measures of anterior-posterior and medial-lateral CoP variability (standard deviation of length of each trajectory across strides) across training speeds and levels of BWS. However, we hypothesized that similar to treadmill walking [[12], [13], [14]], faster motor-assisted ICARE speeds (which lead to an increased lower extremity muscle demands) as well as increased BWS would reduce participants’ CoP control and result in greater nonlinear irregularity of anterior-posterior and medial-lateral CoP motion (SE). Understanding the effect of speed and BWS on movement variability is expected to provide clinicians with essential data to guide selection of training parameters to address not only cardiorespiratory training goals but also movement variability goals to promote adaptive walking skills.
Section snippets
Participants
As this is a secondary analysis of a data set, the participants and methods have been reported previously [18]. In brief, five males and five females (26.8 ± 3.8 years; 174.2 ± 8.5 cm; 80.4 ± 13.2 kg) free from musculoskeletal, neurological, and cardiovascular disorders that might affect walking were recruited from staff at Madonna Rehabilitation Hospitals. After obtaining consent, demographic data were collected. All procedures were reviewed and approved by Madonna’s institutional review board.
Results
Ten individuals participated. Two participants’ data were removed from the repeated measure analysis due to incomplete data acquisition of one condition for each participant. All other data are reported from the eight participants. While ICARE stride length was adjustable, each participant maintained their same self-selected stride length across conditions as evidenced by data displayed on the console. The mean preferred stride length across participants was 1.16 m (SD = 0.18 m;
Discussion
Successfully navigating homes and communities by foot requires that individuals be able to alter and adapt steps to varying speed, surface, and obstacle demands. Promoting variability in training is positively associated with better performance, adaptation, and quicker learning [22], as it helps individuals adapt their behaviors to environmental change. The relationship between variability and motor learning depends on the nature of intrinsic characteristics of the individual as well as task
Financial disclosures
Judith M. Burnfield and Thad W. Buster are inventors of the patented motor-assisted elliptical technology. The patented technology has been licensed to Sports Art for commercial distribution. Madonna Rehabilitation Hospitals receives royalties and a portion of these royalties is shared with inventors. For the remaining authors, no conflicts of interest were declared.
Acknowledgements
The contents of this research report were developed, in part, under a Grant (H133070209; Principal Investigator: Burnfield) from the United States Department of Education, National Institute on Disability and Rehabilitation Research. However, the contents do not necessarily represent the policy of the Department of Education, and endorsement by the Federal Government should not be assumed.
References (30)
- et al.
Measures of frontal plane stability during treadmill and overground walking
Gait Posture
(2010) - et al.
Human movement variability, nonlinear dynamics, and pathology: is there a connection?
Hum. Mov. Sci.
(2011) - et al.
Chapter 8-Gait variability: a theoretical framework for gait analysis and biomechanics
- et al.
Altered dynamic postural control during gait termination following concussion
Gait Posture
(2016) - et al.
Gait speed and gender effects on center of pressure progression during normal walking
Gait Posture
(2013) - et al.
Comparative analysis of speed’s impact on muscle demands during partial body weight support motor-assisted elliptical training
Gait Posture
(2014) - et al.
Investigation of center of pressure during gait and motor-assisted elliptical training in adults
Arch. Phys. Med. Rehabil.
(2018) - et al.
Kinematic variability and local dynamic stability of upper body motions when walking at different speeds
J. Biomech.
(2006) - et al.
Gait training strategies to optimize walking ability in people with stroke: a synthesis of the evidence
Expert Rev. Neurother.
(2007) - et al.
Rehabilitation robots for the treatment of sensorimotor deficits: a neurophysiological perspective
J. Neuroeng. Rehabil.
(2018)