Relevant Biomechanical Variables in Skateboarding: A Literature Review

Skateboarding, once regarded primarily as a means of transportation and entertainment for youth, has become a recognized professional sport, gaining global popularity. With its recent inclusion in the Olympics, a growing imperative exists to comprehensively understand biomechanics explaining skateboarding performance. This literature review seeks to consolidate knowledge within this domain, focusing on experimental and modeling studies about skateboard riding and tricks. The criteria for study selection encompassed content relevance and publication year, spanning from the last two decades and extending further back to 1980 following cross-referencing of seminal works. Peer-reviewed journal articles, conference proceedings, and books were considered, with comprehensive searches conducted on electronic databases, including SCOPUS, PubMed, Scielo, and Taylor & Francis. Comprehending the biomechanical facets of skateboarding is essential in promoting its use and ensuring safety among all practitioners. Insights into factors such as body kinetics, kinematics, and muscle activation represent a foundational step toward understanding the nuances of this sport with implications for both clinical and biomechanical research. Modern data collection systems such as inertial measurement units (IMU) and electromyography (EMG) offer unprecedented insights into human performance during skateboarding, such as joint range of motion, coordination, and muscle activation, whether in casual riding or executing complex tricks and maneuvers. Developing robust modeling approaches also holds promise for enhancing skateboarding training and performance. Crucially, these models can serve as the initial framework for understanding injury mechanisms and implementing strategies to improve performance and mitigate injury risks.

Lower extremity joint-level responses to pelvis perturbation during human walking

The human leg joints play a major role in balance control during walking. They facilitate leg swing, and modulate the ground (re)action forces to prevent a fall. The aim of this study is to provide and explore data on perturbed human walking to gain a better understanding of balance recovery during walking through joint-level control. Healthy walking subjects randomly received anteroposterior and mediolateral pelvis perturbations at the instance of toe-off. The open-source modeling tool OpenSim was used to perform inverse kinematics and inverse dynamics analysis. We found hip joint involvement in accelerating and then halting leg swing, suggesting active preparation for foot placement. Additionally, responses in the stance leg’s ankle and hip joints contribute to balance recovery by decreasing the body’s velocity in the perturbation direction. Modulation also occurs in the plane perpendicular to the perturbation direction, to safeguard balance in both planes. Finally, the recorded muscle activity suggests both spinal and supra-spinal mediated contributions to balance recovery, scaling with perturbation magnitude and direction. The presented data provide a unique and multi-joint insight in the complexity of both frontal and sagittal plane balance control during human walking in terms of joint angles, moments, and power, as well as muscle EMG responses.

Stabilizing skateboard speed-wobble with reflex delay

A simple mechanical model of the skateboard-skater system is analysed, in which the effect of human control is considered by means of a linear proportional-derivative (PD) controller with delay. The equations of motion of this non-holonomic system are neutral delay-differential equations. A linear stability analysis of the rectilinear motion is carried out analytically. It is shown how to vary the control gains with respect to the speed of the skateboard to stabilize the uniform motion. The critical reflex delay of the skater is determined as the function of the speed. Based on this analysis, we present an explanation for the linear instability of the skateboard-skater system at high speed. Moreover, the advantages of standing ahead of the centre of the board are demonstrated from the viewpoint of reflex delay and control gain sensitivity.

The stabilizing properties of foot yaw in human walking

Humans perform a variety of feedback adjustments to maintain balance during walking. These include lateral footfall placement, and center of pressure adjustment under the stance foot, to stabilize lateral balance. A less appreciated possibility would be to steer for balance like a bicycle, whose front wheel may be turned toward the direction of a lean to capture the center of mass. Humans could potentially combine steering with other strategies to distribute balance adjustments across multiple degrees of freedom. We tested whether human balance can theoretically benefit from steering, and experimentally tested for evidence of steering for balance. We first developed a simple dynamic walking model, which shows that bipedal walking may indeed be stabilized through steering-externally rotating the foot about vertical toward the direction of lateral lean for each footfall-governed by linear feedback control. Moreover, least effort (mean-square control torque) is required if steering is combined with lateral foot placement. If humans use such control, footfall variability should show a statistical coupling between external rotation with lateral placement. We therefore examined the spontaneous fluctuations of hundreds of strides of normal overground walking in healthy adults (N=26). We found significant coupling (P=9·10^-8), of 0.54rad of external rotation per meter of lateral foot deviation. Successive footfalls showed a weaker, negative correlation with each other, similar to how a bicycle׳s steering adjustment made for balance must be followed by gradual corrections to resume the original travel direction. Steering may be one of multiple strategies to stabilize balance during walking.

Energetic cost of walking with increased step variability

Step-by-step variations occur during normal human walking, induced in part by imperfect sensorimotor control and naturally occurring random perturbations. These effects might increase energy expenditure during walking, because they differ from the nominal preferred gait, which is typically the most economical, and because of the cost of making active feedback adjustments to maintain gait stability. We tested this hypothesis by artificially inducing greater step variability through visual perturbations from a virtual reality display, and measuring the effect on energy expenditure. Young healthy adult subjects (N=11) walked on a treadmill while viewing a virtual hallway, to which virtual perturbations were applied in fore-aft or medio-lateral directions. The greatest effect on gait was achieved with medio-lateral visual perturbations, which resulted in a 65% increase in step width variability and a 5.9% increase (both P<0.05) in net metabolic rate compared to walking without perturbations. Perturbations generally induced greater variability in both step width and (to a lesser degree) step length, and also induced slightly wider and (to a lesser degree) shorter mean steps. Each of these measures was found to correlate significantly with each other, regardless of perturbation direction and magnitude. They also correlated with metabolic rate (P<0.05 for each separate measure), despite explaining only a modest proportion of overall energetic variations (R(2)<0.40). Step variability increases with some gait disorders and with increasing age. Our results suggest that imperfect sensorimotor control may contribute to the increased metabolic cost of walking observed with such conditions.

EFFECTS OF FOOT SHAPE ON ENERGETIC EFFICIENCY AND DYNAMIC STABILITY OF PASSIVE DYNAMIC BIPED WITH UPPER BODY

Passive dynamic walking has been developed as a possible explanation for the efficiency of the human gait. In this paper, we investigate the effects of foot shape on energetic efficiency and dynamic stability of passivity-based bipeds with upper body. Three walking models with point feet, round feet and flat feet were presented. Each model has an upper body constrained to keep midway between legs. We use computer simulations to find which foot shapes are indeed optimal in view of energetic efficiency and dynamic stability for a passive dynamic biped with upper body. Simulation results indicate that feet improve both the energetic efficiency and dynamic stability of passive dynamic bipeds.

Is slow walking more stable?

Several efforts have been made to study gait stability using measures derived from nonlinear time-series analysis. The maximum finite time Lyapunov exponent (lambda(max)) quantifies how a system responds to an infinitesimally small perturbation. Recent studies suggested that slow walking leads to lower lambda(max) values, and thus is more stable than fast walking, but these studies suffer from methodological limitations. We studied the effects of walking speed on the amount of kinematic variability and stability in human walking. Trunk motions of 15 healthy volunteers were recorded in 3D during 2 min of treadmill walking at different speeds. From those time series, maximum Lyapunov exponents, indicating short-term and long-term divergence (lambda(S-stride) and lambda(L-stride)), and mean standard deviation (MeanSD) were calculated. lambda(S-stride) showed a linear decrease with increasing speed for forward-backward (AP) movements and quadratic effects (inverted U-shaped) for medio-lateral (ML) and up-down (VT) movements. lambda(L-stride) showed a quadratic effect (inverted U-shaped) of walking speed for AP movements, a linear decrease for ML movements, and a linear increase for VT movements. Moreover, positive correlations between lambda(S) and MeanSD were found for all directions, while lambda(L-stride) and MeanSD were correlated negatively in the AP direction. The different effects of walking speed on lambda(S-stride) and lambda(L-stride) for the different planes suggest that slow walking is not necessarily more stable than fast walking. The absence of a consistent pattern of correlations between lambda(L-stride) and MeanSD over the three directions suggests that variability and stability reflect, at least to a degree, different properties of the dynamics of walking.

Comparison of Mechanical Design and Energy Consumption of Adaptable, Passive-compliant Actuators

Different, adaptable, passive-compliant actuators have been developed recently such as the antagonistic setup of two Series Elastic Actuators, the Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator, the Actuator with Mechanically Adjustable Series Compliance, and the Variable Stiffness Actuator. The main purpose of these designs is to reduce the energy consumption of walking/running robots and prostheses. This paper presents a design formulation to link the different mechanical designs together, and a study on the power consumption of these actuators.

Ankle Actuation for Limit Cycle Walkers

Limit Cycle Walkers are bipeds that exhibit a stable cyclic gait without requiring local controllability at all times during gait. Well-known example are McGeer's “Passive Dynamic Walkers”, but the concept expands to actuated bipeds as involved in this study. Current state-of-the-art Limit Cycle Walkers excel in being very energy efficient, but their ability to handle disturbances (i.e. disturbance rejection) is still limited. A way to improve this ability while maintaining low energy consumption is the use of ankle actuation, which has so far seen few applications in this type of walker. In this paper we study the effect of (1) applying (passive) stiffness in the ankle joint, (2) applying control in the stance ankle based only on local sensor information and (3) modulating ankle push-off. For all three strategies the paper shows how they influence energy use and disturbance rejection of a simple point mass walking model, a more realistic model and a physical prototype. We find that applying a passive ankle spring that results in premature heel rise is energetically optimal and gives an actuation pattern that largely resembles that of humans. Local stance ankle control and ankle push-off modulation can improve the disturbance rejection of a Limit Cycle Walker by at least 60%, without increasing its energy use. These findings are substantiated by showing that our prototype is able to handle large disturbances such as a step-down of 5% of its leg length, while walking efficiently at a mechanical cost of transport of 0.09.