Spatiotemporal gait parameter changes due to exposure to vertical whole-body vibration.

BACKGROUND: Vertical whole-body vibration (vWBV) during work, recreation, and transportation can have detrimental effects on physical and mental health. Studies have shown that lateral vibration at low frequencies (<3 Hz) can result in changes to spatiotemporal gait parameters. There are few studies which explore spatiotemporal gait changes due to vertical vibration at higher frequencies (> 3 Hz). This study seeks to assess the effect of vWBV on spatiotemporal gait parameters at a greater range of frequencies (≤ 30 Hz). METHODS: Stride Frequency (SF), Stride Length (SL), and Center of Pressure velocity (CoPv) was measured in seven male subjects (23 ± 4 years, 1.79 ± 0.05 m, 73.9 ± 9.7 kg) during In-Place Walking and nine male subjects (29 ± 7 years, 1.78 ± 0.07 m, 77.8 ± 9.9 kg; mean ± SD) during Treadmill Walking while exposed to vWBV. Load cells measured ground reaction forces during In-Place Walking and sensorized insoles acquired under-foot pressure during Treadmill Walking. Statistical tests included a one-way repeated-measures ANOVA, post-hoc two way paired T-tests, statistical power (1-ß), correlation (R2), and effect size (Cohen's d). RESULTS: While statistical significance was not found for changes in SF, SL, or Mean CoPv, small to large effects were found in all measured spatiotemporal parameters of both setups. During Treadmill Walking, vWBV was correlated with a decrease in SF (R2 = 0.925), an increase in SL (R2 = 0.908), and an increase in Mean CoPv (R2 = 0.921) and Max CoPv (R2 = 0.952) with a significant increase (p < 0.0083) in Max CoPv at frequencies of 8 Hz and higher. SIGNIFICANCE: Study results demonstrated that vWBV influences spatiotemporal gait parameters at frequencies greater than previously studied.

Four degree-of-freedom lumped parameter model of the foot-ankle system exposed to vertical vibration from 10 to 60 Hz with varying centre of pressure conditions.

Modelling the foot-ankle system (FAS) while exposed to foot-transmitted vibration (FTV) is essential for designing inhibition methods to prevent the effects of vibration-induced white-foot. K-means analysis was conducted on a data set containing vibration transmissibility from the floor to 24 anatomical locations on the right foot of 21 participants. The K-means analysis found three locations to be sufficient for summarising the FTV response. A three segment, four degrees-of-freedom lumped parameter model of the FAS was designed to model the transmissibility response at three locations when exposed to vertical vibration from 10 to 60 Hz. Reasonable results were found at the ankle, midfoot, and toes in the natural standing position (mean-squared error (ε) = 0.471, 0.089, 0.047) and forward centre of pressure (COP) (ε = 0.539, 0.058, 0.057). However, when the COP is backward, the model does not sufficiently capture the transmissibility response at the ankle (ε = 1.09, 0.219, 0.039). Practitioner summary The vibration transmissibility response of the foot-ankle system (FAS) was modelled with varying centre of pressure (COP) locations. Modelling the FAS using three transmissibility locations and two foot segments (rearfoot and forefoot) demonstrated reasonable results in a natural standing and forward COP position to test future intervention strategies. Abbreviations: COP: centre of pressure; DOF: degrees-of-freedom; FAS: foot-ankle system; FTV: foot-transmitted vibration; HAVS: hand-arm vibration syndrome; LDV: laser Doppler vibrometer; LP: lumped-parameter; VWT: vibration-induced white-toes; WBV: whole-body vibration.

Vibration transmissibility and apparent mass changes from vertical whole-body vibration exposure during stationary and propelled walking.

Whole-Body Vibration (WBV) is an occupational hazard affecting employees working with transportation, construction or heavy machinery. To minimize vibration-induced pathologies, ISO identified WBV exposure limits based on vibration transmissibility and apparent mass studies. The ISO guidelines do not account for variations in posture or movement. In our study, we measured the transmissibility and apparent mass at the mouth, lower back, and leg of participants during stationary and propelled walking. Stationary walking transmissibility was significantly higher at the lumbar spine and bite bar at 5 and 10 Hz compared to all higher frequencies while the distal tibia was lower at 5 Hz compared to 10 and 15 Hz. Propelled walking transmissibility was significantly higher at the bite bar and knee at 2 Hz than all higher frequencies. These results vary from previously published transmissibility values for static participants, showing that ISO standards should be adjusted for active workers.

Active tuning of stroke-induced vibrations by tennis players.

This paper investigates how tennis players control stroke-induced vibration. Its aim is to characterise how a tennis player deals with entering vibration waves or how he/she has the ability to finely adjust them. A specific experimental procedure was designed, based on simultaneously collecting sets of kinematic, vibration and electromyographic data during forehand strokes using various commercial rackets and stroke intensities. Using 14 expert players, a wide range of excitations at spectral and temporal levels were investigated. Energetic and spectral descriptors of stroke-induced vibration occurring at the racket handle and at the player's wrist and elbow were computed. Results indicated that vibrational characteristics are strongly governed by grip force and to a lower extent by the racket properties. Grip force management drives the amount of energy, as well as its distribution, into the forearm. Furthermore, hand-grip can be assimilated to an adaptive filter which can significantly modify the spectral parameters propagating into the player's upper limb. A significant outcome is that these spectral characteristics are as much dependent on the player as on the racket. This contribution opens up new perspectives in equipment manufacture by underlining the need to account for player/racket interaction in the design process.

The effects of player grip on the dynamic behaviour of a tennis racket.

The aim of this article is to characterise the extent to which the dynamic behaviour of a tennis racket is dependent on its mechanical characteristics and the modulation of the player's grip force. This problem is addressed through steps involving both experiment and modelling. The first step was a free boundary condition modal analysis on five commercial rackets. Operational modal analyses were carried out under "slight", "medium" and "strong" grip force conditions. Modal frequencies and damping factors were then obtained using a high-resolution method. Results indicated that the dynamic behaviour of a racket is not only determined by its mechanical characteristics, but is also highly dependent on the player's grip force. Depending on the grip force intensity, the first two bending modes and the first torsional mode frequencies respectively decreased and increased while damping factors increased. The second step considered the design of a phenomenological hand-gripped racket model. This model is fruitful in that it easily predicts the potential variations in a racket's dynamic behaviour according to the player's grip force. These results provide a new perspective on the player/racket interaction optimisation by revealing how grip force can drive racket dynamic behaviour, and hence underlining the necessity of taking the player into account in the racket design process.

A model of harp plucking.

In this paper, a model of the harp plucking is developed. It is split into two successive time phases, the sticking and the slipping phases, and uses a mechanical description of the human finger's behavior. The parameters of the model are identified through measurements of the finger/string displacements during the interaction. The validity of the model is verified using a configurable and repeatable robotic finger, enhanced with a silicone layer. A parametric study is performed to investigate the influence of the model's parameters on the free oscillations of the string. As a result, a direct implementation of the model produces an accurate simulation of a string response to a given finger motion, as compared to experimental data. The set of parameters that govern the plucking action is divided into two groups: Parameters controlled by the harpist and parameters intrinsic to the plucking. The former group and to a lesser extent the latter highly influence the initial conditions of the string vibrations. The simulations of the string's free oscillations highlight the large impact the model parameters have on the sound produced and therefore allows the understanding of how different players on the same instrument can produce a specific/personal sound quality.

Experimentally based description of harp plucking.

This paper describes an experimental study of string plucking for the classical harp. Its goal is to characterize the playing parameters that play the most important roles in expressivity, and in the way harp players recognize each other, even on isolated notes--what we call the "acoustical signature" of each player. We have designed a specific experimental setup using a high-speed camera that tracks some markers on the fingers and on the string. This provides accurate three-dimensional positioning of the finger and of the string throughout the plucking action, in different musical contexts. From measurements of ten harp players, combined with measurements of the soundboard vibrations, we extract a set of parameters that finely control the initial conditions of the string's free oscillations. Results indicate that these initial conditions are typically a complex mix of displacement and velocity, with additional rotation. Although remarkably reproducible by a single player--and the more so for professional players--we observe that some of these control parameters vary significantly from one player to another.