Zero- vs. one-dimensional, parametric vs. non-parametric, and confidence interval vs. hypothesis testing procedures in one-dimensional biomechanical trajectory analysis.

Biomechanical processes are often manifested as one-dimensional (1D) trajectories. It has been shown that 1D confidence intervals (CIs) are biased when based on 0D statistical procedures, and the non-parametric 1D bootstrap CI has emerged in the Biomechanics literature as a viable solution. The primary purpose of this paper was to clarify that, for 1D biomechanics datasets, the distinction between 0D and 1D methods is much more important than the distinction between parametric and non-parametric procedures. A secondary purpose was to demonstrate that a parametric equivalent to the 1D bootstrap exists in the form of a random field theory (RFT) correction for multiple comparisons. To emphasize these points we analyzed six datasets consisting of force and kinematic trajectories in one-sample, paired, two-sample and regression designs. Results showed, first, that the 1D bootstrap and other 1D non-parametric CIs were qualitatively identical to RFT CIs, and all were very different from 0D CIs. Second, 1D parametric and 1D non-parametric hypothesis testing results were qualitatively identical for all six datasets. Last, we highlight the limitations of 1D CIs by demonstrating that they are complex, design-dependent, and thus non-generalizable. These results suggest that (i) analyses of 1D data based on 0D models of randomness are generally biased unless one explicitly identifies 0D variables before the experiment, and (ii) parametric and non-parametric 1D hypothesis testing provide an unambiguous framework for analysis when one׳s hypothesis explicitly or implicitly pertains to whole 1D trajectories.JOURNAL OF BIOMECHANICS. 48(7):1277-1285 (2015)journal articl

Two-way ANOVA for scalar trajectories, with experimental evidence of non-phasic interactions

Kinematic and force trajectories are often normalized in time, with mean and variance summary statistic trajectories reported. It has been shown elsewhere, for simple one-factor experiments, that statistical testing can be conducted directly on those summary statistic trajectories using Random Field Theory (RFT). This technical note describes how RFT extends to two-factor designs, and how bizarre "non-phasic interactions" can occur in multi-factor experiments. We reanalyzed a public dataset detailing stance phase knee flexion during walking in (a) patellofemoral pain vs. controls, and (b) females vs. males using both a full model (with interaction effect) and a main-effects-only model. In both models the main effect of PAIN failed to reach significance at alpha=0.05. The main effect of GENDER reached significance over 5-40% stance (p=0.0005), but only for the full model. The interaction effect (in the full model) reached significance over 0-15% of stance (p=0.030), and resulted from greater flexion in females but decreased flexion in males in PFP vs. controls. Thus there was a non-phasic interaction in which a non-significant interaction (over 20-40% stance) suppressed the main effect of GENDER. Similarly, if we had only analyzed 20-40% stance, we would have committed Type II error by failing to reject the null PAIN-GENDER interaction hypothesis. The possible presence of non-phasic interactions implies that trajectory analyses must be conducted at the whole-trajectory level, because a failure to do so will generally miss non-phasic interactions if present. (C) 2014 Elsevier Ltd. All rights reserved.ArticleJOURNAL OF BIOMECHANICS. 48(1):186-189 (2015)journal articl

HOW DOES WHOLE BODY BALANCE CONTROL INTERACT WITH STROKE PERFORMANCE DURING THE TENNIS SERVE?

The purpose of this study was to investigate whether there is an interaction between mechanisms used to control whole body balance and racket performance. Fourteen experienced tennis players (nine males and five females; age, 21.5±3.9 yr; height, 1.7± 0.1 m; body mass 65.8± 8.1 kg) completed 10 successful tennis serves. Twelve optoelectronic cameras were used to collect kinematic data at 200 Hz (BTS bioengineering, Milan, Italy). Linear regression using 1D Statistical Parametric Mapping was used to identify interactions between the extrapolated centre of mass (XCoM) displacement in the anteroposterior direction and the changes in arms/trunk segment angular momentum, and peak anterior-posterior racket velocity. Overall, no meaningful relationships were found, except for a small time interval during the forward swing phase in which a greater increase in trunk angular momentum was associated with increased maximum racket velocity

THE ROLE OF TARGET LOCATION ON THE INTERACTION BETWEEN POSTURAL BALANCE MECHANISMS AND END-EFFECTOR PERFORMANCE IN THE TENNIS SERVE

The purpose of this study was to evaluate the relationship between end-effector (tennis racket) performance and postural balance across 4 serving locations. Eleven right-handed experienced tennis players participated in this study. Participants completed 10 successful tennis serves each to 4 serving locations. 12 optoelectronic cameras at 200 Hz (BTS bioengineering, Milan, Italy) were used to collect whole-body kinematic data. Statistical parametric mapping (SPM) with regression was used to identify the relationship between postural balance control (extrapolated centre of mass displacement and changes in arms/trunk angular momentum in forward/backward direction; 1D data) and end-effector performance (maximum racket forward velocity, 0D data) across the four serving locations. The results showed no systematic relationship between postural balance control mechanisms and end-effector performance across 4 different serving locations. It was concluded that serving to different locations likely involves different balance control mechanisms to adjust for target-specific serve technique constraints. For practical application, we found no evidence that balance control and end-effector performance are tightly related within elite tennis serve performance and that these could be trained separately

Vector field statistical analysis of kinematic and force trajectories

When investigating the dynamics of three-dimensional multi-body biomechanical systems it is often difficult to derive spatiotemporally directed predictions regarding experimentally induced effects. A paradigm of 'non-directed' hypothesis testing has emerged in the literature as a result. Non-directed analyses typically consist of ad hoc scalar extraction, an approach which substantially simplifies the original, highly multivariate datasets (many time points, many vector components). This paper describes a commensurately multivariate method as an alternative to scalar extraction. The method, called 'statistical parametric mapping' (SPM), uses random field theory to objectively identify field regions which co-vary significantly with the experimental design. We compared SPM to scalar extraction by re-analyzing three publicly available datasets: 3D knee kinematics, a ten-muscle force system, and 3D ground reaction forces. Scalar extraction was found to bias the analyses of all three datasets by failing to consider sufficient portions of the dataset, and/or by failing to consider covariance amongst vector components. SPM overcame both problems by conducting hypothesis testing at the (massively multivariate) vector trajectory level, with random field corrections simultaneously accounting for temporal correlation and vector covariance. While SPM has been widely demonstrated to be effective for analyzing 3D scalar fields, the current results are the first to demonstrate its effectiveness for 1D vector field analysis. It was concluded that SPM offers a generalized, statistically comprehensive solution to scalar extraction's oversimplification of vector trajectories, thereby making it useful for objectively guiding analyses of complex biomechanical systems.ArticleJOURNAL OF BIOMECHANICS. 46(14):2394-2401 (2013)journal articl

Postural adjustments in catching: on the interplay between segment stabilization and equilibrium control

The purpose of this study was to investigate postural adjustments in one-handed ball catching. Specifically, the functional role of anticipatory postural adjustments (APA) during the initial arm raising and subsequent postural adjustments (SPA) for equilibrium control and ball-hand impact were scrutinized. Full-body kinematics and kinetics allowed an analysis of the mechanical consequences of raising up the arm and preparing for ball-hand impact. APA for catching were suggested to be for segment stabilization. SPA had a functional role for equilibrium control by an inverted pendulum mechanism but were also involved in preparing for the impact of the ball on the hand, which was illustrated by an increased postural response at the end of the movement. These results were compared with raising up the arm in a well-studied reaction-time task, for which an additional counter rotation equilibrium mechanism was observed. Together, our findings demonstrate that postural adjustments should be investigated in relation to their specific functional task constraints, rather than generalizing the functional role of these postural adjustments over different tasks

DO MULTI-PLANAR ACL INJURY RISK VARIABLES RANK INDIVIDUALS MORE CONSISTENTLY ACROSS TASKS THAN UNI-PLANAR VARIABLES?

The ACL injury mechanism is multi-planar, yet rarely are multi-planar variables examined in an injury risk context. This study examines if multi-planar variables rank individuals more consistently across multiple tasks than uni-planar variables. Forty-four female athletes performed bilateral drop vertical jumps, single-leg hops, single-leg drop vertical jumps and sidestep tasks on their dominant leg. Uni-planar (KMab) and multi-planar (KMnsag) variables of the knee were extracted and correlated between tasks. Participants was ranked according to KMab and KMnsag, and then grouped into quintiles for each task. When variables are consistently ranked across tasks, a movement signature is identified. In total, uni-planar movement signatures were identified more than multi-planar movement signatures. However, both undesirable multi-planar and uni-planar movement signatures were identified in unique participants. Multi-planar and uni-planar variables are both important when screening for undesirable movements

Influence of balance surface on ankle stabilizing muscle activity in subjects with chronic ankle instability

Objective: To evaluate the effect of surface type on muscle activity of ankle stabilizing muscles in subjects with chronic ankle instability. Design: Case controlled, repeated measures study design. Subjects: 28 subjects with chronic ankle instability and 28 healthy controls. Methods: Subjects performed a barefooted single legged stance on uni-axial and multidirectional unstable surfaces. Muscle activity of the mm. peroneus longus/brevis, tibialis anterior, gastrocnemius medialis was registered using surface electromyography. Mixed model analysis was used to explore differences in muscle activity between subjects with chronic ankle instability and controls, and the effect of surface type on muscle activity levels within subjects with chronic ankle instability. Results: No differences were found between subjects with chronic ankle instability and healthy controls. Within subjects with chronic ankle instability, balancing along a frontal axis and on the BOSU evoked overall highest muscle activity level and the firm surface the least. Balancing on the firm surface showed the lowest tibialis anterior/peroneus longus ratio, followed by balancing along a frontal axis and on the Airex pad. Conclusions: Clinicians can use these findings to improve the focus of their balance training program by gradually progressing in difficulty level based on muscle activation levels taking cocontraction ratio’s into account

HOW DO TENNIS PLAYERS CONTROL THEIR BALANCE DURING THE SERVE?

The purpose of this study was to investigate how postural balance is manifested in high level tennis players, and whether movement variability is phase dependent. Twelve experienced tennis players (8 males and 4 females; age 21.5±4.11 years; height 174.75±6.06 cm; body mass 66.83±8.12 kg) completed 10 tennis successful serves. Whole-body kinematics were recorded and whole-body extrapolated centre of mass trajectories calculated. Within-subject variation was presented temporally to evaluate phase-dependent differences. Overall, our results showed individual balance control preferences and a progressive increase of within-subject variability throughout the serve movement. This knowledge will help trainers and coaches identify learning and performance needs associated to whole-body balance control

ISBS 2022 CONFERENCE PROCEEDINGS TITLE AND FOREWARD

The ISBS is an international society totally dedicated to biomechanics in sports, whose primary purposes are: To provide a forum for the exchange of ideas for sports biomechanics researchers, coaches and teachers. To bridge the gap between researchers and practitioners. To gather and disseminate information and materials on biomechanics in sports. Papers underwent a double blinded review process. Each paper in these proceedings has been reviewed by at least two members of the scientific committee