Dario Cazzola

A modified prebind engagement process reduces biomechanical loading on front row players during scrummaging:a cross-sectional study of 11 elite teams.

Aim Biomechanical studies of the rugby union scrum have typically been conducted using instrumented scrum machines, but a large-scale biomechanical analysis of live contested scrummaging is lacking. We investigated whether the biomechanical loading experienced by professional front row players during the engagement phase of live contested rugby scrums could be reduced using a modified engagement procedure. Methods Eleven professional teams (22 forward packs) performed repeated scrum trials for each of the three engagement techniques, outdoors, on natural turf. The engagement processes were the 2011/2012 (referee calls crouch-touch-pause-engage), 2012/2013 (referee calls crouch-touch-set) and 2013/2014 (props prebind with the opposition prior to the ‘Set’ command; PreBind) variants. Forces were estimated by pressure sensors on the shoulders of the front row players of one forward pack. Inertial Measurement Units were placed on an upper spine cervical landmark (C7) of the six front row players to record accelerations. Players’ motion was captured by multiple video cameras from three viewing perspectives and analysed in transverse and sagittal planes of motion. Results The PreBind technique reduced biomechanical loading in comparison with the other engagement techniques, with engagement speed, peak forces and peak accelerations of upper spine landmarks reduced by approximately 20%. There were no significant differences between techniques in terms of body kinematics and average force during the sustained push phase. Conclusions Using a scrum engagement process which involves binding with the opposition prior to the engagement reduces the stresses acting on players and therefore may represent a possible improvement for players’ safety.

The cost of transport of human running is not affected, as in walking, by wide acceleration/deceleration cycles.

Although most of the literature on locomotion energetics and biomechanics is about constant-speed experiments, humans and animals tend to move at variable speeds in their daily life. This study addresses the following questions: 1) how much extra metabolic energy is associated with traveling a unit distance by adopting acceleration/deceleration cycles in walking and running, with respect to constant speed, and 2) how can biomechanics explain those metabolic findings. Ten males and ten females walked and ran at fluctuating speeds (5 ± 0, ± 1, ± 1.5, ± 2, ± 2.5 km/h for treadmill walking, 11 ± 0, ± 1, ± 2, ± 3, ± 4 km/h for treadmill and field running) in cycles lasting 6 s. Field experiments, consisting of subjects following a laser spot projected from a computer-controlled astronomic telescope, were necessary to check the noninertial bias of the oscillating-speed treadmill. Metabolic cost of transport was found to be almost constant at all speed oscillations for running and up to ±2 km/h for walking, with no remarkable differences between laboratory and field results. The substantial constancy of the metabolic cost is not explained by the predicted cost of pure acceleration/deceleration. As for walking, results from speed-oscillation running suggest that the inherent within-stride, elastic energy-free accelerations/decelerations when moving at constant speed work as a mechanical buffer for among-stride speed fluctuations, with no extra metabolic cost. Also, a recent theory about the analogy between sprint (level) running and constant-speed running on gradients, together with the mechanical determinants of gradient locomotion, helps to interpret the present findings.

The biomechanics of race walking: Literature overview and new insights

Abstract This review aims to provide both researchers and coaches with a comprehensive overview of race walking biomechanics and to point out new viable route for future analyses. The examined literature has been divided into three categories according to the method of analysis: kinematics, ground reaction forces and joint power/efficiency. From an overall view, race walking athletes seem to adhere to the ‘straightened knee’ rule, but at race speed they do not observe the ‘no-flight time’ rule. The coach-oriented analysis highlights that stride length (SL) is more important than stride frequency (SF) for increasing speed and it is mainly obtained by ankle and hip joint power. Moreover, kinematic differences (SF, SL and flight time) between male and female athletes were shown. Also, we found that the maximal speed prediction according to dynamic similarity theory with walking (Froude number) is not applicable as the 3D trajectory of the body centre of mass does not follow an arc of circumference as in walking. The analysed literature shows some shortcomings: (1) the data collection is often unreliable because of the mixture of gender and performance level and (2) the analysis has sometimes been performed on a limited number of strides and speeds. These limitations lead to a scattered and incomplete gait description and a biased application of the results. The research strategy adopted so far is promising but further rigorous analyses need to be approached to obtain a fully comprehensive picture of race walking and to provide coaches with consistent results and reference values.

Spinal muscle activity in simulated rugby union scrummaging is affected by different engagement conditions.

Biomechanical studies of rugby union scrummaging have focused on kinetic and kinematic analyses, while muscle activation strategies employed by front‐row players during scrummaging are still unknown. The aim of the current study was to investigate the activity of spinal muscles during machine and live scrums. Nine male front‐row forwards scrummaged as individuals against a scrum machine under “crouch‐touch‐set” and “crouch‐bind‐set” conditions, and against a two‐player opposition in a simulated live condition. Muscle activities of the sternocleidomastoid, upper trapezius, and erector spinae were measured over the pre‐engagement, engagement, and sustained‐push phases. The “crouch‐bind‐set” condition increased muscle activity of the upper trapezius and sternocleidomastoid before and during the engagement phase in machine scrummaging. During the sustained‐push phase, live scrummaging generated higher activities of the erector spinae than either machine conditions. These results suggest that the pre‐bind, prior to engagement, may effectively prepare the cervical spine by stiffening joints before the impact phase. Additionally, machine scrummaging does not replicate the muscular demands of live scrummaging for the erector spinae, and for this reason, we advise rugby union forwards to ensure scrummaging is practiced in live situations to improve the specificity of their neuromuscular activation strategies in relation to resisting external loads.

Cervical spine injuries: A whole-body musculoskeletal model for the analysis of spinal loading

Cervical spine trauma from sport or traffic collisions can have devastating consequences for individuals and a high societal cost. The precise mechanisms of such injuries are still unknown as investigation is hampered by the difficulty in experimentally replicating the conditions under which these injuries occur. We harness the benefits of computer simulation to report on the creation and validation of i) a generic musculoskeletal model (MASI) for the analyses of cervical spine loading in healthy subjects, and ii) a population-specific version of the model (Rugby Model), for investigating cervical spine injury mechanisms during rugby activities. The musculoskeletal models were created in OpenSim, and validated against in vivo data of a healthy subject and a rugby player performing neck and upper limb movements. The novel aspects of the Rugby Model comprise i) population-specific inertial properties and muscle parameters representing rugby forward players, and ii) a custom scapula-clavicular joint that allows the application of multiple external loads. We confirm the utility of the developed generic and population-specific models via verification steps and validation of kinematics, joint moments and neuromuscular activations during rugby scrummaging and neck functional movements, which achieve results comparable with in vivo and in vitro data. The Rugby Model was validated and used for the first time to provide insight into anatomical loading and cervical spine injury mechanisms related to rugby, whilst the MASI introduces a new computational tool to allow investigation of spinal injuries arising from other sporting activities, transport, and ergonomic applications. The models used in this study are freely available at simtk.org and allow to integrate in silico analyses with experimental approaches in injury prevention.

Time-based calibrations of pressure sensors improve the estimation of force signals containing impulsive events

Piezoresistive pressure sensors are widely used in biomechanics applications involving both static and dynamic loading conditions. The overall accuracy of these sensors has been reported previously in the literature, and multiple linear or polynomial custom calibrations have been proposed to enhance sensors’ performance mainly in low dynamics conditions. The aim of this technical note was to propose a ‘point-to-point’ time-based method to improve Tekscan F-Scan sensor calibration procedures in reconstructing a force signal with variable dynamic content and duration, using an application-specific loading pattern, characterised by an initial impact followed by a slow dynamic phase. The performance of the proposed calibration procedure was compared with four methods divided into time-based calibrations (‘point-to-point’, ‘log time–based’ and Tekscan ‘step’) and linear calibrations (‘drop-ball’ and Tekscan ‘single-point’). The ‘point-to-point’ calibration was the only method providing accurate force estimation over the entire duration, showing an inaccuracy of about 10% both in impact and slow dynamic phases. Tekscan default calibrations (‘step’ and ‘single-point’) underestimated the criterion force by ~60% over the impact phase but performed better in the slow dynamic phase (~20% of inaccuracy). ‘Log time–based’ and ‘drop-ball’ performed well during the impact phase (~11%) but overestimated the slow dynamic phase by ~170%. For this reason, we recommend ‘point-to-point’ calibration for estimation of forces which are characterised by an initial impulsive event and a subsequent slow-changing load. These findings highlight the importance of selecting the most appropriate calibration with respect to signal dynamics, in terms of loading range, loading pattern and impact duration.

Can coordination variability identify performance factors and skill level in competitive sport? The case of race walking

Background Marginal changes in the execution of competitive sports movements can represent a significant change for performance success. However, such differences may emerge only at certain execution intensities and are not easily detectable through conventional biomechanical techniques. This study aimed to investigate if and how competition standard and progression speed affect race walking kinematics from both a conventional and a coordination variability perspective. Methods Fifteen experienced athletes divided into three groups (elite, international, and national) were studied while race walking on a treadmill at two different speeds (12.0 and 15.5 km/h). Basic gait parameters, the angular displacement of the pelvis and lower limbs, and the variability in continuous relative phase between six different joint couplings were analyzed. Results Most of the spatio-temporal, kinematic, and coordination variability measures proved sensitive to the change in speed. Conversely, non-linear dynamics measures highlighted differences between athletes of different competition standard when conventional analytical tools were not able to discriminate between different skill levels. Continuous relative phase variability was higher for national level athletes than international and elite in two couplings (pelvis obliquity—hip flex/extension and pelvis rotation—ankle dorsi/plantarflexion) and gait phases (early stance for the first coupling, propulsive phase for the second) that are deemed fundamental for correct technique and performance. Conclusion Measures of coordination variability showed to be a more sensitive tool for the fine detection of skill-dependent factors in competitive race walking, and showed good potential for being integrated in the assessment and monitoring of sports motor abilities.

Specific tackling situations affect the biomechanical demands experienced by rugby union players

Abstract Tackling in Rugby Union is an open skill which can involve high-speed collisions and is the match event associated with the greatest proportion of injuries. This study aimed to analyse the biomechanics of rugby tackling under three conditions: from a stationary position, with dominant and non-dominant shoulder, and moving forward, with dominant shoulder. A specially devised contact simulator, a 50-kg punch bag instrumented with pressure sensors, was translated towards the tackler (n = 15) to evaluate the effect of laterality and tackling approach on the external loads absorbed by the tackler, on head and trunk motion, and on trunk muscle activities. Peak impact force was substantially higher in the stationary dominant (2.84 ± 0.74 kN) than in the stationary non-dominant condition (2.44 ± 0.64 kN), but lower than in the moving condition (3.40 ± 0.86 kN). Muscle activation started on average 300 ms before impact, with higher activation for impact-side trapezius and non-impact-side erector spinae and gluteus maximus muscles. Players’ technique for non-dominant-side tackles was less compliant with current coaching recommendations in terms of cervical motion (more neck flexion and lateral bending in the stationary non-dominant condition) and players could benefit from specific coaching focus on non-dominant-side tackles.

Pre-binding prior to full engagement improves loading conditions for front row players in contested rugby union scrums

We investigated the effect of a “PreBind” engagement protocol on the biomechanics of contested Rugby Union scrummaging at different playing levels. “PreBind” requires front‐row props to take a bind on opposing players prior to the engagement, and to maintain the bind throughout the scrum duration. Twenty‐seven teams from five different playing levels performed live scrums under realistic conditions. Video analysis, pressures sensors, and inertial measurement units measured biomechanical outcomes as teams scrummaged following different engagement protocols: the CTPE (referee calls “crouch‐touch‐pause‐engage”), the CTS (“crouch‐touch‐set”), and the PreBind (“crouch‐bind‐set”) variants. PreBind reduced the set‐up distance between the packs (−27%) and the speed at which they came into contact by more than 20%. The peak biomechanical stresses acting on front rows during the engagement phase were decreased in PreBind by 14–25% with respect to CTPE and CTS, without reducing the capability to generate force in the subsequent sustained push. No relevant main effects were recorded for playing level due to within‐group variability and there were no interaction effects between playing level and engagement protocol. Pre‐binding reduced many mechanical quantities that have been indicated as possible factors for chronic and acute injury, and may lead to safer engagement conditions without affecting subsequent performance.

On the Estimation Accuracy of the 3D Body Center of Mass Trajectory during Human Locomotion: Inverse vs. Forward Dynamics

The dynamics of body center of mass (BCoM) 3D trajectory during locomotion is crucial to the mechanical understanding of the different gaits. Forward Dynamics (FD) obtains BCoM motion from ground reaction forces while Inverse Dynamics (ID) estimates BCoM position and speed from motion capture of body segments. These two techniques are widely used by the literature on the estimation of BCoM. Despite the specific pros and cons of both methods, FD is less biased and considered as the golden standard, while ID estimates strongly depend on the segmental model adopted to schematically represent the moving body. In these experiments a single subject walked, ran, (uni- and bi-laterally) skipped, and race-walked at a wide range of speeds on a treadmill with force sensors underneath. In all conditions a simultaneous motion capture (8 cameras, 36 markers) took place. 3D BCoM trajectories computed according to five marker set models of ID have been compared to the one obtained by FD on the same (about 2,700) strides. Such a comparison aims to check the validity of the investigated models to capture the “true” dynamics of gaits in terms of distance between paths, mechanical external work and energy recovery. Results allow to conclude that: (1) among gaits, race walking is the most critical in being described by ID, (2) among the investigated segmental models, those capturing the motion of four limbs and trunk more closely reproduce the subtle temporal and spatial changes of BCoM trajectory within the strides of most gaits, (3) FD-ID discrepancy in external work is speed dependent within a gait in the most unsuccessful models, and (4) the internal work is not affected by the difference in BCoM estimates.