History, philosophy, and value of mechanical models in sports science and engineering

This article is giving an introduction to JSET’s special section on mechanical models. It explains why mechanical models represent essential tools in sports engineering, defines their potential application fields and proposes a possible categorization. Further two short examples of mechanical models with corresponding literature and an overview of the manuscripts included in this special section are given.

A cluster analysis of basketball players for each of the five traditionally defined positions

Determining the players’ playing styles and bringing the right players together are very important for winning in basketball. This study aimed to group basketball players into similar clusters according to their playing styles for each of the traditionally defined five positions (point guard (PG), shooting guard (SG), small forward (SF), power forward (PF), and center (C)). This way, teams would be able to identify their type of players to help them determine what type of players they should recruit to build a better team. The 17 game-related statistics from 15 seasons of the National Basketball Association (NBA) were analyzed using a hierarchical clustering method. The cluster validity indices (CVIs) were used to determine the optimum number of groups. Based on this analysis, four clusters were identified for PG, SG, and SF positions, while five clusters for PF position and six clusters for C position were established. In addition to the definition of the created clusters, their individual achievements were examined based on three performance indicators: adjusted plus-minus (APM), average points differential, and the percentage of clusters on winning teams. This study contributes to the evaluation of team compatibility, which is a significant part of winning, as it allows one to determine the playing styles for each position, while examining the success of position pair combinations.

Laboratory assessment of a head impact sensor for youth soccer ball heading impacts using an anthropomorphic test device

Recent advances in technology have enabled the development of instrumented equipment, which estimate the head impact kinematics of athletes in vivo. One such headband-mounted impact sensor is the SIM-G (Triax Technologies, Norwalk, CT, USA), which has been previously used to investigate the biomechanics of soccer heading by human subjects. Previous studies have evaluated the accuracy of the SIM-G for pure rotation and pendulum, impulse hammer and drop rig impacts. The current study used a soccer ball heading model to evaluate the accuracy of the SIM-G. A soccer ball was projected at the head of an anthropomorphic test device (ATD) representing a 10-year-old to replicate the heading maneuver at various impact sites, angles and speeds previously identified in youth soccer. Linear regression revealed that the SIM-G sensor overestimated the peak angular velocity and linear acceleration recorded by the ATD headform by approximately 44% and 105%, respectively. Tests in which the ball directly contacted the SIM-G sensor resulted in the largest peak linear accelerations. Glancing impacts were significantly associated with a decrease in percentage error of the SIM-G sensor peak angular velocity data relative to the ATD reference data. While it may not demonstrate accuracy in estimating the magnitudes of head impacts, the SIM-G remains a useful tool to provide estimates of head impact exposure for soccer players.

Ageing of climbing ropes with and without hydrophobic coating

The effects of 4 months of weather exposure on the ageing of dynamic climbing ropes made of polyamide 6 were studied and differences between ropes with and without hydrophobic coating were examined. The polyamide degradation of the rope yarns was studied using infrared spectroscopy and a quasi-static tensile test. The number of falls to failure and the maximum force on the climber in a fall were evaluated with a drop test according to the UIAA 101 standard. Moreover, changes in the length of the ropes due to weathering were measured. The following results were found. After 4 months of weathering, sheath yarns of the coated rope showed a greater decrease in breaking force than those of the uncoated rope, which might be due to reactions of polyamide with radicals formed during the photo-induced oxidation of the coating. In contrast, the core yarns from the uncoated rope showed a greater decrease in breaking force than those from the coated rope, probably due to prolonged exposure of the uncoated core to water with possibly dissolved atmospheric acids. Furthermore, the decrease in the number of falls to failure was greater in the uncoated than in the coated rope. This difference was explained by a mechanism of changes in radial pressure of the sheath on the core. Regarding the maximum force on the climber, no significant changes due to ageing were observed during the drop test. Thus, it was concluded that 4 months of weather exposure do not pose a safety risk for climbing ropes, but the negative effect of coating on the ageing of polyamide might be detrimental when it comes to static personal safety equipment, such as slings or accessory cords.

Softball bat inertial considerations and their effects on female softball swing mechanics and estimated bat performance

This paper discusses the role of the inertial properties in the design and selection of the softball bat, and their interrelated effects on female softball swing mechanics and bat performance. The study was performed using 14 collegiate subjects whose swings were analyzed via a computer model, which included methods for estimating post-impact ball velocity. The model was driven kinematically by subject swing recordings using two different trial bats with markedly different inertial properties. Using this method, the following information was determined: subjects altered their linear kinetic inputs while applying consistent angular kinetic inputs to maintain nearly consistent trajectories; subjects compensated for increased bat inertia by modification of the bat instantaneous center-of-rotation trajectory ( ICR); and swing trajectory influenced the bat’s inertial feel, actual, and ideal impact locations, and batted-ball velocity. Subsequently, the mass properties of 27 collegiate level bats were input into the model for each subject trial. Using this information, the relative changes in kinetic inputs were quantified and the performance of the bats was estimated. Results showed that bat inertial properties varied considerably, and independent manipulation of individual properties was evident. In addition, subject kinetic inputs and bat performance measures were most affected by changes in mass center (CG) location, mass, and CG inertia. A modified definition of IGRIP based upon the ICR was presented, and practical implications for designers and practitioners were offered based upon these findings.

A destructible headform for the assessment of sports impacts

Commercially available headforms, such as the Hybrid-III and EN 960 headforms, have been used effectively to investigate the mechanics of head impacts. These headforms may result in accelerations that are unrepresentative of a human head in some impact scenarios. This may be important when considering impacts that produce areas of high pressure, since skull deformation and resonance excitation may influence the dynamic response. The National Operating Committee on Standards for Athletic Equipment (NOCSAE) headform may produce a more suitable response during these types of impacts due to the more representative skull component. However, permanent deformation may occur in some unprotected impact scenarios, resulting in the entire headform needing to be replaced. This paper outlines the development of a novel, modular and destructible headform (LU headform) that can be used in potentially destructive testing, where individual components can be replaced. The LU headform was modelled after a UK 50th percentile male. The inertial properties of the LU headform were within 6% of those observed in humans. The skull simulant properties were within the range of values reported for human tissue in two build orientations, but lower in one build orientation. The lowest and highest resonance frequencies observed in the headform model were within 5% of those observed in humans. Drop and projectile tests were conducted in line with previous cadaver tests with the observed accelerations within the range reported for post-mortem human subjects. The LU headform offers a practical means of simulating head dynamics during localised unprotected impacts or in protected impacts where local deformation and/or resonance frequency excitation remains possible.

The range of bicycle helmet performance at real world impact locations

In the United States, all bicycle helmets must comply with the standard created by the Consumer Product Safety Commission (CPSC). In this standard, bike helmets are only required to by tested above an established test line. Unregulated helmet performance below the test line could pose an increased risk of head injury to riders. This study quantified the impact locations of damaged bike helmets from real-world accidents and tested the most commonly impacted locations under CPSC bike helmet testing protocol. Ninety-five real-world impact locations were quantified. The most common impact locations were side-middle (31.6%), rear boss-rim (13.7%), front boss-rim (9.5%), front boss-middle (9.5%), and rear boss-middle (9.5%). The side-middle, rear boss-rim, and front boss (front boss-middle and front boss-rim regions combined) were used for testing. Two of the most commonly impacted regions were below the test line (front boss-rim and rear boss-rim). Twelve purchased helmet models were tested under CPSC protocol at each location for a total of 36 impacts. An ANOVA test showed that impact location had a strong influence on the variance of peak linear acceleration (PLA) ( p = 0.002). A Tukey HSD post hoc test determined that PLA at the side-middle (214.9 ± 20.8 g) and front boss (228.0 ± 39.6 g) locations were significantly higher than the PLA at the rear boss-rim (191.5 ± 24.2 g) location. The highest recorded PLA (318.8 g) was at the front boss-rim region. This was the only test that exceeded the 300 g threshold. This study presented a method for quantifying real-world impact locations of damaged bike helmets. Higher variance in helmet performance was found at the regions on or below the test line than at the region above the test line.

An effective, low-cost method to improve the movement velocity measurement of a smartphone app during the bench press exercise

Some studies have reported considerable errors in the movement velocity measurement when using the My Lift app. This study aimed to investigate whether these errors may be related to the use of a range of movement (ROM) statically measured prior to the movement (ROMMYLIFT) instead of ROM dynamically monitored. Ten young adults performed two repetitions of the bench press exercise on a Smith machine with loads that allowed two velocity conditions (above and below 0.6 m s−1). The exercises were monitored by the My Lift app, a magnet and a rotary encoder. After, 15 older adults performed the same exercise at different percentages of 1RM, monitored by the My Lift app and a magnet. The results revealed that ROM dynamically obtained by encoder (reference method) with the mean velocity above (0.497 ± 0.069 m) and below (0.450 ± 0.056 m) 0.6 m s−1 were quite different ( p < 0.05; large effect) from the ROMMYLIFT (0.385 ± 0.040 m). These errors provided highly biased and heteroscedastic mean velocity measurements (mean errors approximately 22%). The errors observed in adults were also observed in the older participants, except for loads equal to 85% of 1RM. The magnet method proved to be valid, presenting measurements very close to the encoder (mean errors approximately 1.7%; r > 0.99). In conclusion, the use of ROMMYLIFT is inadequate, as the higher the movement velocity, the higher the errors, both for young and older adults. Thus, to improve the measurement of the My Lift app, it is recommended that the magnet method be used in conjunction with the app to more accurately determine the ROM.

The ADR Encoder is a reliable and valid device to measure barbell mean velocity during the Smith machine bench press exercise

Velocity-based training is a contemporary resistance training method, which uses lifting velocity to prescribe and assess the effects of training. However, the high cost of velocity monitoring devices can limit their use among strength and conditioning professionals. Therefore, this study aimed to examine the reliability and concurrent validity of an affordable linear position transducer (ADR Encoder) for measuring barbell velocity during the Smith machine bench press exercise. Twenty-eight resistance-trained males performed two blocks of six repetitions in a single session. Each block consisted of two repetitions at 40%, 60%, and 80% of their estimated one-repetition maximum. The mean velocity of the lifting phase was simultaneously recorded with the ADR Encoder and a gold-standard linear velocity transducer (T-Force® System). Both devices demonstrated high reliability for measuring mean velocity (ADR Encoder: CVrange = 2.80%–6.40% and ICCrange = 0.78–0.82; T-Force® System: CVrange = 3.27%–6.62% and ICCrange = 0.77–0.81). The ADR Encoder provided mean velocity at 40%1RM with a higher reliability than the T-Force® System (CVratio = 1.17), but the reliability did not differ between devices at higher loads (60%1RM–80%1RM) (CVratio ≤ 1.08). No fixed or proportional bias was observed for the different loads using least-products regression analysis, while the Bland–Altman plots revealed low systematic bias (0.01 m·s−1) and random errors (0.03 m·s−1). However, heteroscedasticity of the errors was observed between both devices ( R2 = 0.103). The high reliability and validity place the ADR Encoder as a low-cost device for accurately measuring mean velocity during the Smith machine bench press exercise.

Sensitivity of cycling Motion Performance Indicators (MPIs) to incremented load and their relationship with performance in professional cyclists

There is a lack of research assessing Motion Performance Indicators (MPIs), which have been recently made commercially available. Therefore, this study explored: (1) the influence of incremented exercise on MPIs and; (2) the relationships between MPIs and cycling performance at different intensities during a graded exercise test (GXT) in professional cyclists. Thirty-six professional cyclists performed GXT until exhaustion with their own bikes attached to a cycle ergometer. MPIs were collected using a real-time motion capture system based on inertial measurement units at 100 Hz of sample rate. Data were extracted from intensities of the GXT when lactate thresholds (LT1, LT2) and peak power (POpeak) were determined. Results showed that only Pelvic Angle ( p < 0.01, d > 1.15) and Pelvic Rotation ( p < 0.01, d > 1.37) were sensitive to increases in exercise intensity (i.e. greater inclination and increased rotation at greater power). Multivariate liner regression analyses showed that a reduced range of movement (ROM) for the upper legs at sub-maximum intensities (LT1 and LT2) was associated with greater power production ( r2 > 0.21), whilst a reduced ROM for the right foot was associated with greater POpeak ( r2 = 0.20). In conclusion, changes in movement patterns were limited to a greater inclination and rotation of the pelvis at maximum power without changes in other MPIs throughout the GXT. Cyclists who produced greater power presented less ROM for their upper legs at LT1 and LT2 whilst at POpeak and greater power production was moderately associated with less ROM for the right foot. Coaches may be able to use MPI to analyze for excess ROM, particularly at higher exercise intensities, as this seems to increase inefficiencies and limit power production.