Vertical stiffness asymmetries during drop jumping are related to ankle stiffness asymmetries

Asymmetry in vertical stiffness has been associated with increased injury incidence and impaired performance. The determinants of vertical stiffness asymmetry have not been previously investigated. Eighteen healthy males performed three unilateral drop jumps during which vertical stiffness and joint stiffness of the ankle and knee were calculated. Reactive strength index was also determined during the jumps using the ratio of flight time to ground contact time. ‘Moderate’ differences in vertical stiffness (t17 = 5.49; P < 0.001), ‘small’ differences in centre of mass displacement (t17 = -2.19; P = 0.043) and ‘trivial’ differences in ankle stiffness (t17 = 2.68; P = 0.016) were observed between stiff and compliant limbs. A model including ankle stiffness and reactive strength index symmetry angles explained 79% of the variance in vertical stiffness asymmetry (R2 = 0.79; P < 0.001). None of the symmetry angles were correlated to jump height or reactive strength index. Results suggest that asymmetries in ankle stiffness may play an important role in modulating vertical stiffness asymmetry in recreationally trained males

Relationships between lower-body muscle structure and, lower-body strength, explosiveness and eccentric leg stiffness in adolescent athletes

The purpose of the present study was to determine whether any relationships were present between lower-body muscle structure and, lower-body strength, variables measured during a counter-movement jump (CMJ) and squat jump (SJ), and eccentric leg stiffness, in adolescent athletes. Thirty junior male (n = 23) and female (n = 7) surfing athletes (14.8 ± 1.7 y; 1.63 ± 0.09 m; 54.8 ± 12.1 kg) undertook lower-body muscle structure assessment with ultrasonography and performed a; CMJ, SJ and an isomet-ric mid-thigh pull (IMTP). In addition, eccentric leg stiffness was calculated from variables of the CMJ and IMTP. Moderate to very large relationships (r = 0.46-0.73) were identified be-tween the thickness of the vastus lateralis (VL) and lateral gas-trocnemius (LG) muscles, and VL pennation angle and; peak force (PF) in the CMJ, SJ and IMTP. Additionally, moderate to large relationships (r = 0.37-0.59) were found between eccentric leg stiffness and; VL and LG thickness, VL pennation angle, and LG fascicle length, with a large relationship (r = 0.59) also present with IMTP PF. These results suggest that greater thick-ness of the VL and LG were related to improved maximal dy-namic and isometric strength, likely due to increased hypertro-phy of the extensor muscles. Furthermore, this increased thickness was related to greater eccentric leg stiffness, as the associated enhanced lower-body strength likely allowed for greater neuromuscular activation, and hence less compliance, during a stretch-shortening cycle

Vibration serviceability of footbridges under human-induced excitation : a literature review

Increasing strength of new structural materials and longer spans of new footbridges, accompanied with aesthetic requirements for greater slenderness, are resulting in more lively footbridge structures. In the past few years this issue attracted great public attention. The excessive lateral sway motion caused by crowd walking across the infamous Millennium Bridge in London is the prime example of the vibration serviceability problem of footbridges. In principle, consideration of footbridge vibration serviceability requires a characterisation of the vibration source, path and receiver. This paper is the most comprehensive review published to date of about 200 references which deal with these three key issues. The literature survey identified humans as the most important source of vibration for footbridges. However, modelling of the crowd-induced dynamic force is not clearly defined yet, despite some serious attempts to tackle this issue in the last few years. The vibration path is the mass, damping and stiffness of the footbridge. Of these, damping is the most uncertain but extremely important parameter as the resonant behaviour tends to govern vibration serviceability of footbridges. A typical receiver of footbridge vibrations is a pedestrian who is quite often the source of vibrations as well. Many scales for rating the human perception of vibrations have been found in the published literature. However, few are applicable to footbridges because a receiver is not stationary but is actually moving across the vibrating structure. During footbridge vibration, especially under crowd load, it seems that some form of human–structure interaction occurs. The problem of influence of walking people on footbridge vibration properties, such as the natural frequency and damping is not well understood, let alone quantified. Finally, there is not a single national or international design guidance which covers all aspects of the problem comprehensively and some form of their combination with other published information is prudent when designing major footbridge structures. The overdue update of the current codes to reflect the recent research achievements is a great challenge for the next 5–10 years

Do stiffness and asymmetries predict change of direction performance?

Change of direction speed (CODS) underpins performance in a wide range of sports but little is known about how stiffness and asymmetries affect CODS. Eighteen healthy males performed unilateral drop jumps to determine vertical, ankle, knee and hip stiffness, and a CODS test to evaluate left and right leg cutting performance during which ground reaction force data were sampled. A step-wise regression analysis was performed to ascertain the determinants of CODS time. A two-variable regression model explained 63% (R-2 = 0.63; P = 0.001) of CODS performance. The model included the mean vertical stiffness and jump height asymmetry determined during the drop jump. Faster athletes (n = 9) exhibited greater vertical stiffness (F = 12.40; P = 0.001) and less asymmetry in drop jump height (F = 6.02; P = 0.026) than slower athletes (n = 9); effect sizes were both "large" in magnitude. Results suggest that overall vertical stiffness and drop jump height asymmetry are the strongest predictors of CODS in a healthy, non-athletic population

The relationship between stiffness, asymmetries and change of direction speed

A thesis submitted to the University of Bedfordshire, in fulfilment of the requirements for the degree of Doctor of PhilosophyChange of direction speed (CODS) is an important determinant of performance in many sports. Greater stiffness of the lower limb should be beneficial to CODS, but this had not been well investigated. The purpose of this thesis was to establish the relationship between vertical stiffness, vertical stiffness asymmetries and CODS, with a view to augmenting CODS performance. The pilot study and studies 1-2 sought to determine the most reliable and ecologically valid method to assess stiffness in athletes required to perform changes of direction. The pilot study reported that the use of ultrasonography to determine Achilles tendon stiffness did not demonstrate appropriate reliability for inclusion in subsequent studies. Coefficients of variation (CVs) in excess of 27% were reported during an isometric plantar flexion task. Study 1 reported that CVs for vertical stiffness were lower when assessed during unilateral drop jumping (~7%) than during bilateral drop jumping (~12%) or bilateral hopping (~14%). Study 2 reported that the expression of vertical stiffness (P = 0.033) and vertical stiffness symmetry angle (P = 0.006) was significantly different across three performance tasks: unilateral drop jumping, bilateral drop jumping and bilateral hopping. Asymmetry percentages between compliant and stiff limbs were 5.6% (P < 0.001; d: 0.22), 23.3% (P = 0.001; d = 0.86) and 12.4% (P = 0.001; d = 0.39), respectively. Given the findings of studies 1 and 2, this thesis demonstrated the reliability and validity of a novel method by which to assess vertical stiffness - the unilateral drop jump. This task was used in subsequent studies to measure vertical stiffness. Study 3 sought to determine if vertical stiffness and vertical stiffness asymmetries influenced CODS performance determined during a 90o cutting task. Multiple regression analyses reported that mean vertical stiffness and asymmetry in jump height explained 63% (r2 = 0.63; P = 0.001) of CODS performance. Study 3 was the first investigation to demonstrate the importance of vertical stiffness to CODS performance. Study 4 sought to determine if acute exercise interventions designed to augment vertical stiffness would improve CODS. Unilateral and bilateral ‘stiffness’ interventions were evaluated against a control condition. CODS performances following the unilateral intervention were significantly faster than control (1.7%; P= 0.011; d = -1.08), but not significantly faster than the bilateral intervention (1.0% faster; P = 0.14; d = -0.59). Versus control, vertical stiffness was 14% greater (P = 0.049; d = 0.39) following the unilateral intervention. Study 4 demonstrated that a novel unilateral ‘stiffness’ intervention improved vertical stiffness and CODS performance. This highlights that the potential applicability of unilateral stiffness interventions in the pre-performance preparation of athletes

Lower limb stiffness testing in athletic performance: a critical review

Stiffness describes the resistance of a body to deformation. In regards to athletic performance, a stiffer leg-spring would be expected to augment performance by increasing utilisation of elastic energy. Two-dimensional spring-mass and torsional spring models can be applied to model whole-body (vertical and/or leg stiffness) and joint stiffness. Various tasks have been used to characterise stiffness, including hopping, gait, jumping, sledge ergometry and change of direction tasks. Appropriate levels of reliability have been reported in most tasks, although vary between investigations. Vertical stiffness has demonstrated the strongest reliability across tasks and may be more sensitive to changes in high-velocity running performance than leg stiffness. Joint stiffness demonstrates the weakest reliability, with ankle stiffness more reliable than knee stiffness. Determination of stiffness has typically necessitated force plate analyses, however, validated field-based equations permit determination of whole-body stiffness without force plates. Vertical, leg and joint stiffness measures have all demonstrated relationships with performance measures. Greater stiffness is typically demonstrated with increasing intensity (i.e. running velocity or hopping frequency). Greater stiffness is observed in athletes regularly subjecting the limb to high ground reaction forces (i.e. sprinters). Careful consideration should be given to the most appropriate assessment of stiffness on a team/individual basis

Lower extremity stiffness: considerations for testing, performance enhancement and injury risk

Force-deformation characteristics of the lower limb have been associated with athletic performance and may modulate the risk of injury. In-spite of these known associations, measurements of lower extremity stiffness are not commonly administered by strength and conditioning coaches. This review provides an overview of the available literature pertaining to the effects of lower extremity stiffness on physical performance and injury risk. Practical methods of monitoring and training stiffness are also discussed. The cumulative body of evidence indicates that increases in lower extremity stiffness are associated with heightened performance in athletic tasks such as hopping, jumping, throwing, endurance running, sprinting and changing direction. Relationships with injury are less conclusive as both excessive and insufficient limb stiffness have been postulated to increase risk. Thus, the ‘optimal’ level of stiffness appears to be dependent on the anthropometry, and physical capabilities of the athlete, in addition to sport-specific activity demands. Training interventions can positively enhance lower extremity stiffness, including isometric, eccentric and isotonic strength training and plyometrics. Complex training also appears to provide a potent stimulus and may be more effective than the use of singular training modes. For plyometric activities, it is recommended that coaches use a developmental sequence of exercises with increasing eccentric demand to provide an appropriate stimulus based on the training age and technical competency of the athlete

Evaluation of mechanical load in the musculoskeletal system : development of experimental and modeling methodologies for the study of the effect of exercise in human models

Doutoramento em Motricidade Humana, na especialidade de BiomecânicaA major concern of Biomechanics research is the evaluation of the mechanical load and power that the human body develops and endorses when performing high to moderate sport activities. With the purpose of increasing performance and reducing the risk of injury, substantial advances were accomplished to pursuit this goal, either on the laboratory techniques as well as modelling and simulation. Traditionally, the main focus was the assessment of kinematics, kinetics and electromyography data to describe the main mechanics and neuromuscular behaviour, when performing a certain movement. The use of methodologies that enable the quantification of the effect of a particular joint moment of force in the entire body or the contribution of an individual muscle force to accelerate the centre of mass of the body is quite relevant in biomechanical analysis. This is particularly important when dealing with explosive movements such as those that occur in sports activities, or in the clinical field when dealing with abnormal movement. At the same time, the advances in imaging technology allows us the use of some of those techniques to gather subject-specific information, particularly the muscle architectural parameters that are crucial to the production of force, such as muscle volume, muscle physiological cross-section area and muscle pennation angle. In the course of this dissertation, we investigated the use and/or combination of different methodologies to study the effect of mechanical load in the lower limb musculoskeletal system during a cyclic stretch-shortening exercise. We aimed at using an integrated approach to better characterize the behaviour of the musculoskeletal system when subjected to this type of mechanical load.RESUMO: Uma das principais preocupações da investigação em Biomecânica é a avaliação da carga mecânica que o corpo desenvolve e que consegue suportar quando realiza ações desportivas com nível de desempenho de moderado a elevado. Com o objetivo de melhorar a performance mas reduzindo o risco de lesão, têm sido realizados avanços significativos quer nas técnicas laboratoriais e equipamentos, quer nas técnicas de modelação e simulação. A investigação tradicional em biomecânica tem o seu foco na avaliação da cinemática, cinética e função neuromuscular para descrever a mecânica do corpo e o comportamento neuromuscular, durante a execução de um determinado movimento. No entanto, a utilização de metodologias que permitam a quantificação do efeito de um determinado momento de força articular em todos os segmentos corporais ou a contribuição de um momento de força muscular individual na aceleração do centro de massa do corpo é bastante relevante na análise biomecânica. Isto é particularmente importante quando se lida com movimentos explosivos, tais como os que ocorrem em actividades desportivas, ou no âmbito clínico quando se tratam de condições não normais ou patológicas. Ao mesmo tempo, os avanços na tecnologia de imagem permitem a utilização de algumas destas técnicas para recolher informações específicas do sujeito, nomeadamente no que diz respeito aos parâmetros arquitectónicos do músculo, que são cruciais para a produção da força, tal como o volume muscular, a área de secção transversal fisiológica ou o ângulo de penação. No decurso deste trabalho, foi investigada a utilização e/ou combinação de diferentes metodologias para estudar o efeito da carga mecânica no sistema musculo-esquelético do membro inferior durante um exercício de alongamento-encurtamento realizado de forma cíclica. O principal objetivo foi utilizar uma abordagem integrada para melhor caracterizar o comportamento do sistema músculo-esquelético, quando submetido a este tipo de carga mecânica.FCT - Fundação para a Ciência e a Tecnologi

Lower extremity stiffness: considerations for testing, performance enhancement and injury risk

Force-deformation characteristics of the lower limb have been associated with athletic performance and may modulate the risk of injury. In-spite of these known associations, measurements of lower extremity stiffness are not commonly administered by strength and conditioning coaches. This review provides an overview of the available literature pertaining to the effects of lower extremity stiffness on physical performance and injury risk. Practical methods of monitoring and training stiffness are also discussed. The cumulative body of evidence indicates that increases in lower extremity stiffness are associated with heightened performance in athletic tasks such as hopping, jumping, throwing, endurance running, sprinting and changing direction. Relationships with injury are less conclusive as both excessive and insufficient limb stiffness have been postulated to increase risk. Thus, the ‘optimal’ level of stiffness appears to be dependent on the anthropometry, and physical capabilities of the athlete, in addition to sport-specific activity demands. Training interventions can positively enhance lower extremity stiffness, including isometric, eccentric and isotonic strength training and plyometrics. Complex training also appears to provide a potent stimulus and may be more effective than the use of singular training modes. For plyometric activities, it is recommended that coaches use a developmental sequence of exercises with increasing eccentric demand to provide an appropriate stimulus based on the training age and technical competency of the athlete