The effects of applying external loads on human jumping mechanics

The effects of external loading on jumping mechanics have been extensively investigated; however, review of the body of knowledge revealed a number of methodological issues, as well as a common omission of key mechanical theory, with limited studies investigating the effects of changing the position of the external load. Therefore, the primary aim of this thesis was to investigate the effects of changing the position of the external load and training status on system centre of mass and lower limb joint mechanics during countermovement jumping, with a particular emphasis on power output. Section 3 (Mundy, Lake, Carden, Smith, & Lauder, 2016a) established that the force platform method and the combined method cannot be used interchangeably within practice for measuring power output, and argued that the force platform method should be adopted as the criterion method. Using the force platform method, section 4 (Mundy, Smith, Lauder, & Lake, 2016b) established that the effects of barbell loading on system centre of mass mechanics reported are often overemphasised, and it was argued that investigating the complex interaction between the underpinning force, temporal and spatial components is of interest to practitioners. Section 5 demonstrated that practitioners may overcome the constraints of barbell loading by changing the position of the external load to arms’ length using a hexagonal barbell, which facilitated greater system centre of mass and lower limb joint mechanics in strength-power trained athletes. However, as the effects of external load have been posited to be population specific, section 6 demonstrated that regardless of the magnitude of the external load, strength-power trained athletes produced significantly greater hip joint peak power outputs than their recreationally trained counterparts. As such, the findings of the thesis support the hypothesis that practitioners should prescribe the position of external loading that maximise hip joint peak power output. In conclusion, this thesis has not only made significant steps towards providing a standardised method of measuring power output, but it has also offered a revealing insight into the effects of training status, as well as the effects of changing the position and magnitude of the external load on system centre of mass and lower limb joint mechanics during countermovement jumping

Do the peak and mean force methods of assessing vertical jump force asymmetry agree?

The aim of this study was to assess agreement between peak and mean force methods of quantifying force asymmetry during the countermovement jump (CMJ). Forty-five men performed four CMJ with each foot on one of two force plates recording at 1000 Hz. Peak and mean were obtained from both sides during the braking and propulsion phases. The dominant side was obtained for the braking and propulsion phase as the side with the largest peak or mean force and agreement was assessed using percentage agreement and the kappa coefficient. Braking phase peak and mean force methods demonstrated a percentage agreement of 84% and a kappa value of 0.67 (95% confidence limits: 0.45 to 0.90), indicating substantial agreement. Propulsion phase peak and mean force methods demonstrated a percentage agreement of 87% and a kappa value of 0.72 (95% confidence limits: 0.51 to 0.93), indicating substantial agreement. While agreement was substantial, side-to-side differences were not reflected equally when peak and mean force methods of assessing CMJ asymmetry were used. These methods should not be used interchangeably, but rather a combined approach should be used where practitioners consider both peak and mean force to obtain the fullest picture of athlete asymmetry

Investigating Player Selection within UK Academy Soccer: The Application of Objective and Subjective Assessments in Detecting Talent

Talent selection and development in academy soccer is highly dependent on coach intuition. Given such reliance upon subjective inputs, a greater understanding towards the utility of coach intuition may prove invaluable. The present study investigated coach agreement, the associations between subjective and objective outcomes and prominent traits highlighted within player (de)selection. Academy players (n = 45, age = 14 ± 2yrs) and coaches (n = 10, age = 31 ± 5yrs) were recruited from a professional soccer academy. Objective assessments included tactical and psychological surveys, physical assessments (linear sprints, change of direction and jumping tasks) and performance analysis (performance assessment for team sports). Coach subjective player gradings were collected using a visual analogue scale aligned to the objective assessments. Lead and assistant coaches demonstrated poor-to-moderate agreements in perceived player skills (ICC = 0.48 to 0.76) and fair to almost perfect agreement in player (de)selection (ICC= 0.23-1.00, P <.001 to .26). However, coach agreement reduced as players aged. Likewise, a maturation related bias was present whereby biologically older players were selected over their lesser mature players. Moreover, coach intuition demonstrated a strong predictive capability to select players, whilst the study was incapable of distinguishing exclusive traits related to selection outcome

The Influence of Recruitment Age and Anthropometric and Physical Characteristics on the Development Pathway of English Academy Football Players

Purpose: This study aims firstly to investigate the influence of recruitment age on retention and release across the development pathway. Secondly, the study aims to explore the influence of anthropometric and physical characteristics on retention and release at different ages throughout the development pathway and the likelihood of obtaining a professional contract. Methods: Following ethics approval, a cross-sectional study tracking 4 cohorts of players over 5 years assessed 76 male youth football players (11-16 years) from an English football academy on three occasions annually in anthropometry, countermovement jump height, linear (30m and 15m) and multidirectional sprint time. Players were categorised based on their start and release date. Results: Starting early (i.e. before U12) in an academy was a key indicator of obtaining a professional contract, representing 87% of the players signed. Bayesian regression models suggest that the majority of differences in physical characteristics between players that were released and retained are trivial, small and / or uncertain. Players who attained a professional contract at 18 had slower 15m and 30m sprint times at U13-U15 (P>0=0.87–0.99), slower multidirectional sprint times at U14 (P>0=0.99) and a lower countermovement jump height at U13-U16 (P>0=0.88-0.99) compared with players who did not gain a contract. Conclusion: Players recruited early have an increased likelihood of gaining a professional contract. Physical assessments lack utility when used in isolation as a talent identification tool

Development and Reliability of Countermovement Jump Performance in Youth Athletes at Pre-, Circa- and Post-Peak Height Velocity

The purpose of this study was to establish the intrasession reliability of various outcome, propulsion and braking phase countermovement jump (CMJ) variables and to compare the mean differences in youth athletes at different stages of maturity. Thirty male participants, aged 10-16 years, were grouped as either pre-, circa- or post-peak height velocity (PHV) according to their percentage of predicted adult height. All participants performed 3 CMJ trials on a force plate, sampling at 1000 Hz. A one-way ANOVA identified statistically significant differences between maturity groups for all CMJ variables (P<0.05) excluding propulsion peak rate of force development (RFD), braking peak velocity and countermovement depth. Post-hoc analysis revealed that the significant differences in CMJ variables were between the pre- to post- and circa- to post-PHV groups (P <0.05), with moderate to very large effect sizes. Relative and absolute reliability improved with maturity as the post-PHV group demonstrated superior reliability scores (ICC = 0.627-0.984; CV% = 3.25-21.55) compared to circa- (ICC = 0.570-0.998; CV% = 1.82-20.05) and pre-PHV groups (ICC= 0.851-0.988; CV% = 2.16-14.12). In summary, these results suggest that the biggest differences in CMJ performance are observed between preto post- and circa- to post-PHV, and that careful consideration is warranted when selecting variables in youth athletes at pre- and circa-PHV, given the lower reliability scores observed

The Validity of Hawkin Dynamics Wireless Dual Force Plates for Measuring Countermovement Jump and Drop Jump Variables

Force plate testing is becoming more commonplace in sport due to the advent of commercially available, portable, and affordable force plate systems (i.e., hardware and software). Following the validation of the Hawkin Dynamics Inc. (HD) proprietary software in recent literature, the aim of this study was to determine the concurrent validity of the HD wireless dual force plate hardware for assessing vertical jumps. During a single testing session, the HD force plates were placed directly atop two adjacent Advanced Mechanical Technology Inc. in-ground force plates (the “gold standard”) to simultaneously collect vertical ground reaction forces produced by 20 participants (27 ± 6 years, 85 ± 14 kg, 176.5 ± 9.23 cm) during the countermovement jump (CMJ) and drop jump (DJ) tests (1000 Hz). Agreement between force plate systems was determined via ordinary least products regression using bootstrapped 95% confidence intervals. No bias was present between the two force plate systems for any of the CMJ and DJ variables, except DJ peak braking force (proportional bias) and DJ peak braking power (fixed and proportional bias). The HD system may be considered a valid alternative to the industry gold standard for assessing vertical jumps because fixed or proportional bias was identified for none of the CMJ variables (n = 17) and only 2 out of 18 DJ variables

Building for the Future: A Systematic Review of the Effects of Eccentric Resistance Training on Measures of Physical Performance in Youth Athletes

Background: Eccentric resistance training is recognised as an effective stimulus for enhancing measures of muscular strength and power in adult populations; however, its value in youth athletes is currently not well understood. Objective: The aim of this systematic review was to critically appraise the effects of eccentric resistance training on measures of physical performance (i.e. muscular strength, jump, sprint and change of direction) in youth athletes 18 years of age and under. Methods: Original journal articles published between 1950 and June 2022 were retrieved from electronic search engines of PubMed, SPORTDiscus and Google Scholar’s advanced search option. Full journal articles investigating the acute and chronic effects of eccentric resistance training on measures of physical performance in youth athletes (i.e. a person 18 years of age or under who competes in sport) were included. The methodological quality and bias of each study were assessed prior to data extraction using a modified Downs and Black checklist. Results: The search yielded 749 studies, of which 436 were duplicates. Three-hundred studies were excluded based upon title and abstract review and a further 5 studies were removed following the modified Downs and Black checklist. An additional 14 studies were identified during backward screening. Accordingly, 22 studies were included in our systematic review. The Nordic hamstring exercise and flywheel inertial training were the most frequently used eccentric resistance training methods in youth athletes. Improvements in physical performance following the Nordic hamstring exercise are dependent upon an increase in the breakpoint angle, rather than training volume (sets and repetitions), and are further elevated with the addition of hip extension exercises or high-speed running. A minimum of 3 familiarisation trials is necessary to elicit meaningful adaptations following flywheel inertial training. Furthermore, an emphasis should be placed upon decelerating the rotating flywheel during the final one to two thirds of the eccentric phase, rather than gradually throughout the entire eccentric phase. Conclusions: The findings of this systematic review support the inclusion of eccentric resistance training in youth athletes to improve measures of muscular strength, jump, sprint and change of direction performance. The current eccentric resistance training methods are predominantly limited to the Nordic hamstring exercise and flywheel inertial training; however, the efficacy of accentuated eccentric loading to improve jump performance warrants attention in future investigations

Experimental Evidence for the Effects of Calcium and Vitamin D on Bone: A Review

Animal models fed low calcium diets demonstrate a negative calcium balance and gross bone loss while the combination of calcium deficiency and oophorectomy enhances overall bone loss. Following oophorectomy the dietary calcium intake required to remain in balance increases some 5 fold, estimated to be approximately 1.3% dietary calcium. In the context of vitamin D and dietary calcium depletion, osteomalacia occurs only when low dietary calcium levels are combined with low vitamin D levels and osteoporosis occurs with either a low level of dietary calcium with adequate vitamin D status or when vitamin D status is low in the presence of adequate dietary calcium intake. Maximum bone architecture and strength is only achieved when an adequate vitamin D status is combined with sufficient dietary calcium to achieve a positive calcium balance. This anabolic effect occurs without a change to intestinal calcium absorption, suggesting dietary calcium and vitamin D have activities in addition to promoting a positive calcium balance. Each of the major bone cell types, osteoblasts, osteoclasts and osteocytes are capable of metabolizing 25 hydroxyvitamin D (25D) to 1,25 dihydroxyvitamin D (1,25D) to elicit biological activities including reduction of bone resorption by osteoclasts and to enhance maturation and mineralization by osteoblasts and osteocytes. Each of these activities is consistent with the actions of adequate circulating levels of 25D observed in vivo