A novel approach for athlete profiling: the unilateral dynamic strength index

The dynamic strength index (DSI) provides a ratio of the peak force an athlete can produce in both isometric and ballistic tasks. While the DSI measured during bilateral tests has been examined, unilateral DSI scores have not been reported to date and thus was the aim of the present study. Twenty-eight recreational sport athletes performed three trials of a unilateral isometric squat and countermovement jump (CMJ) to measure peak force in each task across two separate test sessions. The unilateral DSI was calculated using both left vs. right and dominant vs. non-dominant limbs. Good to excellent reliability was shown in the isometric squat (ICC = 0.86-0.96; CV ≤ 5.7%) and the CMJ (ICC = 0.83-0.93; CV ≤ 5.8%) on both limbs. The DSI showed moderate to good reliability (ICC = 0.71-0.79; CV = 7.54-11.9%). DSI scores of 0.52-0.55 and 0.55-0.59 were reported on the left and right limbs respectively, with no significant differences reported between limbs. A significant difference (p = 0.04) was seen for the CMJ between left and right during the second test session only. The dominant and non-dominant limbs reported mean DSI scores of 0.53-0.57, and significant differences were evident between limbs in both the isometric squat and CMJ (p < 0.01). The present study provides normative data for the unilateral DSI and indicates acceptable levels of reliability, while the consistency of individual measures of peak force can be considered good when quantified unilaterally

A framework to guide practitioners when selecting metrics during the countermovement and drop jump tests

Researchers and practitioners have highlighted the necessity to monitor jump strategy metrics as well as the commonly reported outcome measures during the countermovement (CMJ) and drop jump (DJ) tests. However, there is a risk of confusion for practitioners, given the vast range of metrics that now seem to be on offer via analysis software when collecting data from force platforms. As such, practitioners may benefit from a framework that can help guide metric selection for commonly used jump tests, which is the primary purpose of this article. To contextualise the proposed framework, we have provided two examples for how this could work: one for the CMJ and one for the DJ, noting that these tests are commonly utilized by practitioners during routine testing across a range of sport performance and clinical settings

Seasonal variation of physical performance and inter-limb asymmetry in professional cricket athletes

The aims of the present study were to: 1) determine the seasonal variation of physical performance in professional cricket players, 2) determine the seasonal variation of inter-limb asymmetries in the same cohort of professional players. Fifteen male professional cricket players (age: 20.60 ± 1.59 years; height: 1.82 ± 0.08 m; body mass: 78.70 ± 11.23 kg) performed unilateral countermovement jumps (CMJ), unilateral drop jumps, 10m sprints and 505 change of direction (COD) speed tests at pre (March), mid (June) and end (September) of the 2018 season. Inter-limb asymmetry was quantified in the unilateral CMJ (jump height and concentric impulse), unilateral drop jump (jump height and reactive strength index – RSI) and 505 (total time and COD deficit). Significant changes (p < 0.05) were evident for the following tests: unilateral CMJ (ES range = 0.67 to 1.00), 505 on the right leg (ES = 0.70), 10m (ES range = -1.39 to 0.70) and COD deficit (ES range = 0.70 to 0.80), with the largest changes evident for 10m sprint. No significant differences were evident in drop jump performance throughout the season. For the magnitude of asymmetry, significant changes in jump height asymmetry from the unilateral CMJ were evident from mid to end of season (ES = 0.72). For the direction of asymmetry, levels of agreement ranged from poor to substantial in the unilateral CMJ (Kappa = -0.21 to 0.72), fair to substantial in the unilateral drop jump (Kappa range = 0.33 to 0.74) and slight to moderate during the 505 test (Kappa range = 0.06 to 0.44), with RSI showing noticeably better results than other tests or metrics. These data shows that the largest changes in performance scores throughout the season came from the 10m test, which practitioners may wish to consider implementing if not doing so already. Furthermore, both unilateral jump tests showed their use for asymmetry interpretation, which practitioners may wish to consider implementing in to their test batteries. Specifically, jump height asymmetry during the unilateral CMJ was the only metric to exhibit meaningful changes between time points, whilst RSI was the metric that exhibited more consistent limb dominance characteristics for the direction of asymmetry

Interlimb asymmetries: the need for an individual approach to data analysis

It has been shown that the magnitude of inter-limb asymmetries varies depending on the test selected; however, literature relating to whether asymmetries always favour the same limb is scarce. The aim of the present study was to determine whether inter-limb asymmetries always favoured the same side for common metrics across unilateral strength and jumping-based tests. Twenty-eight recreational sport athletes performed unilateral isometric squats, single leg countermovement jumps (SLCMJ) and single leg broad jumps (SLBJ) with asymmetries in peak force compared across all tests, and eccentric and concentric impulse asymmetries compared between jumps. Mean asymmetries for all tests were low (≤ -5.3%) and all inter-limb differences for jump tests favoured the left limb, whilst asymmetries during the isometric squat favoured the right limb. Despite the low mean asymmetry values, individual data highlighted substantially greater differences. Levels of agreement for asymmetries were computed via the Kappa coefficient and ranged from slight to substantial (< 0.01 – 0.79), although concentric impulse asymmetries for jump tests was the only comparison to result in substantial levels of agreement. With asymmetries rarely being present on the same side across tests, these results show that a more individual approach to reporting asymmetries is required, which should help practitioners when designing targeted training interventions for their reduction

Effects of a resistance training intervention on the strength-deficit of elite young soccer players

The aim of this study was to examine the effects of a traditional resistance training scheme on the relative strength (RS), relative peak-force (RPF), strength deficit (SDef), and vertical jump and sprint abilities in elite young soccer players. Thirty-five under-20 soccer players from two professional clubs were assessed before and after a 4-week competitive period. One team performed 12 sessions of a structured resistance training program and the other maintained their regular soccer-specific training and competitive routines. The resistance training program consisted of performing half-squat and jump squat exercises at distinct percentages of the one-repetition maximum. Both teams performed pre- and post-measurements in the following order: (1) countermovement jump (CMJ), (2) 20-m sprint, and (3) half-squat one-repetition maximum to determine the RS, RPF, and SDef. A two-way analysis of variance was used to test for group x time interaction among variables. Effect sizes (ES) and 95% confidence intervals (CI) were also calculated. Group x time interactions were demonstrated for RS ([ES [95%CI] = 1.21 [0.57; 1.85], P = 0.001), RPF (ES [95%CI] = 1.18 [0.52; 1.80], P = 0.001), SDef (ES [95%CI] = 0.86 [0.01; 1.71], P = 0.04), and CMJ (ES [95%CI] = 0.64 [0.28; 0.99], P = 0.001); whereas a non-significant interaction was observed for 20-m sprint performance (ES [95%CI] = 0.02 [-0.32; 0.36], P = 0.85). Traditional strength-power oriented training resulted in improved maximum strength performance and CMJ ability but, paradoxically, increased the SDef. As a consequence, stronger athletes are not necessarily able to use greater percentages of their peak-force against relatively lighter loads

Drop jump asymmetry is associated with reduced sprint and change-of-direction speed performance in adult female soccer players

Studies that examine the effects of inter-limb asymmetry on measures of physical performance are scarce, especially in adult female populations. The aim of the present study was to establish the relationship between inter-limb asymmetry and speed and change-of-direction speed (CODS) in adult female soccer players. Sixteen adult players performed a preseason test battery consisting of unilateral countermovement jump (CMJ), unilateral drop jump (DJ), 10 m, 30 m, and 505 CODS tests. Inter-limb asymmetry was calculated using a standard percentage difference equation for jump and CODS tests, and Pearson's r correlations were used to establish a relationship between asymmetry and physical performance as well as asymmetry scores themselves across tests. Jump-height asymmetry from the CMJ (8.65%) and DJ (9.16%) tests were significantly greater (p < 0.05) than asymmetry during the 505 test (2.39%). CMJ-height asymmetry showed no association with speed or CODS. However, DJ asymmetries were significantly associated with slower 10 m (r = 0.52; p < 0.05), 30 m (r = 0.58; p < 0.05), and 505 (r = 0.52⁻0.66; p < 0.05) performance. No significant relationships were present between asymmetry scores across tests. These findings suggest that the DJ is a useful test for detecting existent between-limb asymmetry that might in turn be detrimental to speed and CODS performance. Furthermore, the lack of relationships present between different asymmetry scores indicates the individual nature of asymmetry and precludes the use of a single test for the assessment of inter-limb differences

Practices of strength and conditioning coaches in professional sports:A systematic review

The practices of strength and conditioning (S&C) coaches have been reported in various professional sports. This study aims to comprehensively assess this available evidence to help establish whether theoretical, practical, and evidence-based guidelines align with the practices employed by these experienced S&C coaches. Three databases were searched (PubMed, SPORTDiscus, and Cochrane) until November 2020. Studies surveying the practices of S&C coaches in professional sports using a survey design with common questions, written in English, and published in peer-reviewed journals were reviewed. Eight studies (n = 318 S&C coaches) were finally included. All studies adapted a similar survey, providing a strong basis for comparison between sports. Periodization strategies were widely used (89%), with training volume consistently reduced during the in-season period. Olympic weightlifting was commonly used across sports, except in baseball (29%). Plyometric exercises were predominantly prescribed for speed development (74%) and lower body power (68%), which were mostly programed as complex training (45%) and conducted all year round (52%). Flexibility exercises were mostly performed before practice (83%) for 6–10 min (40%). Physical tests were mainly conducted during the pre-season period (66%), with body composition (86%) being the most used test. S&C coaches generally adhered to current guidelines and research in S&C concerning training prescription and physical testing. Whereas, Intersport differences were also noted and further discussed. Results of this study can be used by S&C coaches to plan, implement, and review their professional practices. Furthermore, may inform the development of general and sport-specific guidelines, and future research in S&C

The effect of load placement on the power production characteristics of three lower extremity jumping exercises

The purpose of this study was to compare the power production characteristics of the jump squat (JS), hexagonal barbell jump (HEXJ), and jump shrug (JShrug) across a spectrum of relative loads. Fifteen resistance-trained men completed three testing sessions where they performed repetitions of either the JS, HEXJ, or JShrug at body mass (BM) or with 20, 40, 60, 80, or 100% of their BM. Relative peak power (PPRel), relative force at PP (FPP), and velocity at PP (VPP) were compared between exercises and loads. In addition, power-time curves at each load were compared between exercises. Load-averaged HEXJ and JShrug PPRel were statistically greater than the JS (both p < 0.01), while no difference existed between the HEXJ and the JShrug (p = 1.000). Load-averaged JShrug FPP was statistically greater than both the JS and the HEXJ (both p < 0.001), while no statistical difference existed between the JS and the HEXJ (p = 0.111). Load averaged JS and HEXJ VPP were statistically greater than the JShrug (both p < 0.01). In addition, HEXJ VPP was statistically greater than the JS (p = 0.009). PPRel was maximized at 40, 40, and 20% BM for the JS, HEXJ, and JShrug, respectively. The JShrug possessed statistically different power-time characteristics compared to both the JS and the HEXJ during the countermovement and propulsion phases. The HEXJ and the JShrug appear to be superior exercises for PPRel compared to the JS. The HEXJ may be considered a more velocity-dominant exercise, while the JShrug may be a more force-dominant one

Change of direction deficit in national team rugby union players: is there an Influence of playing position?

The aim of this study was to investigate the change of direction (COD) ability and deficits of elite rugby union players, discriminating between position (backs and forwards), and between "faster and slower players", in multiple COD tasks. Twenty-four male rugby union players from the Brazilian senior National team completed the following assessments: Squat and countermovement jumps; drop jump; standing long jump, horizontal triple jumps; 40-m linear sprint; Pro-agility, L-Drill, and Zig-zag COD tests; and squat 1-repetition maximum. The differences between backs and forwards and between faster and slower performers were examined using magnitude-based inferences. Backs were faster (in both linear and COD speed tests) and jumped higher than forwards. Moreover, they generated an inferior sprint momentum. No differences were found in COD deficit between playing positions. However, when dividing the sample by median split, faster players outperformed their slower counterparts in all power⁻speed variables and presented higher COD deficits. These results suggest that separating rugby players by playing position might not discriminate players with different COD skills and that the median split analysis is more sensitive to identifying these differences. Furthermore, the present data indicate that faster rugby players are less efficient at changing direction and tolerating higher approach velocities in COD maneuvers