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Background If interplanetary travel is to be successful over the coming decades, it is essential that countermeasures to minimize deterioration of the musculoskeletal system are as effective as possible, given the increased duration of spaceflight associated with such missions. The aim of this review, therefore, is to determine the magnitude of deconditioning of the musculoskeletal system during prolonged spaceflight and recommend possible methods to enhance the existing countermeasures. Methods A literature search was conducted using PubMed, Ovid and Scopus databases. 5541 studies were identified prior to the removal of duplicates and the application of the following inclusion criteria: (1) group means and standard deviations for pre- and post-spaceflight for measures of strength, muscle mass or bone density were reported (or provided by the corresponding author when requested via e-mail), (2) exercise-based countermeasures were included, (3) the population of the studies were human, (4) muscle function was assessed and (5) spaceflight rather than simulated spaceflight was used. The methodological quality of the included studies was evaluated using a modified Physiotherapy Evidence Database (PEDro) scale for quality, with publication bias assessed using a failsafe N (Rosenthal method), and consistency of studies analysed using I 2 as a test of heterogeneity. Secondary analysis of studies included Hedges’ g effect sizes, and between-study differences were estimated using a random-effects model. Results A total of 11 studies were included in the meta-analyses. Heterogeneity of the completed meta-analyses was conducted revealing homogeneity for bone mineral density (BMD) and spinal muscle size (Tau 2  < 0.001; I 2  = 0.00%, p  > 0.05), although a high level of heterogeneity was noted for lower body force production (Tau 2  = 1.546; I 2  = 76.03%, p  < 0.001) and lower body muscle mass (Tau 2  = 1.386; I 2  = 74.38%, p  < 0.001). The estimated variance (≤ -0.306) for each of the meta-analyses was significant ( p  ≤ 0.033), for BMD (− 0.48 to − 0.53, p  < 0.001), lower body force production (− 1.75, p  < 0.001) and lower body muscle size (− 1.98, p  < 0.001). Spaceflight results in small reductions in BMD of the femur (Hedges g  = − 0.49 [− 0.69 to – 0.28]), trochanter (Hedges g  = − 0.53 [− 0.77 to – 0.29]), and lumbo-pelvic region (Hedges g  = − 0.48 [− 0.73 to – 0.23]), but large decreases in lower limb force production (Hedges g  = − 1.75 [− 2.50 to – 0.99]) and lower limb muscle size (Hedges g  = − 1.98 [− 2.72 to – 1.23]). Conclusions Current exercise countermeasures result in small reductions in BMD during long-duration spaceflight. In contrast, such exercise protocols do not alleviate the reductions in muscle function or muscle size, which may be attributable to the low to moderate loads reported by crewmembers and the interference effect associated with concurrent training. It is recommended that higher-load resistance exercise and the use of high-intensity interval training should be investigated, to determine if such modifications to the reported training practices result in more effective countermeasures to the deleterious effect of long-duration spaceflight on the muscular system.

The Nordic hamstring exercise (NHE) has commonly been investigated in isolation, however, within practice multiple modalities are commonly incorporated. However, the NHE has a low level of compliance within sport, with sprinting being potentially being preferred. The present study aimed to observe the effect of a lower-limb program with either additional NHE or sprinting on the modifiable risk factors hamstring strain injury (HSI) and athletic performance. 38 collegiate athletes were randomly assigned into three groups: control standardised lower-limb training program ( n = 10 (2 female, 8 male), age = 23.50 ± 2.95 years, height = 1.75 ± 0.09 m, mass 77.66 ± 11.82 kg), additional NHE ( n = 15 (7 female, 8 male), age = 21.40 ± 2.64 years, height = 1.74 ± 0.04 m, mass 76.95 ± 14.20 kg) and additional sprinting ( n = 13 (4 female, 9 male), age = 22.15 ± 2.54 years, height = 1.74 ± 0.05 m, mass 70.55 ± 7.84 kg). All participants performed a standardised lower-limb training program twice per week for seven weeks, including Olympic lifting derivatives, squatting movements, and the Romanian deadlift, with experimental groups performing with either additional sprinting or NHE. Bicep femoris architecture, eccentric hamstring strength, jump performance, lower-limb maximal strength and sprint ability were measured pre and post. All training groups demonstrated significant ( p < 0.001), small-moderate increases in Bicep femoris architecture ( g = 0.60–1.22 ) , with significant ( p < 0.001), small-large increases in absolute and relative eccentric peak force ( g = 0.60–1.84 ) . Significant and small increases were observed in take-off velocity and mean propulsion force ( p < 0.02, g = 0.47–0.64), with non-significant and small increases for both the sprint and control training groups for mean propulsion force ( p > 0.05, g = 0.42–0.50). Nordic and sprint training groups had significant and small increases in peak absolute and relative net force ( p < 0.001, g = 0.44–0.60). The control group had a non-significant trivial increase in absolute peak net force ( p > 0.05, g = 0.22), with a significant and small increase in relative peak relative net force ( p = 0.034, g = 0.48). Significant and small decreases for the NHE and sprinting training groups was observed for 0–10 m, 0–20 m, and 10–20 m sprint time ( p < 0.010, g = 0.47–0.71). Performing multiple modalities, with either additional NHE or sprinting, as part of a complete resistance training program was superiorly effective for measures of modifiable risk factors HSI, with similar increases observed in measures of athletic performance derived from the standardised lower-limb training program.

Female soccer players have been identified as presenting with low energy availability (LEA), though the prevalence of LEA may be overestimated given inaccuracies associated with self‐reporting dietary intakes. Accordingly, we aimed to quantify total daily energy expenditure (TDEE) via the doubly labelled water (DLW) method, energy intake (EI) and energy availability (EA). Adolescent female soccer players (n = 45; 16 ± 1 years) completed a 9–10 day ‘training camp’ representing their national team. Absolute and relative TDEE was 2683 ± 324 and 60 ± 7 kcal kg−1 fat free mass (FFM), respectively. Mean daily EI was lower (P < 0.01) when players self‐reported using the remote food photography method (RFPM) (2047 ± 383 kcal day−1) over a 3‐day period versus DLW derived EI estimates accounting for body mass (BM) changes (2545 ± 518 kcal day−1) over 7–8 days, representing a mean daily Δ of 499 ± 526 kcal day−1 and 22% error when using the RFPM. Estimated EA was different (P < 0.01) between methods (DLW: 48 ± 14 kcal kg−1 FFM, range: 22–82; RFPM: 37 ± 8 kcal kg−1 FFM, range: 22–54), such that prevalence of LEA (<30 kcal kg−1 FFM) was lower in DLW compared with RFPM (5% vs. 15%, respectively). Data demonstrate the potential to significantly underestimate EI when using self‐report methods. This approach can therefore cause a misrepresentation and an over‐prevalence of LEA, which is the underlying aetiology of ‘relative energy deficiency in sport’ (REDs). What is the central question of this study? Do self‐reported dietary intakes (via remote food photography method, RFPM) overestimate low energy availability (LEA) prevalence in female soccer players compared with energy intake evaluation from the doubly labelled water (DLW) method? What is the main finding and its importance? Estimated energy availability is greater with the DLW method compared with RFPM, such that the prevalence of LEA is greater when self‐reporting dietary intakes. Accordingly, data demonstrate the potential to misrepresent the prevalence of LEA, an underlying factor in the aetiology of ‘relative energy deficiency in sport’ (REDs).

Background Although performance of the Nordic hamstring exercise (NHE) has been shown to elicit adaptations that may reduce hamstring strain injury (HSI) risk and occurrence, compliance in NHE interventions in professional soccer teams is low despite a high occurrence of HSI in soccer. A possible reason for low compliance is the high dosages prescribed within the recommended interventions. The aim of this review was to investigate the effect of NHE-training volume on eccentric hamstring strength and biceps femoris fascicle length adaptations. Methods A literature search was conducted using the SPORTDiscus, Ovid, and PubMed databases. A total of 293 studies were identified prior to application of the following inclusion criteria: (1) a minimum of 4 weeks of NHE training was completed; (2) mean ± standard deviation (SD) pre- and post-intervention were provided for the measured variables to allow for secondary analysis; and (3) biceps femoris muscle architecture was measured, which resulted in 13 studies identified for further analysis. The TESTEX criteria were used to assess the quality of studies with risk of bias assessment assessed using a fail-safe N (Rosenthal method). Consistency of studies was analysed using I 2 as a test of heterogeneity and secondary analysis of studies included Hedges’ g effect sizes for strength and muscle architecture variables to provide comparison within studies, between-study differences were estimated using a random-effects model. Results A range of scores (3–11 out of 15) from the TESTEX criteria were reported, showing variation in study quality. A ‘low risk of bias’ was observed in the randomized controlled trials included, with no study bias shown for both strength or architecture ( N  = 250 and 663, respectively; p  < 0.001). Study consistency was moderate to high for strength ( I 2  = 62.49%) and muscle architecture ( I 2  = 88.03%). Within-study differences showed that following interventions of ≥ 6 weeks, very large positive effect sizes were seen in eccentric strength following both high volume ( g  = 2.12) and low volume ( g  = 2.28) NHE interventions. Similar results were reported for changes in fascicle length ( g  ≥ 2.58) and a large-to-very large positive reduction in pennation angle ( g  ≥ 1.31). Between-study differences were estimated to be at a magnitude of 0.374 ( p  = 0.009) for strength and 0.793 ( p  < 0.001) for architecture. Conclusions Reducing NHE volume prescription does not negatively affect adaptations in eccentric strength and muscle architecture when compared with high dose interventions. These findings suggest that lower volumes of NHE may be more appropriate for athletes, with an aim to increase intervention compliance, potentially reducing the risk of HSI.

Background In-season competition and tournaments for team sports can be both long and congested, with some sports competing up to three times per week. During these periods of time, athletes need to prepare technically, tactically and physically for the next fixture and the short duration between fixtures means that, in some cases, physical preparation ceases, or training focus moves to recovery as opposed to progressing adaptations. Objective The aim of this review was to investigate the effect of training frequency on muscular strength to determine if a potential method to accommodate in-season resistance training, during busy training schedules, could be achieved by utilizing shorter more frequent training sessions across a training week. Methods A literature search was conducted using the SPORTDiscus, Ovid, PubMed and Scopus databases. 2134 studies were identified prior to application of the following inclusion criteria: (1) maximal strength was assessed, (2) a minimum of two different training frequency groups were included, (3) participants were well trained, and finally (4) compound exercises were included within the training programmes. A Cochrane risk of bias assessment was applied to studies that performed randomized controlled trials and consistency of studies was analysed using I 2 as a test of heterogeneity. Secondary analysis of studies included Hedges’ g effect sizes ( g ) and between-study differences were estimated using a random-effects model. Results Inconsistency of effects between pre- and post-intervention was low within-group ( I 2  = 0%), and moderate between-group ( I 2  ≤ 73.95%). Risk of bias was also low based upon the Cochrane risk of bias assessment. Significant increases were observed overall for both upper ( p  ≤ 0.022) and lower ( p  ≤ 0.008) body strength, pre- to post-intervention, when all frequencies were assessed. A small effect was observed between training frequencies for upper ( g  ≤ 0.58) and lower body ( g  ≤ 0.45). Conclusion Over a 6–12-week period, there are no clear differences in maximal strength development between training frequencies, in well-trained populations. Such observations may permit the potential for training to be manipulated around competition schedules and volume to be distributed across shorter, but more frequent training sessions within a micro-cycle rather than being condensed into 1–2 sessions per week, in effect, allowing for a micro-dosing of the strength stimuli.

Eccentric strength training can reduce the risk of hamstring strain injury (HSI) occurrence; however, its implementation can be impacted by athlete compliance and prescription. The aim of this review was to investigate the effects of intervention compliance, consistency and modality, on the prevention of HSIs among athletes. A literature search was conducted. 868 studies were identified prior to the application of the exclusion criteria which resulted in 13 studies identified. Random effects models were used to produce log odds ratios and 95% confidence intervals. Very high (>75.1%), moderate-high (50.1–75%), low-moderate (25.1–50%) and very low (<25%) and <1-, 1.01–3.00-, >3.01-weeks/session were used as thresholds of compliance and consistency, respectively. Modality was also observed. A positive effect on HSI incidence -0.61 (−1.05 to −0.17), favoring the intervention treatments (Z = −2.70, p = 0.007). There were non-significant, large differences between compliance (p = 0.203, Z = −1.272) and consistency (p = 0.137, Z = −1.488), with increased compliance and consistency showing greater effectiveness. A significant difference between intervention modalities was observed (p < 0.001, Z = −4.136), with eccentric interventions being superiorly effective. Compliance of >50.1% and consistent application with <3 weeks/session having positive effects on HSI incidence. Training interventions that can achieve high levels of compliance, and can be consistently performed, should be the objective of future practice.

Micro-dosing of resistance training is “the division of total volume within a micro-cycle, across frequent, short duration, repeated bouts” as defined within this thesis, and is a concept built on a foundation of well-established training approaches, methods, and theories. Despite drawing influence from many other aspects of resistance training, micro-dosing is still a relatively new term and has only recently begun to be explicitly investigated as a programming strategy. There may be considerable acute benefits of utilising micro-dosing, however, considering the lack of previously published data on the topic, our aim was to lay the foundations and determine whether performing micro-dosing had a similar chronic effect to training adaptations as a traditional approach. This thesis, therefore, includes an investigation comparing the effects of micro-dosing the Nordic hamstring exercises (NHE) as a ‘proof of concept’, prior to a comparison of micro-dosing and traditional approaches to lower body strength training. Following both a systematic review and meta-analysis of the appropriate NHE prescription, and reliability of field-based hamstring strength measurements, a comparison of micro-dosing and traditional prescriptions of the NHE was investigated across a single micro-cycle and a 9-week intervention. The findings of both of these studies indicate that there were no meaningful differences between the micro-dosing and traditional groups. A further systematic review and meta-analyses was carried to determine the effect of resistance training frequency, in well-trained athletes, and potential implications for in-season resistance training, with training frequency appearing to have a trivial effect on lower-body strength increases. Finally, a randomised cross-over feasibility study was conducted, with accompanying between-session reliability of performance measures, in which both micro-dosing and traditional groups followed a 5-week, in-season, strength training intervention. Greater improvements were observed in the micro-dosing group for force production characteristics (g = 0.62-0.64), sprint (g = 0.31-0.58), and change of direction (g = 0.57-1.25) performance. In contrast, there were no meaningful differences in countermovement jump performance between groups. It would therefore appear that micro-dosing can achieve similar, if not superior, training adaptations in comparison to a traditional approach to in-season resistance training. One reason for the micro-dosing group potentially providing superior training adaptations could be due to greater compliance/adherence to training, as the micro-dosing group demonstrated small to moderately greater compliance (g = 0.47- 0.72).

The Nordic hamstring exercise (NHE) has commonly been investigated in isolation, however, within practice multiple modalities are commonly incorporated. However, the NHE has a low level of compliance within sport, with sprinting being potentially being preferred. The present study aimed to observe the effect of a lower-limb program with either additional NHE or sprinting on the modifiable risk factors hamstring strain injury (HSI) and athletic performance. 38 collegiate athletes were randomly assigned into three groups: control standardised lower-limb training program (n = 10 (2 female, 8 male), age = 23.50 ± 2.95 years, height = 1.75 ± 0.09 m, mass 77.66 ± 11.82 kg), additional NHE (n = 15 (7 female, 8 male), age = 21.40 ± 2.64 years, height = 1.74 ± 0.04 m, mass 76.95 ± 14.20 kg) and additional sprinting (n = 13 (4 female, 9 male), age = 22.15 ± 2.54 years, height = 1.74 ± 0.05 m, mass 70.55 ± 7.84 kg). All participants performed a standardised lower-limb training program twice per week for seven weeks, including Olympic lifting derivatives, squatting movements, and the Romanian deadlift, with experimental groups performing with either additional sprinting or NHE. Bicep femoris architecture, eccentric hamstring strength, jump performance, lower-limb maximal strength and sprint ability were measured pre and post. All training groups demonstrated significant (p < 0.001), small-moderate increases in Bicep femoris architecture (g = 0.60–1.22), with significant (p < 0.001), small-large increases in absolute and relative eccentric peak force (g = 0.60–1.84). Significant and small increases were observed in take-off velocity and mean propulsion force (p < 0.02, g = 0.47–0.64), with non-significant and small increases for both the sprint and control training groups for mean propulsion force (p > 0.05, g = 0.42–0.50). Nordic and sprint training groups had significant and small increases in peak absolute and relative net force (p < 0.001, g = 0.44–0.60). The control group had a non-significant trivial increase in absolute peak net force (p > 0.05, g = 0.22), with a significant and small increase in relative peak relative net force (p = 0.034, g = 0.48). Significant and small decreases for the NHE and sprinting training groups was observed for 0–10 m, 0–20 m, and 10–20 m sprint time (p < 0.010, g = 0.47–0.71). Performing multiple modalities, with either additional NHE or sprinting, as part of a complete resistance training program was superiorly effective for measures of modifiable risk factors HSI, with similar increases observed in measures of athletic performance derived from the standardised lower-limb training program.

between the studies assessed for