The Stretch-Shortening Cycle

A Meta-Analysis of Plyometric Training in Female Youth: Its Efficacy and Shortcomings in the Literature

Moran, J, Clark, CCT, Ramirez-Campillo, R, Davies, MJ, and Drury, B. A meta-analysis of plyometric training in female youth: its efficacy and shortcomings in the literature. J Strength Cond Res 33(7): 1996-2008, 2019-This meta-analysis characterized female youths' adaptability to plyometric training (PT). A second objective was to highlight the limitations of the body of literature with a view to informing future research. Fourteen studies were included in the final analysis. The effect size (ES = Hedges' g) for the main effect of vertical jump performance was "small" (ES = 0.57, 95% confidence interval: 0.21-0.93). Effect sizes were larger in younger (<15 years; ES = 0.78 [0.25-1.30] vs. 0.31 [-0.18 to 0.80]), shorter (<163 cm; ES = 1.03 [0.38-1.68] vs. 0.25 [-0.20 to 0.70]), and lighter (<54 kg; ES = 1.14 [0.39-1.89] vs. 0.26 [-0.15 to 0.67]) participants. Programming variables seemed to influence adaptive responses with larger effects in interventions which were longer (8 weeks; ES = 1.04 [0.35-1.72] vs. 0.24 [-0.11 to 0.59]), had greater weekly training frequency (>2; ES = 1.22 [0.18-2.25] vs. 0.37 [0.02-0.71]), and whose sessions were of longer duration (≥30 minutes ES = 1.16 [0.14-2.17] vs. 0.33 [0.03-0.63]). More than 16 sessions per program (0.85 [0.18-1.51]) was more effective than exactly 16 sessions (0.46 [0.08-0.84]) which, in turn, was more effective than less than 16 (0.37 [-0.44 to 1.17]). These findings can inform the prescription of PT in female youth.

Self-Myofascial Release Effect With Foam Rolling on Recovery After High-Intensity Interval Training

The goal of this experiment was to assess the impact of self-myofascial massage with the aid of a foam roller on a lower limb immediately after high-intensity interval training (HIIT), using the Tabata protocol (20 s work/10 s rest, repeated 8 times), according to selected recovery variables. The method used Tabata squats (20 s of air squats/10 s of rest, repeated 8 times), after which the subject performed three series of self-myofascial massage with a foam roller on one leg, the other leg being used as the control. Biomechanical lower limb performance was assessed through a squat jump, a countermovement jump, and a hopping on the spot test. Flexibility was assessed through the active and passive range of motion at the hip, knee, and ankle. Pain was measured by recording the delay of muscle soreness (DOMS). Measurements were recorded immediately after the workout, then 24 and 48 h later. Twenty healthy males participated in the study. The results revealed no effect on jumping performance, in terms of height, leg stiffness, power or force output. Additionally, HIIT had a significant impact on muscle damage, as revealed by the reduction in performance 48 h later (−9.7% for the countermovement height). The self-myofascial release decreased DOMS by 50% for the massaged leg compared with 20% for the control leg and increased the hip range of motion by approximately 4.2% for the massaged leg in comparison with the unmassaged leg. This experiment reveals the poor effect of self-myofascial release on regaining the initial value of performance but could be useful for reducing DOMS after high-intensity interval training.

The Increased Effectiveness of Loaded Versus Unloaded Plyometric Jump Training in Improving Muscle Power, Speed, Change of Direction, and Kicking-Distance Performance in Prepubertal Male Soccer Players

PURPOSE

To examine the effects of loaded (LPJT) versus unloaded plyometric jump training (UPJT) programs on measures of muscle power, speed, change of direction (CoD), and kicking-distance performance in prepubertal male soccer players.

METHODS

Participants (N = 29) were randomly assigned to a LPJT group (n = 13; age = 13.0 [0.7] y) using weighted vests or UPJT group (n = 16; age = 13.0 [0.5] y) using body mass only. Before and after the intervention, tests for the assessment of proxies of muscle power (ie, countermovement jump, standing long jump); speed (ie, 5-, 10-, and 20-m sprint); CoD (ie, Illinois CoD test, modified 505 agility test); and kicking-distance were conducted. Data were analyzed using magnitude-based inferences.

RESULTS

Within-group analyses for the LPJT group showed large and very large improvements for 10-m sprint time (effect size [ES] = 2.00) and modified 505 CoD (ES = 2.83) tests, respectively. For the same group, moderate improvements were observed for the Illinois CoD test (ES = 0.61), 5- and 20-m sprint time test (ES = 1.00 for both the tests), countermovement jump test (ES = 1.00), and the maximal kicking-distance test (ES = 0.90). Small enhancements in the standing long jump test (ES = 0.50) were apparent. Regarding the UPJT group, small improvements were observed for all tests (ES = 0.33-0.57), except 5- and 10-m sprint time (ES = 1.00 and 0.63, respectively). Between-group analyses favored the LPJT group for the modified 505 CoD (ES = 0.61), standing long jump (ES = 0.50), and maximal kicking-distance tests (ES = 0.57), but not for the 5-m sprint time test (ES = 1.00). Only trivial between-group differences were shown for the remaining tests (ES = 0.00-0.09).

CONCLUSION

Overall, LPJT appears to be more effective than UPJT in improving measures of muscle power, speed, CoD, and kicking-distance performance in prepubertal male soccer players.

Medial Gastrocnemius Muscle Activity during Single-Leg Hopping to Exhaustion

This study described changes in leg muscle activation characteristics during exhaustive single-leg hopping. Twenty-seven healthy men performed trials (132 hops/min) to exhaustion, without a target height, to a target height with visual feedback and target height with tactile feedback. Mean muscle activation amplitude of the medial gastrocnemius (MG) decreased during the anticipatory period while duration of MG activity was maintained when hopping to a target height and contrasted the changes during hopping without a target height. Changes to MG activity were specific to whether the hopping height had been maintained or not. Changes during the anticipatory period of MG activity, indicative of adaptation in descending motor pathways, implicate utility of a motor learning strategy to allow completion of an exhaustive task.

Analysis of skeletal-muscle condition after excessive loading of the lower legs by sequential magnetic resonance imaging

OBJECTIVES

To determine the changes of muscle conditions of lower leg after stretch shortening cycle exercises by MRI.

SUBJECTS AND METHODS

This study included 20 healthy male adult volunteers. After conducting control MRI, each subject performed 3 sets of exercise loading, each set consisting of 100 repetitions of rebound jumping on one foot. MRIs were performed immediately after exercise loading (0 h), 6, 30, and 168 h later. After constructing T1/T2/Fractional Anisotropy (FA)/Apparent Diffusion Coefficient (ADC) maps, the changes of three skeletal muscles of the leg (the tibialis anterior [TA], soleus [SOL], and gastrocnemius [GA]) were quantitatively evaluated in each map at each time point.

RESULTS

The T1 and T2 values were prolonged after exercise loading, and there was a delay in the recovery of T1 at 6 and 30 h after exercise loading, as compared to those of T2 values over time. The ADC values were elevated in all three muscles immediately after exercise loading, then recovered more slowly than T1 and T2, and still had not returned to baseline 168 h after exercise loading. The FA value decreased in all three muscles after exercise loading, with the greatest decrease occurring immediately after exercise loading. As with the ADC values, the FA values were slow to recover from the decrease, and had not returned to baseline levels 168 h post-loading.

CONCLUSION

The delay of T1 value recovery suggested that the T1 value may reflect the muscle condition like fatigue and damage. Changes in the ADC and FA values over time suggested that structural changes such as minute muscular injuries can be detected by diffusion-weighted MRI. Meanwhile, the changes observed in the T1 and T2 values suggested that the measured relaxation time data reflected not only the water volume in the muscle, but also the muscle condition after exercise loading.

Regular changes in foot strike pattern during prolonged downhill running do not influence neuromuscular, energetics, or biomechanical parameters

Research has suggested that a high variability in foot strike pattern during downhill running is associated with lower neuromuscular fatigue of the plantar flexors (PF). Given the popularity of trail running, we designed an intervention study to investigate whether a strategy with regular changes in foot strike pattern during downhill running could reduce the extent of fatigue on neuromuscular, energetics and biomechanical parameters as well as increase an uphill time-to-exhaustion trial (TTE) performance. Fourteen experienced trail runners completed two interventional conditions (separated by 15 days) in a pseudo-randomised and counter-balanced order that consisted of 2.5-h of treadmill graded running with (switch condition) or without (control condition) a change between fore- and rear-foot strike pattern every 30 s during the downhill sections. Pre and Post, neuromuscular tests were performed to assess PF central and peripheral fatigue. Energy cost of running was assessed using an indirect calorimetry system and biomechanical gait parameters were acquired with an instrumented treadmill. TTE was performed after both the graded running conditions. There were not significant condition × time interactions (p ≥ .085) for any of the variables considered, and TTE was not different between the two conditions (p = .755). A deliberate strategy to alternate between foot strike patterns did not reduce the extent of fatigue during prolonged graded running. We suggest that it is not the ability to switch between foot strike patterns that minimises fatigue; rather the ability to adapt foot strike pattern to the terrain and therefore a better running technique.

Accelerating Recovery from Exercise-Induced Muscle Injuries in Triathletes: Considerations for Olympic Distance Races

The triathlon is one of the fastest developing sports in the world due to expanding participation and media attention. The fundamental change in Olympic triathlon races from a single to a multistart event is highly demanding in terms of recovery from and prevention of exercise-induced muscle injures. In elite and competitive sports, ultrastructural muscle injuries, including delayed onset muscle soreness (DOMS), are responsible for impaired muscle performance capacities. Prevention and treatment of these conditions have become key in regaining muscular performance levels and to guarantee performance and economy of motion in swimming, cycling and running. The aim of this review is to provide an overview of the current findings on the pathophysiology, as well as treatment and prevention of, these conditions in compliance with clinical implications for elite triathletes. In the context of DOMS, the majority of recovery interventions have focused on different protocols of compression, cold or heat therapy, active regeneration, nutritional interventions, or sleep. The authors agree that there is a compelling need for further studies, including high-quality randomized trials, to completely evaluate the effectiveness of existing therapeutic approaches, particularly in triathletes. The given recommendations must be updated and adjusted, as further evidence emerges.

Eccentric Muscle Contractions: Risks and Benefits

Eccentric contractions, characterized by the lengthening of the muscle-tendon complex, present several unique features compared with other types of contractions, which may lead to unique adaptations. Due to its specific physiological and mechanical properties, there is an increasing interest in employing eccentric muscle work for rehabilitation and clinical purposes. However, unaccustomed eccentric exercise is known to cause muscle damage and delayed pain, commonly defined as “Delayed-Onset Muscular Soreness” (DOMS). To date, the most useful preventive strategy to avoid these adverse effects consists of repeating sessions involving submaximal eccentric contractions whose intensity is progressively increased over the training. Despite an increased number of investigations focusing on the eccentric contraction, a significant gap still remains in our understanding of the cellular and molecular mechanisms underlying the initial damage response and subsequent adaptations to eccentric exercise. Yet, unraveling the molecular basis of exercise-related muscle damage and soreness might help uncover the mechanistic basis of pathological conditions as myalgia or neuromuscular diseases. In addition, a better insight into the mechanisms governing eccentric training adaptations should provide invaluable information for designing therapeutic interventions and identifying potential therapeutic targets.

A Novel Method of Assessment for Monitoring Neuromuscular Fatigue in Australian Rules Football Players

Purpose: To compare the sensitivity of a submaximal run test (SRT) with a countermovement-jump test (CMJ) to provide an alternative method of measuring neuromuscular fatigue (NMF) in high-performance sport. Methods: A total of 23 professional and semiprofessional Australian rules football players performed an SRT and CMJ test prematch and 48 and 96 h postmatch. Variables from accelerometers recorded during the SRT were player load 1D up (vertical vector), player load 1D side (mediolateral vector), and player load 1D forward (anteroposterior vector). Meaningful difference was examined through magnitude-based inferences (effect size [ES]), with reliability assessed as typical error of measurements expressed as coefficient of variance. Results: A small decrease in CMJ height, ES −0.43 ± 0.39 (likely), was observed 48 h postmatch before returning to baseline 96 h postmatch. This was accompanied by corresponding moderate decreases in the SRT variables player load 1D up, ES −0.60 ± 0.51 (likely), and player load 1D side, ES −0.74 ± 0.57 (likely), 48 h postmatch before also returning to prematch baseline. Conclusion: The results suggest that in the presence of NMF, players use an alternative running profile to produce the same external output (ie, time). This indicates that changes in accelerometer variables during an SRT can be used as an alternative method of measuring NMF in high-performance Australian rules football and provides a flexible option for monitoring changes in the recovery phase postmatch.

Assisted jumping: A possible method of incorporating high-velocity exercise in older populations

In the past, older adults were traditionally deemed too weak or fragile to participate in high-intensity exercise, but more recent research indicates that not only is high-intensity exercise not dangerous in this population, but it may in fact be a preferable form of exercise over other, less-intense alternatives. However, many seniors still do not participate in high-intensity exercise despite the mounting evidence that it can improve a number of physiological functions and ultimately increase quality of life. As health, sport, and medical professionals, we must continuously ask ourselves how we can apply our lab-based findings in real-life scenarios, and in the case of older adults, we must find a way to circumvent some of their most common reasons for not exercising, which can include a lack of time, a lack of know-how, a lack of motivation, a fear that high-intensity exercise is unsafe, and a perception that high-intensity exercise is too difficult. Therefore, introducing quick, simple, safe, and perceptually easy exercises may result in immediate health and functional benefits and may serve as a gateway exercise to usher older adults into the realm of high-intensity training. Specifically, assisted jumping could serve this purpose. In this article, the ideas behind the theory and practice of assisted jumping are set forth, providing an evidence-based hypothesis from which future researchers can build on to implement high-speed, high-power, high-intensity exercise in older populations in both research and practical settings.

The Effect of Varying Plyometric Volume on Stretch-Shortening Cycle Capability in Collegiate Male Rugby Players

Jeffreys, MA, De Ste Croix, MBA, Lloyd, RS, Oliver, JL, and Hughes, JD. The effect of varying plyometric volume on stretch-shortening cycle capability in collegiate male rugby players. J Strength Cond Res 33(1): 139-145, 2019-The purpose of this study was to identify the effectiveness of low and high volume plyometric loads on developing stretch-shortening cycle capability in collegiate rugby players. A between-group repeated measures design was used. Thirty-six subjects (age 20.3 ± 1.6 years, mass 91.63 ± 10.36 kg, stature 182.03 ± 5.24 cm) were randomly assigned to one of 3 groups: a control group (CG), a low volume plyometric group (LPG), or a high volume plyometric group (HPG). Data were collected from a force plate, and measures of reactive strength index (RSI) and leg stiffness were calculated from jump height, contact time, and flight time. A significant between-group × time (F = 4.01, p ≤ 0.05) interaction effect for RSI was observed. Bonferroni post hoc analysis indicated that both the LPG training group (p = 0.002) and HPG training group (p = 0.009) were significantly higher than the control group. No significant interaction effect between time × group was observed for leg stiffness (F = 1.39, p = 0.25). This study has demonstrated that it is possible to improve reactive strength capabilities through the use of a low volume plyometric program. The low volume program elicited the same performance improvement in RSI as a high volume program while undertaking a lower dose. This suggests that strength and conditioning coaches may be able to benefit from the ability to develop more time-efficient and effective plyometric programs.

Short Trail Running Race: Beyond the Classic Model for Endurance Running Performance

PURPOSE

This study aimed to examine the extent to which the classical physiological variables of endurance running performance (maximal oxygen uptake (V˙O2max), %V˙O2max at ventilatory threshold (VT), and running economy (RE)) but also muscle strength factors contribute to short trail running (TR) performance.

METHODS

A homogeneous group of nine highly trained trail runners performed an official TR race (27 km) and laboratory-based sessions to determine V˙O2max, %V˙O2max at VT, level RE (RE0%) and RE on a +10% slope, maximal voluntary concentric and eccentric knee extension torques, local endurance assessed by a fatigue index (FI), and a time to exhaustion at 87.5% of the velocity associated with V˙O2max. A simple regression method and commonality analysis identifying unique and common coefficients of each independent variable were used to determine the best predictors for the TR race time (dependent variable).

RESULTS

Pearson correlations showed that FI and V˙O2max had the highest correlations (r = 0.91 and r = -0.76, respectively) with TR performance. The other selected variables were not significantly correlated with TR performance. The analysis of unique and common coefficients of relative V˙O2max, %V˙O2max at VT, and RE0% provides a low prediction of TR performance (R = 0.48). However, adding FI and RE on a +10% slope (instead of RE0%) markedly improved the predictive power of the model (R = 0.98). FI and V˙O2max showed the highest unique (49.8% and 20.4% of total effect, respectively) and common (26.9% of total effect) contributions to the regression equation.

CONCLUSIONS

The classic endurance running model does not allow for meaningful prediction of short TR performance. Incorporating more specific factors into TR such as local endurance and gradient-specific RE testing procedures should be considered to better characterize short TR performance.

Effect of high volume stretch-shortening cycle exercise on vertical leg stiffness and jump performance

The study aims were to investigate the effects of muscle damage induced by high-volume stretch-shortening cycle exercise on countermovement jump (CMJ) performance and vertical leg stiffness during squat and landing phases. Changes in the peak torque of knee extensor muscles, soreness, plasma CK activity, vertical leg stiffness, and CMJ characteristics were measured in recreationally active men after 50, 100, 150 and 200 drop jumps (DJs) and again 24 h later. The isometric voluntary peak torque after 50 DJs decreased by 22 ± 12% (p < 0.01). However, during the squat phase of CMJ eccentric peak power and rate of force development together with the concentric mean force during the push-off phase increased after 50 DJs (all p < 0.05). After 200 DJs, jump height and concentric peak power during push-off were decreased (p < 0.05). At 24 h after exercise, jump height, peak eccentric force, the rate of eccentric force development and the mean concentric force in CMJ and vertical leg stiffness in the squat all remained decreased (p < 0.05), while vertical leg stiffness in the landing phase was increased (p < 0.05). Muscle damaging exercise volume is associated with changes in vertical leg stiffness and CMJ performance.

Effects of myostatin on the mechanical properties of muscles during repeated active lengthening in the mouse

The aim of the present study was to investigate how myostatin dysfunction affects fast and slow muscle stiffness and viscosity during severe repeated loading. Isolated extensor digitorum longus (EDL) and soleus muscles of young adult female mice of the BEH (dysfunctional myostatin) and BEH+/+ (functional myostatin) strains were subjected to 100 contraction-stretching loading cycles during which contractile and mechanical properties were assessed. BEH mice exhibited greater exercise-induced muscle damage, although the effect was muscle- and age-dependent and limited to the early phases of simulated exercise. The relative reduction of the EDL muscle isometric force recorded during the initial 10-30 loading cycles was greater in BEH mice than in BEH+/+ mice and exceeded that of the soleus muscle of either strain. The induced damage was associated with lower muscle stiffness. The effects of myostatin on the mechanical properties of muscles depend on muscle type and maturity.

Explosive Push-ups: From Popular Simple Exercises to Valid Tests for Upper-Body Power

Zalleg, D, Ben Dhahbi, A, Dhahbi, W, Sellami, M, Padulo, J, Souaifi, M, Bešlija, T, and Chamari, K. Explosive push-ups: From popular simple exercises to valid tests for upper-body power. J Strength Cond Res 34(10): 2877-2885, 2020-The purpose of this study was to assess the logical and ecological validity of 5 explosive push-up variations as a means of upper-body power assessment, using the factorial characterization of ground reaction force-based (GRF-based) parameter outputs. Thirty-seven highly active commando soldiers (age: 23.3 ± 1.5 years; body mass: 78.7 ± 9.7 kg; body height: 179.7 ± 4.3 cm) performed 3 trials of 5 variations of the explosive push-up in a randomized-counterbalanced order: (a) standard countermovement push-up, (b) standard squat push-up, (c) kneeling countermovement push-up, (d) kneeling squat push-up, and (e) drop-fall push-up. Vertical GRF was measured during these exercises using a portable force plate. The initial force-supported, peak-GRF and rate of force development during takeoff, flight time, impact force, and rate of force development impact on landing were measured. A significant relationship between initial force-supported and peak-GRF takeoff was observed for the countermovement push-up (CMP) exercises (standard countermovement push-up, kneeling countermovement push-up, and drop-fall push-up) and squat push-up (SP) exercises (standard squat push-up and kneeling squat push-up) (r = 0.58 and r = 0.80, respectively; p < 0.01). Furthermore, initial force supported was also negatively correlated to a significant degree with flight time for both CMP and SP (r = -0.74 and r = -0.80; p < 0.01, respectively). Principal component analysis (PCA) showed that the abovementioned 6 GRF-based variables resulted in the extraction of 3 significant components, which explained 88.9% of the total variance for CMP, and 2 significant components, which explained 71.0% of the total variance for SP exercises. In summary, the PCA model demonstrated a great predictive power in accounting for GRF-based parameters of explosive push-up exercises, allowing for stronger logical and ecological validity as tests of upper-body power. Furthermore, it is possible to adjust the intensity level of the push-up exercise by altering the starting position (i.e., standard vs. kneeling).

Following a Long-Distance Classical Race the Whole-Body Kinematics of Double Poling by Elite Cross-Country Skiers Are Altered

Introduction: Although short-term (approximately 10-min) fatiguing DP has been reported not to alter the joint kinematics or displacement of the centre of mass (COM) of high-level skiers, we hypothesize that prolonged DP does change these kinematics, since muscular strength is impaired following endurance events lasting longer than 2 h.

Methods: During the 58-km Marcialonga race in 2017, the fastest 15 male skiers were videofilmed (100 fps, FHD resolution in the sagittal plane) on two 20-m sections (inclines: 0.7 ± 0.1°) 48 km apart (i.e., 7 and 55 km from the start), approximating 50- km Olympic races. The cameras were positioned perpendicular to and about 40 m from the middle of each section and spatial dimensions adjusted for each individual track skied. Pole and joint kinematics, as well as displacement of the COM during two DP cycles were assessed.

Results: The 10 skiers who fulfilled our inclusion criteria finished the race in 2 h 09 min 19 s ± 28 s. Displacements of the joints and COM were comparable to previous observations on skiers roller skiing on a flat treadmill at similar speeds in the laboratory. 55 km after the start, cycle velocity and length were lower (P < 0.001 and P = 0.002, respectively) and the angular range of elbow joint flexion during the initial part of the poling phase reduced, while shoulder angle was greater during the first 35% of the DP cycle (all P < 0.05). Moreover, the ankle angle was increased and forward displacement of the COM reduced during the first 80% of the cycle.

Conclusion: Prolonged DP reduced the forward displacement of the COM and altered arm kinematics during the early poling phase. The inefficient utilization of COM observed after 2 h of competition together with potential impairment of the stretch-shortening of arm extensor muscles probably attenuated generation of poling force. To minimize these effects of fatigue, elite skiers should focus on maintaining optimal elbow and ankle kinematics and an effective forward lean during the propulsive phase of DP.

Reactive Strength Index: A Poor Indicator of Reactive Strength?

Purpose: To assess the relationships between reactive strength measures and associated kinematic and kinetic performance variables achieved during drop jumps. A secondary aim was to highlight issues with the use of reactive strength measures as performance indicators. Methods: Twenty-eight national- and international-level sprinters, 14 men and 14 women, participated in this cross-sectional analysis. Athletes performed drop jumps from a 0.3-m box onto a force platform with dependent variables contact time (CT), landing time, push-off time, flight time, jump height (JH), reactive strength index (RSI, calculated as JH/CT), reactive strength ratio (RSR, calculated as flight time/CT), and vertical leg-spring stiffness recorded. Results: A Pearson correlation test found very high to near-perfect relationships between RSI and RSR (r = .91–.97), with mixed relationships between RSI, RSR, and the key performance variables (men: r = −.86 to −.71 between RSI/RSR and CT, r = .80–.92 between RSI/RSR and JH; women: r = −.85 to −.56 between RSR and CT, r = .71 between RSI and JH). Conclusions: The method of assessing reactive strength (RSI vs RSR) may be influenced by the performance strategies adopted, that is, whether athletes achieve their best reactive strength scores via low CTs, high JHs, or a combination. Coaches are advised to limit the variability in performance strategies by implementing upper and/or lower CT thresholds to accurately compare performances between individuals.

Training Load and Fatigue Marker Associations with Injury and Illness: A Systematic Review of Longitudinal Studies

Background

Coaches, sport scientists, clinicians and medical personnel face a constant challenge to prescribe sufficient training load to produce training adaption while minimising fatigue, performance inhibition and risk of injury/illness.

Objective

The aim of this review was to investigate the relationship between injury and illness and longitudinal training load and fatigue markers in sporting populations.

Methods

Systematic searches of the Web of Science and PubMed online databases to August 2015 were conducted for articles reporting relationships between training load/fatigue measures and injury/illness in athlete populations.

Results

From the initial 5943 articles identified, 2863 duplicates were removed, followed by a further 2833 articles from title and abstract selection. Manual searching of the reference lists of the remaining 247 articles, together with use of the Google Scholar ‘cited by’ tool, yielded 205 extra articles deemed worthy of assessment. Sixty-eight studies were subsequently selected for inclusion in this study, of which 45 investigated injury only, 17 investigated illness only, and 6 investigated both injury and illness. This systematic review highlighted a number of key findings, including disparity within the literature regarding the use of various terminologies such as training load, fatigue, injury and illness. Athletes are at an increased risk of injury/illness at key stages in their training and competition, including periods of training load intensification and periods of accumulated training loads.

Conclusions

Further investigation of individual athlete characteristics is required due to their impact on internal training load and, therefore, susceptibility to injury/illness.

Creatine kinase, neuromuscular fatigue, and the contact codes of football: A systematic review and meta-analysis of pre- and post-match differences

Physiological or performance tests are routinely utilised to assess athletes' recovery. At present, the ideal tool to assess recovery remains unknown. Therefore, the aim of this systematic review was to examine the change in creatine kinase (CK) and neuromuscular function as measured via a countermovement jump (CMJ) following a match in the contact codes of football. A comprehensive search of databases was undertaken with RevMan (V 5.3) used for statistical analysis. Our results demonstrated that CK pre- versus post-match (standardised mean difference (SMD) = 0.90, 95% CI = 0.50 to 1.31, p < .0001), CK pre- versus 24 h post-match (SMD = 1.50, 95% CI = 1.12 to 1.88, p < .00001), and CK pre- versus 48 h post-match all increased significantly (SMD = 0.90, 95% CI = 0.50 to 1.31, p < .0001), while CMJ peak power (PP) pre- versus post-match (SMD = -0.59, 95% CI = -1.12 to -0.06, p = .03), and pre- versus 24 h post-match (SMD = -0.80, 95% CI = -1.31 to -0.28, p = .002) decreased significantly. There was a significant relationship between the change in CK and the change in CMJ PP from immediately pre to immediately post (r = -0.924, p = .025), and between CMJ immediately following a match and 24 h CK change (r = -0.983, p = .017). In conclusion, CK levels increase and performance in the CMJ decreases following a match of a contact code of football. The identification of this relationship may allow coaching staff to implement a standalone measure of recovery.

Determination of Vertical Jump as a Measure of Neuromuscular Readiness and Fatigue

Watkins, CM, Barillas, SR, Wong, MA, Archer, DC, Dobbs, IJ, Lockie, RG, Coburn, JW, Tran, TT, and Brown, LE. Determination of vertical jump as a measure of neuromuscular readiness and fatigue. J Strength Cond Res 31(12): 3305-3310, 2017-Coaches closely monitor training loads and periodize sessions throughout the season to create optimal adaptations at the proper time. However, only monitoring training loads ignores the innate physiological stress each athlete feels individually. Vertical jump (VJ) is widely used as a measure of lower-body power, and has been used in postmatch studies to demonstrate fatigue levels. However, no pretraining monitoring by VJ performance has been previously studied. Therefore, the purpose of this study was to determine the sensitivity of VJ as a measure of readiness and fatigue on a daily sessional basis. Ten healthy resistance-trained males (mass = 91.60 ± 13.24 kg; height = 179.70 ± 9.23 cm; age = 25.40 ± 1.51 years) and 7 females (mass = 65.36 ± 12.29 kg; height = 162.36 ± 5.75 cm; age = 25.00 ± 2.71 years) volunteered to participate. Vertical jump and BRUNEL Mood Assessment (BAM) were measured 4 times: pre-workout 1, post-workout 1, pre-workout 2, and post-workout 2. Workout intensity was identical for both workouts, consisting of 4 sets of 5 repetitions for hang cleans, and 4 sets of 6 repetitions for push presses at 85% 1 repetition maximum (1RM), followed by 4 sets to failure of back squats (BSs), Romanian deadlift, and leg press at 80% 1RM. The major finding was that VJ height decrement (-8.05 ± 9.65 cm) at pre-workout 2 was correlated (r = 0.648) with BS volume decrement (-27.56 ± 24.56%) between workouts. This is important for coaches to proactively understand the current fatigue levels of their athletes and their readiness to resistance training.

Delayed-onset muscle soreness in human masticatory muscles increases inhibitory jaw reflex responses

The effects of masticatory muscles' overloading on jaw-motor control are not yet fully clarified. Therefore, it was tested whether eccentric and concentric exercises of the human masticatory muscles would influence inhibitory jaw reflex responses. Eleven participants (6 males, 5 females) performed 6, 5-minutes bouts of eccentric-concentric contractions. Before, immediately after, 24 hours, 48 hours and 1 week afterwards, visual analogue scale (VAS) scores for jaw muscle fatigue and pain, maximum voluntary bite force (MVBF) and inhibitory jaw reflexes were recorded. Reflex data were analysed with the cumulative sum control chart error box method. Immediate and delayed masticatory muscle fatigue and pain were provoked. Further, 24 hours after the exercises, MVBF tended to decrease (P = .056), suggesting that delayed-onset muscle soreness (DOMS) was provoked in the masticatory muscles. In addition, the inhibitory jaw reflex showed a delayed increase in size 24 hours after the exercise (P < .05). In conclusion, DOMS provoked in the masticatory muscles alters jaw motor control by inducing a delayed increase in the size of the inhibitory jaw reflex.

Monitoring and Managing Fatigue in Basketball

The sport of basketball exposes athletes to frequent high intensity movements including sprinting, jumping, accelerations, decelerations and changes of direction during training and competition which can lead to acute and accumulated chronic fatigue. Fatigue may affect the ability of the athlete to perform over the course of a lengthy season. The ability of practitioners to quantify the workload and subsequent fatigue in basketball athletes in order to monitor and manage fatigue levels may be beneficial in maintaining high levels of performance and preventing unfavorable physical and physiological training adaptations. There is currently limited research quantifying training or competition workload outside of time motion analysis in basketball. In addition, systematic research investigating methods to monitor and manage athlete fatigue in basketball throughout a season is scarce. To effectively optimize and maintain peak training and playing performance throughout a basketball season, potential workload and fatigue monitoring strategies need to be discussed.

The Influence of Growth and Maturation on Stretch-Shortening Cycle Function in Youth

Hopping, skipping, jumping and sprinting are common tasks in both active play and competitive sports. These movements utilise the stretch-shortening cycle (SSC), which is considered a naturally occurring muscle action for most forms of human locomotion. This muscle action results in more efficient movements and helps optimise relative force generated per motor unit recruited. Innate SSC development throughout childhood and adolescence enables children to increase power (jump higher and sprint faster) as they mature. Despite these improvements in physical performance, the underpinning mechanisms of SSC development during maturational years remain unclear. To the best of our knowledge, a comprehensive review of the potential structural and neuromuscular adaptations that underpin the SSC muscle action does not exist in the literature. Considering the importance of the SSC in human movement, it is imperative to understand how neural and structural adaptations throughout growth and maturation can influence this key muscle action. By understanding the factors that underpin functional SSC development, practitioners and clinicians will possess a better understanding of normal development processes, which will help differentiate between training-induced adaptations and those changes that occur naturally due to growth and maturation. Therefore, the focus of this article is to identify the potential underpinning mechanisms that drive development of SSC muscle action and to examine how SSC function is influenced by growth and maturation.

Effect of Jump Interval Training on Kinematics of the Lower Limbs and Running Economy

Ache-Dias, J, Pupo, JD, Dellagrana, RA, Teixeira, AS, Mochizuki, L, and Moro, ARP. Effect of jump interval training on kinematics of the lower limbs and running economy. J Strength Cond Res 32(2): 416-422, 2017-This study analyzed the effects of the addition of jump interval training (JIT) to continuous endurance training (40-minute running at 70% of peak aerobic velocity, 3 times per week for 4 weeks) on kinematic variables and running economy (RE) during submaximal constant-load running. Eighteen recreational runners, randomized into control group (CG) or experimental group (EG) performed the endurance training. In addition, the EG performed the JIT twice per week, which consisted of 4-6 bouts of continuous vertical jumping (30 seconds) with 5-minute intervals. The oxygen consumption (V[Combining Dot Above]O2) during the submaximal test (performed at 9 km·h) was similar before (EG: 38.48 ± 2.75 ml·kg·min; CG: 36.45 ± 2.70 ml·kg·min) and after training (EG: 37.42 ± 2.54 ml·kg·min; CG: 35.81 ± 3.10 ml·kg·min). No effect of training, group, or interaction (p > 0.05) was found for RE. There was no interaction or group effect for the kinematic variables (p > 0.05). Most of the kinematic variables had a training effect for both groups (support time [p ≤ 0.05]; step rate [SR; p ≤ 0.05]; and step length [SL; p ≤ 0.05]). In addition, according to the practical significance analysis (percentage chances of a better/trivial/worse effect), important effects in leg stiffness (73/25/2), vertical stiffness (73/25/2), SR (71/27/2), and SL (64/33/3) were found for the EG. No significant relationship between RE and stiffness were found for EG and CG. In conclusion, the results suggest that JIT induces important changes in the kinematics of the lower limbs of recreational runners, but the changes do not affect RE.

Is inertial flywheel resistance training superior to gravity-dependent resistance training in improving muscle strength? A systematic review with meta-analyses

OBJECTIVE

The primary aim of this systematic review was to determine if inertial flywheel resistance training is superior to gravity-dependent resistance training in improving muscle strength. The secondary aim was to determine whether inertial flywheel resistance training is superior to gravity-dependent resistance training in improving other muscular adaptations.

DESIGN

A systematic review with meta-analyses of randomised and non-randomised controlled trials.

METHODS

We searched MEDLINE, Scopus, SPORTDiscus, Web of Science and Cochrane Central Register of Controlled Trials with no publication date restrictions until November 2016. We performed meta-analyses on randomised and non-randomised controlled trials to determine the standardized mean difference between the effects of inertial flywheel and gravity-dependent resistance training on muscle strength. A total of 76 and 71 participants were included in the primary and secondary analyses, respectively.

RESULTS

After systematic review, we included three randomised and four non-randomised controlled trials. In the primary analysis for the primary outcome muscle strength, the pooled results from randomised controlled trials showed no difference (SMD=-0.05; 95%CI -0.51 to 0.40; p=0.82; I²=0%). In the secondary analyses of the primary outcome, the pooled results from non-randomised controlled trials showed no difference (SMD=0.02; 95%CI -0.45 to 0.49; p=0.93; I²=0%; and SMD=0.03; 95%CI -0.43 to 0.50; p=0.88; I²=0%). Meta-analysis on secondary outcomes could not be performed.

CONCLUSION

Based on the available data, inertial flywheel resistance training was not superior to gravity-dependent resistance training in enhancing muscle strength. Data for other strength variables and other muscular adaptations was insufficient to draw firm conclusions from.

The pain threshold of high-threshold mechanosensitive receptors subsequent to maximal eccentric exercise is a potential marker in the prediction of DOMS associated impairment

Background

Delayed-onset muscle soreness (DOMS) refers to dull pain and discomfort in people after participating in exercise, sport or recreational physical activities. The aim of this study was to detect underlying mechanical thresholds in an experimental model of DOMS.

Methods

Randomised study to detect mechanical pain thresholds in a randomised order following experimentally induced DOMS of the non-dominant arm in healthy participants. Main outcome was the detection of the pressure pain threshold (PPT), secondary thresholds included mechanical detection (MDT) and pain thresholds (MPT), pain intensity, pain perceptions and the maximum isometric voluntary force (MIVF).

Results

Twenty volunteers (9 female and 11 male, age 25.2 ± 3.2 years, weight 70.5 ± 10.8 kg, height 177.4 ± 9.4 cm) participated in the study. DOMS reduced the PPT (at baseline 5.9 ± 0.4 kg/cm²) by a maximum of 1.5 ± 1.4 kg/cm² (-24%) at 48 hours (p < 0.001). This correlated with the decrease in MIVF (r = -0.48, p = 0.033). Whereas subjective pain was an indicator of the early 48 hours, the PPT was still present after 72 hours (r = 0.48, p = 0.036). Other mechanical thresholds altered significantly due to DOMS, but did show no clinically or physiologically remarkable changes.

Conclusions

Functional impairment following DOMS seems related to the increased excitability of high-threshold mechanosensitive nociceptors. The PPT was the most valid mechanical threshold to quantify the extent of dysfunction. Thus PPT rather than pain intensity should be considered a possible marker indicating the athletes’ potential risk of injury.

Effects of Footwear and Fatigue on Running Economy and Biomechanics in Trail Runners

PURPOSE

This study aimed to examine the effects of footwear and neuromuscular fatigue induced by short distance trail running (TR) on running economy (RE) and biomechanics in well-trained and traditionally shod runners.

METHODS

RE, vertical and leg stiffness (Kvert and Kleg), as well as foot strike angle were measured from two 5-min treadmill running stages performed at a speed of 2.5 (with 10% grade, uphill running) and 2.77 m·s (level running) before and after an 18.4-km TR exercise (approximately 90% of maximal heart rate) in runners wearing minimalist shoes (MS), MS plus added mass (MSm), or traditional shoes (TS). Maximal voluntary contraction torque of knee extensors and perceived muscle pain were also evaluated before and after TR.

RESULTS

Maximal voluntary contraction values decreased after TR in all footwear conditions (P < 0.001), indicating the occurrence of neuromuscular fatigue. In the nonfatigued condition, runners exhibited a better RE only during level running in MS and MSm (i.e., combined effects of shoe mass and midsole geometry), in association with significant decreases in foot strike angle (P < 0.05). However, no significant difference in RE was observed between shod conditions after TR during either uphill or level running. Decreases in both Kvert/Kleg and foot strike angle were more pronounced during running in MS and MSm (P < 0.05) compared with TS, whatever the period. Calf pain increased after TR when wearing MS and MSm compared with TS (P < 0.05).

CONCLUSIONS

These findings indicated specific alterations in RE and biomechanics over time during the MS and MSm conditions compared with the TS condition. Future studies are warranted to evaluate the relationship between RE and footwear with fatigue in experienced minimally shod runners.

An examination of the jump-and-lift factors influencing the time to reach peak catch height during a Rugby Union lineout

The goal of an offensive Rugby Union lineout is to throw the ball in a manner that allows your team to maintain possession. Typically, the player catching the ball jumps and is lifted upwards by two teammates, reaching above the opposing player who is competing for the ball also. Despite various beliefs regarding the importance of the jumper's mass and attempted jump height, and lifters' magnitude and point of force application, there is negligible published data on the topic. The squeeze technique is one lifting method commonly employed by New Zealand teams during lineout plays, whereby the jumper initiates the jump quickly and the lifters provide assistance only once the jumper reaches 20-30 cm. While this strategy may reduce cues to the opposition, it might also constrain the jumper and lifters. We developed a model to explore how changes in the jumper's body mass and attempted jump height, and lifters' magnitude and point of force application influence the time to reach peak catch height. The magnitude of the lift force impacted the time-to-reach peak catch height the most; followed by the jumper's (attempted) jump height and body mass; and lastly, the point of lift force application.

Can a Repeated Sprint Ability Test Help Clear a Previously Injured Soccer Player for Fully Functional Return to Activity? A Pilot Study

OBJECTIVE

To investigate the effects of fatigue induced by a repeated sprint ability (RSA) test on the neuromuscular responses of soccer players with a recent history of lower limb injuries (CH) and a matched control group in good fitness condition (GH).

DESIGN

This was a case-control study.

PARTICIPANTS

Nine CH and 9 GH.

INDEPENDENT VARIABLE

Allocation to CH or GH.

MAIN OUTCOME MEASURES

Each player was assessed for blood lactate concentration and jumping performance [squat jump (SJ) and countermovement jump (CMJ)] before/after RSA. Post-RSA rate of perceived exertion (RPE) was obtained. Receiver operating characteristic analysis was performed to calculate RSA sensitivity and specificity in distinguishing between CH and GH. Intraclass correlation coefficient was used to assess reliability.

RESULTS

No baseline differences were found for any variable. ΔSJ before/after RSA was -14 ± 2% and -5 ± 2% in CH and GH, respectively (P < 0.05). ΔCMJ before/after RSA was -15 ± 2% and -7 ± 2% in CH and GH, respectively (P < 0.05). ΔSJ-based and ΔCMJ-based (before/after RSA) area under curve (AUC) resulted in 0.90 ± 0.07 and 0.86 ± 0.09, respectively, with both AUCs differentiating between CH and GH with 77.78% sensitivity and 88.89% specificity. Pooled AUC resulted in 0.88 ± 0.06. Intraclass correlation coefficient was high (0.85/0.97).

CONCLUSIONS

Repeated sprint ability is a simple, low-cost field test potentially able to assist in clinical decision making for return to sport.

Acute Effects of Block Jumps in Female Volleyball Players: The Role of Performance Level

Although the role of jumping ability in female volleyball players is well recognised, the effect of fatigue on this ability is not well known. The aim of the present study was to examine the effect of a series of block jumps (BJ) on jumping ability and whether it varies by performance level. Ten elite (EG) and 11 amateur (AG) female volleyball players performed a fatigue intervention consisting of 45 BJ, being tested for squat jump (SJ), countermovement jump (CMJ) and BJ before and after the intervention. Elasticity index (EI): (100 × (CMJ − SJ)/SJ) and upper limbs coordination index (ULCI): (100 × (BJ − CMJ)/CMJ) were calculated. After the intervention, EG showed a decrease of 4.40% in BJ height (p = 0.04; ES = 0.40), whereas AG presented an increase of 1.27%, which was not significant (p = 0.57; ES = 0.07). However, EG and AG presented no significant differences in SJ (p = 0.965 and p = 0.655) and CMJ (p = 0.742 and p = 0.211) when comparing baseline with post-intervention. Although EI and ULCI showed no significant differences after intervention in any group (AG: p = 0.989 and p = 0.114; EG: p = 0.242 and p = 0.205, respectively), AG presented a medium effect size (ES = 0.50) in EI and a small one in ULCI (ES = 0.37), also EG showed a medium-large effect in ULCI (ES = 0.75). These findings suggest that EG performance in BJ tends to decrease at the end of a specific jump training practice. Therefore, coaches and fitness trainers working with elite volleyball players should focus on exercises to maintain jumping ability during a match.

Effects of Accumulating Work Shifts on Performance-Based Fatigue Using Multiple Strength Measurements in Day and Night Shift Nurses and Aides

Objective This study aimed to examine the effects of accumulating nursing work on maximal and rapid strength characteristics in female nurses and compare these effects in day versus night shift workers. Background Nurses exhibit among the highest nonfatal injury rates of all occupations, which may be a consequence of long, cumulative work shift schedules. Fatigue may accumulate across multiple shifts and lead to performance impairments, which in turn may be linked to injury risks. Method Thirty-seven nurses and aides performed isometric strength-based performance testing of three muscle groups, including the knee extensors, knee flexors, and wrist flexors (hand grip), as well as countermovement jumps, at baseline and following exposure to three 12-hour work shifts in a four-day period. Variables included peak torque (PT) and rate of torque development (RTD) from isometric strength testing and jump height and power output. Results The rigorous work period resulted in significant decreases (-7.2% to -19.2%) in a large majority (8/9) of the isometric strength-based measurements. No differences were noted for the day versus night shift workers except for the RTD at 200 millisecond variable, for which the night shift had greater work-induced decreases than the day shift workers. No changes were observed for jump height or power output. Conclusions A compressed nursing work schedule resulted in decreases in strength-based performance abilities, being indicative of performance fatigue. Application Compressed work schedules involving long shifts lead to functional declines in nurse performance capacities that may pose risks for both the nurse and patient quality of care. Fatigue management plans are needed to monitor and regulate increased levels of fatigue.

Fatigue and Recovery in Soccer: Evidence and Challenges

Background:

Soccer presents physiological, metabolic, physical and psychological demands which can deteriorate players’ performance due to fatigue. The high variability in physiological, metabolic, physical and psychological responses also influences the magnitude of exercise-induced muscle damage, with symptoms negatively affecting neuromuscular function during recovery or subsequent training sessions or matches. Consequently, more precise and consistent knowledge is required in this area to optimize training and performance.

Objective:

Therefore, the purpose is to sum-up current evidence on fatigue and recovery in soccer players, to shed light on factors that can affect players’ performance, and to suggest applications for coaches and further research.

Method:

A comprehensive review of the scientific literature on the field was conducted.

Results:

Physical performance decrements during matches have traditionally been associated with physiological fatigue, but the magnitude of the symptoms in soccer players is unclear and depends on several factors. Moreover, the decline in physical performance during a soccer match is related to specific demands of each match. These could explain inter-individual variability in acute fatigue or training recovery processes when comparing players from the same team. Recovery counteracts the effects of fatigue, both peripheral and central, but there is a lack of consensus about the usefulness of tests used to monitor fatigue and recovery kinetics.

Conclusion:

Although fatigue and recovery in soccer has been extensively studied, there are still uncertainties about the underlying mechanisms because they are influenced by physiological and match-related demands.

Age-Related Variation in Male Youth Athletes' Countermovement Jump After Plyometric Training: A Meta-Analysis of Controlled Trials

Moran, J, Sandercock, GRH, Ramírez-Campillo, R, Meylan, CMP, Collison, J, and Parry, DA. Age-related variation in male youth athletes' countermovement jump after plyometric training: A meta-analysis of controlled trials. J Strength Cond Res 31(2): 552-565, 2017-Recent debate on the trainability of youths has focused on the existence of periods of accelerated adaptation to training. Accordingly, the purpose of this meta-analysis was to identify the age- and maturation-related pattern of adaptive responses to plyometric training in youth athletes. Thirty effect sizes were calculated from the data of 21 sources with studies qualifying based on the following criteria: (a) healthy male athletes who were engaged in organized sport; (b) groups of participants with a mean age between 10 and 18 years; and (c) plyometric-training intervention duration between 4 and 16 weeks. Standardized mean differences showed plyometric training to be moderately effective in increasing countermovement jump (CMJ) height (Effect size = 0.73 95% confidence interval: 0.47-0.99) across PRE-, MID-, and POST-peak height velocity groups. Adaptive responses were of greater magnitude between the mean ages of 10 and 12.99 years (PRE) (ES = 0.91 95% confidence interval: 0.47-1.36) and 16 and 18 years (POST) (ES = 1.02 [0.52-1.53]). The magnitude of adaptation to plyometric training between the mean ages of 13 and 15.99 years (MID) was lower (ES = 0.47 [0.16-0.77]), despite greater training exposure. Power performance as measured by CMJ may be mediated by biological maturation. Coaches could manipulate training volume and modality during periods of lowered response to maximize performance.

The acute effects of an intense stretch-shortening cycle fatigue protocol on the neuromechanical parameters of lower limbs in men and prepubescent boys

The study examined the differences between boys and adults after an intense stretch-shortening cycle fatigue protocol on neuromechanical parameters of the lower limb. Thirteen boys (9-11 years old) and 13 adult men (22-28 years old) were tested for maximal isometric voluntary knee extension torque and drop jump (DJ) performance from 30 cm before and immediately after a fatigue protocol, consisted of 10 × 10 maximum effort vertical jumps. Three-dimensional kinematics, kinetics and electromyographic (EMG) parameters of the lower extremities muscles were recorded during DJs before and after the fatigue test. The results indicated that reduction in maximal isometric torque and jumping performance was significantly higher in adults compared to boys. Vertical ground reaction forces, contact time and maximum knee flexion increased in a greater extend in adults than in boys. In addition, preactivation, EMG agonist activity, knee joint stiffness and stretch reflex decreased more in adults than in boys at all the examined phases of jumping tasks. It is concluded that employed fatigue protocol induced acute reduction in performance and altered motor control during jumping in both age groups. However, the differences in the level of fatigue between the 2 groups could be attributed to neuromuscular, mechanical and kinematic parameters observed between groups.

Mechanical Alterations Associated with Repeated Treadmill Sprinting under Heat Stress

Purpose

Examine the mechanical alterations associated with repeated treadmill sprinting performed in HOT (38°C) and CON (25°C) conditions.

Methods

Eleven recreationally active males performed a 30-min warm-up followed by three sets of five 5-s sprints with 25-s recovery and 3-min between sets in each environment. Constant-velocity running for 1-min at 10 and 20 km.h^-1 was also performed prior to and following sprinting.

Results

Mean skin (37.2±0.7 vs. 32.7±0.8°C; P<0.001) and core (38.9±0.2 vs. 38.8±0.3°C; P<0.05) temperatures, together with thermal comfort (P<0.001) were higher following repeated sprinting in HOT vs. CON. Step frequency and vertical stiffness were lower (-2.6±1.6% and -5.5±5.5%; both P<0.001) and contact time (+3.2±2.4%; P<0.01) higher in HOT for the mean of sets 1–3 compared to CON. Running distance per sprint decreased from set 1 to 3 (-7.0±6.4%; P<0.001), with a tendency for shorter distance covered in HOT vs. CON (-2.7±3.4%; P = 0.06). Mean vertical (-2.6±5.5%; P<0.01), horizontal (-9.1±4.4%; P<0.001) and resultant ground reaction forces (-3.0±2.8%; P<0.01) along with vertical stiffness (-12.9±2.3%; P<0.001) and leg stiffness (-8.4±2.7%; P<0.01) decreased from set 1 to 3, independently of conditions. Propulsive power decreased from set 1 to 3 (-16.9±2.4%; P<0.001), with lower propulsive power values in set 2 (-6.6%; P<0.05) in HOT vs. CON. No changes in constant-velocity running patterns occurred between conditions, or from pre-to-post repeated-sprint exercise.

Conclusions

Thermal strain alters step frequency and vertical stiffness during repeated sprinting; however without exacerbating mechanical alterations. The absence of changes in constant-velocity running patterns suggests a strong link between fatigue-induced velocity decrements during sprinting and mechanical alterations.

Changes in balance ability, power output, and stretch-shortening cycle utilisation after two high-intensity intermittent training protocols in endurance runners

PURPOSE

This study aimed to describe the acute effects of 2 different high-intensity intermittent trainings (HIITs) on postural control, countermovement jump (CMJ), squat jump (SJ), and stretch-shortening cycle (SSC) utilisation, and to compare the changes induced by both protocols in those variables in endurance runners.

METHODS

Eighteen recreationally trained endurance runners participated in this study and were tested on 2 occasions: 10 runs of 400 m with 90 s recovery between running bouts (10 × 400 m), and 40 runs of 100 m with 30 s recovery between runs (40 × 100 m). Heart rate was monitored during both HIITs; blood lactate accumulation and rate of perceived exertion were recorded after both protocols. Vertical jump ability (CMJ and SJ) and SSC together with postural control were also controlled during both HIITs.

RESULTS

Repeated measures analysis revealed a significant improvement in CMJ and SJ during 10 × 400 m (p < 0.05), whilst no significant changes were observed during 40 × 100 m. Indexes related to SSC did not experience significant changes during any of the protocols. As for postural control, no significant changes were observed in the 40 × 100 m protocol, whilst significant impairments were observed during the 10 × 400 m protocol (p < 0.05).

CONCLUSION

A protocol with a higher number of shorter runs (40 × 100 m) induced different changes in those neuromuscular parameters than those with fewer and longer runs (10 × 400 m). Whereas the 40 × 100 m protocol did not cause any significant changes in vertical jump ability, postural control or SSC utilisation, the 10 × 400 m protocol impaired postural control and caused improvements in vertical jumping tests.

Fatigue associated with prolonged graded running

Scientific experiments on running mainly consider level running. However, the magnitude and etiology of fatigue depend on the exercise under consideration, particularly the predominant type of contraction, which differs between level, uphill, and downhill running. The purpose of this review is to comprehensively summarize the neurophysiological and biomechanical changes due to fatigue in graded running. When comparing prolonged hilly running (i.e., a combination of uphill and downhill running) to level running, it is found that (1) the general shape of the neuromuscular fatigue-exercise duration curve as well as the etiology of fatigue in knee extensor and plantar flexor muscles are similar and (2) the biomechanical consequences are also relatively comparable, suggesting that duration rather than elevation changes affects neuromuscular function and running patterns. However, 'pure' uphill or downhill running has several fatigue-related intrinsic features compared with the level running. Downhill running induces severe lower limb tissue damage, indirectly evidenced by massive increases in plasma creatine kinase/myoglobin concentration or inflammatory markers. In addition, low-frequency fatigue (i.e., excitation-contraction coupling failure) is systematically observed after downhill running, although it has also been found in high-intensity uphill running for different reasons. Indeed, low-frequency fatigue in downhill running is attributed to mechanical stress at the interface sarcoplasmic reticulum/T-tubule, while the inorganic phosphate accumulation probably plays a central role in intense uphill running. Other fatigue-related specificities of graded running such as strategies to minimize the deleterious effects of downhill running on muscle function, the difference of energy cost versus heat storage or muscle activity changes in downhill, level, and uphill running are also discussed.

Genetic variation and exercise-induced muscle damage: implications for athletic performance, injury and ageing

Prolonged unaccustomed exercise involving muscle lengthening (eccentric) actions can result in ultrastructural muscle disruption, impaired excitation-contraction coupling, inflammation and muscle protein degradation. This process is associated with delayed onset muscle soreness and is referred to as exercise-induced muscle damage. Although a certain amount of muscle damage may be necessary for adaptation to occur, excessive damage or inadequate recovery from exercise-induced muscle damage can increase injury risk, particularly in older individuals, who experience more damage and require longer to recover from muscle damaging exercise than younger adults. Furthermore, it is apparent that inter-individual variation exists in the response to exercise-induced muscle damage, and there is evidence that genetic variability may play a key role. Although this area of research is in its infancy, certain gene variations, or polymorphisms have been associated with exercise-induced muscle damage (i.e. individuals with certain genotypes experience greater muscle damage, and require longer recovery, following strenuous exercise). These polymorphisms include ACTN3 (R577X, rs1815739), TNF (-308 G>A, rs1800629), IL6 (-174 G>C, rs1800795), and IGF2 (ApaI, 17200 G>A, rs680). Knowing how someone is likely to respond to a particular type of exercise could help coaches/practitioners individualise the exercise training of their athletes/patients, thus maximising recovery and adaptation, while reducing overload-associated injury risk. The purpose of this review is to provide a critical analysis of the literature concerning gene polymorphisms associated with exercise-induced muscle damage, both in young and older individuals, and to highlight the potential mechanisms underpinning these associations, thus providing a better understanding of exercise-induced muscle damage.