***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: The principal objective of this study was to test the hypothesis that acclimatization to moderate altitude ( 2,500 m ) plus training at low altitude ( 1,250 m ) , " living high-training low , " improves sea-level performance in well-trained runners more than an equivalent sea-level or altitude control . Thirty-nine competitive runners ( 27 men , 12 women ) completed 1 ) a 2-wk lead-in phase , followed by 2 ) 4 wk of supervised training at sea level ; and 3 ) 4 wk of field training camp r and  omized to three groups : " high-low " ( n = 13 ) , living at moderate altitude ( 2,500 m ) and training at low altitude ( 1,250 m ) ; " high-high " ( n = 13 ) , living and training at moderate altitude ( 2,500 m ) ; or " low-low " ( n = 13 ) , living and training in a mountain environment at sea level ( 150 m ) . A 5,000-m time trial was the primary measure of performance ; laboratory outcomes included maximal O2 uptake ( VO2 max ) , anaerobic capacity ( accumulated O2 deficit ) , maximal steady state ( MSS ; ventilatory threshold ) , running economy , velocity at VO2 max , and blood compartment volumes . Both altitude groups significantly increased VO2 max ( 5 % ) in direct proportion to an increase in red cell mass volume ( 9 % ; r = 0.37 , P < 0.05 ) , neither of which changed in the control . Five-kilometer time was improved by the field training camp only in the high-low group ( 13.4 + /- 10 s ) , in direct proportion to the increase in VO2 max ( r = 0.65 , P < 0.01 ) . Velocity at VO2 max and MSS also improved only in the high-low group . Four weeks of living high-training low improves sea-level running performance in trained runners due to altitude acclimatization ( increase in red cell mass volume and VO2 max ) and maintenance of sea-level training velocities , most likely accounting for the increase in velocity at VO2 max and MSS To assess the impact of ' top-up ' normoxic or hypoxic repeat-sprint training on sea-level repeat-sprint ability , thirty team sport athletes were r and omly split into three groups , which were matched in running repeat-sprint ability ( RSA ) , cycling RSA and 20 m shuttle run performance . Two groups then performed 15 maximal cycling repeat-sprint training sessions over 5 weeks , in either normoxia ( NORM ) or hypoxia ( HYP ) , while a third group acted as a control ( CON ) . In the post-training  cycling RSA test , both NORM ( 13.6 % ; p = 0.0001 , and 8.6 % ; p = 0.001 ) and HYP ( 10.3 % ; p = 0.007 , and 4.7 % ; p = 0.046 ) significantly improved overall mean and peak power output , respectively , whereas CON did not change ( 1.4 % ; p = 0.528 , and -1.1 % ; p = 0.571 , respectively ) ; with only NORM demonstrating a moderate effect for improved mean and peak power output compared to CON . Running RSA demonstrated no significant between group differences ; however , the mean sprint times improved significantly from pre- to post-training for CON ( 1.1 % ) , NORM ( 1.8 % ) , and HYP ( 2.3 % ) . Finally , there were no group differences in 20 m shuttle run performance . In conclusion , ' top-up ' training improved performance in a task-specific activity ( i.e. cycling ) ; however , there was no additional benefit of conducting this ' top-up ' training in hypoxia , since cycle RSA improved similarly in both HYP and NORM conditions . Regardless , the ' top-up ' training had no significant impact on running RSA , therefore the use of cycle repeat-sprint training should be discouraged for team sport athletes due to limitations in specificity . Key points'Top-up ' repeat-sprint training performed on a cycle ergometer enhances cycle repeat-sprint ability compared to team sport training only in football players . The addition of moderate hypoxia to repeat-sprint training provides no additional performance benefits to sea-level repeat-sprint ability or endurance performance than normoxic repeat-sprint training.'Top-up ' cycling repeat-sprint training provides no significant additional benefit to running RSA or endurance performance than team sport training only , and therefore running based repeat-sprint interventions are recommended for team sport athletes The purpose s of the present study were to investigate if a ) shuttle-run sprint training performed in a normobaric hypoxia chamber of limited size ( 4.75x2.25 m ) is feasible , in terms of producing the same absolute training load , when compared to training in normoxia , and b ) if such training improves the repeated sprint ability ( RSA ) and the Yo-Yo intermittent recovery ( YYIR ) test outcome in young elite soccer players . Players of an elite soccer training Centre ( age : 15.3 ± 0.5 years , height : 1.73 ± 0.07 m , body mass : 62.6 ± 6.6 kg ) were r and omly assigned to a hypoxia or a normoxia training group . Within a 5-week period , players , who were not informed about the hypoxia intervention , performed at least 7 sessions of identical shuttle-run sprint training either in a normal training room ( FiO2 = 20.95 % ) or in a hypoxic chamber ( FiO2 = 14.8 % ; approximately 3300 m ) , both equipped with the same floor . Each training session comprised 3 series of 5x10s back and forth sprints ( 4.5 m ) performed at maximal intensity . Recovery time between repetitions was 20s and between series 5min . Before and after the training period the RSA ( 6 x 40 m shuttle sprint with 20 s rest between shuttles ) and the YYIR test were performed . The size of the chamber did not restrict the training intensity of the sprint training ( both groups performed approximately 8 shuttles during 10s ) . Training in hypoxia result ed in a lower fatigue slope which indicates better running speed maintenance during the RSA test ( p = 0.024 ) . YYIR performance increased over time ( p = 0.045 ) without differences between groups ( p > 0.05 ) . This study showed that training intensity of the shuttle-run sprint training was not restricted in a hypoxic chamber of limited size which indicates that such training is feasible . Furthermore , hypoxia compared to normoxia training reduced the fatigue slope during the RSA test in youth soccer players . Key PointsShuttle-run sprint training is feasible in hypoxic chambers of limited size ( i.e. , 4.75x2.25m).Hypoxia sprint training ( RSH ) , in comparison to normoxia training , might lead to better running speed maintenance during the repeated sprint ability test PURPOSE The purpose of this study was to assess research aim ed at measuring performance enhancements that affect success of individual elite athletes in competitive events . ANALYSIS Simulations show that the smallest worthwhile enhancement of performance for an athlete in an international event is 0.7 - 0.4 of the typical within-athlete r and om variation in performance between events . Using change in performance in events as the outcome measure in a crossover study , research ers could delimit such enhancements with a sample of 16 - 65 athletes , or with 65 - 260 in a fully controlled study . Sample size for a study using a valid laboratory or field test is proportional to the square of the within-athlete variation in performance in the test relative to the event ; estimates of these variations are therefore crucial and should be determined by repeated- measures analysis of data from reliability studies for the test and event . Enhancements in test and event may differ when factors that affect performance differ between test and event ; overall effects of these factors can be determined with a validity study that combines reliability data for test and event . A test should be used only if it is valid , more reliable than the event , allows estimation of performance enhancement in the event , and if the subjects replicate their usual training and dietary practice s for the study ; otherwise the event itself provides the only dependable estimate of performance enhancement . Publication of enhancement as a percent change with confidence limits along with an analysis for individual differences will make the study more applicable to athletes . Outcomes can be generalized only to athletes with abilities and practice s represented in the study . CONCLUSION estimates of enhancement of performance in laboratory or field tests in most previous studies may not apply to elite athletes in competitive events This study aim ed to determine changes in spring-mass model ( SMM ) characteristics , plantar pressures , and muscle activity induced by the repetition of sprints in soccer-specific conditions ; i.e. , on natural grass with soccer shoes . Thirteen soccer players performed 6 × 20 m sprints interspersed with 20 s of passive recovery .  Plantar pressure distribution was recorded via an insole pressure recorder device divided into nine areas for analysis . Stride temporal parameters allowed to estimate SMM characteristics . Surface electromyographic activity  was monitored for vastus lateralis , rectus femoris , and biceps femoris muscles . Sprint time , contact time , and total stride duration lengthened from the first to the last repetition ( + 6.7 , + 12.9 , and + 9.3 % ; all P < 0.05 ) , while flight time , swing time , and stride length remained constant . Stride frequency decrease across repetitions approached significance ( −6.8 % ; P = 0.07 ) . No main effect of the sprint number or any significant interaction between sprint number and foot region was found for maximal force , mean force , peak pressure and mean pressure ( all P > 0.05 ) . Center of mass vertical displacement increased ( P < 0.01 ) with time , together with unchanged ( both P > 0.05 ) peak vertical force and leg compression . Vertical stiffness decreased ( −15.9 % ; P < 0.05 ) across trials , whereas leg stiffness changes were not significant ( −5.9 % ; P > 0.05 ) . Changes in root mean square activity of the three tested muscles over sprint repetitions were not significant . Although repeated sprinting on natural grass with players wearing soccer boots impairs their leg-spring behavior ( vertical stiffness ) , there is no substantial concomitant alterations in muscle activation levels or plantar pressure patterns Blood flow to dynamically contracting myocytes is regulated to match O(2 ) delivery to metabolic dem and . The red blood cell ( RBC ) itself functions as an O(2 ) sensor , contributing to the control of O(2 ) delivery by releasing the vasodilators ATP and S-nitrosohaemoglobin with the offloading of O(2 ) from the haemoglobin molecule . Whether RBC number is sensed remains unknown . To investigate the role of RBC number , in isolation and in combination with alterations in blood oxygenation , on muscle and systemic perfusion , we measured local and central haemodynamics during  one-legged knee-extensor exercise ( approximately 50 % peak power ) in 10 healthy males under conditions of normocythaemia ( control ) , anaemia , anaemia + plasma volume expansion ( PVX ) , anaemia + PVX + hypoxia , polycythaemia , polycythaemia + hyperoxia and polycythaemia + hypoxia , which changed either RBC count alone or both RBC count and oxyhaemoglobin . Leg blood flow ( LBF ) , cardiac output ( Q ) and vascular conductance did not change with either anaemia or polycythaemia alone . However , LBF increased with anaemia + PVX ( 28 + /- 4 % ) and anaemia + PVX + hypoxia ( 46 + /- 6 % ) and decreased with polycythaemia + hyperoxia ( 18 + /- 5 % ) . LBF and Q with anaemia + PVX + hypoxia ( 8.0 + /- 0.5 and 15.8 + /- 0.7 l min(-1 ) , respectively ) equalled those during maximal knee-extensor exercise . Collectively , LBF and vascular conductance were intimately related to leg arterial-venous ( a-v ) O(2 ) difference ( r(2)= 0.89 - 0.93 ; P < 0.001 ) , suggesting a pivotal role of blood O(2 ) gradients in muscle microcirculatory control . The systemic circulation accommo date d to the changes in muscle perfusion . Our results indicate that , when coping with severe haematological challenges , local regulation of skeletal muscle blood flow and O(2 ) delivery primarily senses alterations in the oxygenation state of haemoglobin and , to a lesser extent , alterations in the number of RBCs and haemoglobin molecules 
    Output:
    
Conclusion Based on current scientific literature , RSH induces greater improvement for mean repeated-sprint performance during sea-level repeated sprinting than RSN .