P J Lander, 1,2 R J Butterly, 2 A M Edwards 3. Original article

Transcription

1 Self-paed exerise is less physially hallenging than enfored onstant pae exerise of the same intensity: influene of omplex entral metaboli ontrol P J Lander, 1,2 R J Butterly, 2 A M Edwards 3 Original artile 1 UCOL Institute of Tehnology, Palmerston North, New Zealand; 2 Leeds Metropolitan University, Carnegie Faulty of Sport & Eduation, Leeds, UK; 3 Institute of Sport & Exerise Siene, James Cook University, Cairns, Australia Correspondene to: Dr A M Edwards, Institute of Sport & Exerise Siene, James Cook University, Cairns, Queensland, Australia; andrew.edwards@ju.edu.au Aepted 29 January 2009 Published Online First 5 February 2009 ABSTRACT Objetive: To examine whether self-paing redues the physiologial hallenge of performing 5000 m rowing ergometry exerise in omparison with a mathedintensity exerise ondition in whih a onstant effort paing strategy is enfored. Methods: Nine healthy well-trained male partiipants volunteered to partiipate in three 5000 m rowing onditions (two submaximal and one maximal onditions) in an individualised order. In the submaximal onditions, partiipants were required to (1) perform 5000 m at a onstant rating of pereived exertion (RPE 15-Hard) (SubRPE) or (2) perform 5000 m at an enfored onstant pae equivalent to the mean power output (PO) of the SubRPE ondition (SubEXT). A maximal ondition (MaxTT) was inluded to disguise the purpose of the study and to failitate an element of randomisation in the test sequene. Dynami intratest responses were assessed every 30s: PO, VO 2, iemg, ore (T ) and skin temperatures (T sk ). Results: There was no differene between performane times of the two submaximal trials. The mean PO represented (SD 8.88)% (SubRPE) and (8.84)% (SubEXT) of the mean MaxTT power output. T (SubRPE:38.46 (0.23)uC, SubEXT:38.72 (0.36)uC; p,0.01), post-test BLa (SubRPE:5.24 (2.18), SubEXT:6.19 (2.51) mmol/l; p,0.05) and iemg (p,0.05) were signifiantly elevated in SubEXT ompared with SubRPE. There were no differenes in the dynamis of HR or VO 2 between SubEXT and SubRPE. The intratest stroketo-stroke variability of power output was signifiantly greater in the SubRPE ondition ompared with SubEXT (p,0.01). Conlusions: Enfored onstant paed exerise presents a signifiantly greater physiologial hallenge than selfpaed exerise. The ability to dynamially self-pae effort via manipulations of power output during exerise is an important behavioural response to homeostati hallenges and thus forms an integral part of a omplex entral regulatory proess. The ability to aurately self pae an exerise bout is an important feature of rae and time trial performanes. 1 3 Self-paed exerise bouts are known to demonstrate onsiderable intratrial flutuations of power output, 4 and it is unlikely that this is simply due to random misjudgements of pae. It is probable that these flutuations of power output are important behavioural responses during exerise at times when homeostasis is hallenged. 5 However, the importane of this observation requires researhers to onsider the brain as a (entral) feature of paing and the development of fatigue. Until reently, 6 8 it had ommonly been viewed that exerise of maximal intensity progressively indued a derease in fore prodution towards a terminal endpoint of fatigue at whih the immediate essation of exerise was a neessary onsequene. 9 This theory has often been used to attribute fatigue to impaired peripheral musle ontratile funtion, through either exessive aumulation of metaboli aidosis or the depletion of intramusular fuels. 10 However, suh peripheral fatigue annot easily explain all observations during endurane exerise, 11 in partiular those where performane improves in the end stages of a self-paed exerise bout Several ontemporary researh studies have suggested that disreet alterations in pae are mediated through entral neural ontrol, by whih musle reruitment is manipulated as part of a regulatory proess to maintain a reserve of motor units and thus avoid atastrophi fatigue. 6 8 Aording to this entral (governor) model, the regulation of exerise intensity (power output) is a behavioural response to both feedbak information from peripheral reeptors and feedforward (antiipatory) mehanisms whih regulate exerise intensity to avoid the development of bodily harm Consequently, flutuations in power output during exerise may be an important feature of a regulatory proess, based on information from various peripheral systems (eg, musle, respiratory, metaboli reeptors) within a omplex metaboli ontrol system. Previous work has shown biologial variation to be an important feature in submaximal exerise. 15 However, relatively few studies have thoroughly examined both the dynami physiologial and thermoregulatory responses to exerise in relation to the onept of paing With the development of fast-response tehnologies, it is now feasible to examine the onept of paing in more dynami experimental onditions than was previously pratial. For example, it is possible that thermoregulatory fators suh as ore and skin temperatures are dynamially related to the pereption of effort during exerise through whih alterations in paing are linked to temperature regulation and/or musle reruitment patterns. Nevertheless, there urrently remains a lak of empirial data in whih dynami responses have been evaluated. Br J Sports Med 2009;43: doi: /bjsm

2 We propose that the inter-relationship between onsious pereptions of effort (RPE) and subonsious metaboli ontrol (mediating musle reruitment) will result in physiologially meaningful non-random flutuations of power output 4 in selfpaed exerise, while enfored mathed-intensity onstant paed exerise will result in adverse physiologial responses. The aim of this study was onsequently to ompare physiologial responses to a 5000 m rowing exerise at a mathed intensity in whih the partiipants were (1) able to voluntarily flutuate power output (self-paed) while performing exerise at a fixed rating of pereived exertion (RPE) or (2) were required to maintain a mathed-intensity (enfored) onstant power output. METHODOLOGY Partiipants Nine healthy, well-trained male partiipants agreed to take part in this study (table 1). All were informed of the proedures in advane, and informed onsent was provided prior to any data olletion. The study was approved by the Central Regional Ethis Committee of New Zealand. All partiipants were rereational gymnasium users, and eah reeived tehnial advie from a qualified rowing oah on using the rowing ergometer during a 2-week familiarisation period. Preliminary testing At the beginning of the study, all partiipants performed a standardised familiarisation trial whih onsisted of a fourstage inremental protool. The initial stage required partiipants to work for 4 min at RPE 11 (light), and eah subsequent stage inreased in intensity and dereased in time (3 min: RPE 13 (moderate), 2 min: RPE 15 (hard), 1 min: RPE 19 (very very hard)). This familiarisation protool was subsequently used as a standardised priming exerise in eah of the trials (fig 1). Rowing ergometry was seleted as a useful exerise modality for this study, as the power output attained (and effort) from eah rowing stroke during the bout an be easily influened by sensations of fatigue through up- and downregulation of effort when pulling the rowing handle. It is therefore highly sensitive to flutuations in power during the test. In all onditions, the air resistane of the ergometer flywheel was standardised by using the damper lever to apply a predetermined drag fator 130 (10 26 Nms ). On a separate and subsequent oasion all partiipants performed an inremental exerise test to volitional exhaustion on a Conept II rowing ergometer (Model D: Conept II, Tauranga, New Zealand) in our exerise laboratory for the determination of maximal aerobi power (VO 2 max). Oxygen uptake (Cortex MetaMax 3B, Cortex Biophysik, Leipzig, Germany) and power output (RowPro v2.006 software; Digital Rowing, Boston, Massahusetts) were ontinuously monitored stroke-to-stroke. Power output was visible via the Conept II display unit at all times. Experimental proedures Eah subjet ompleted three 5000 m rowing trials in three different experimental onditions in an individualised order. Condition 1 (Submaximal Self-paed using RPE sale; SubRPE) required the partiipants to omplete 5000 m at a onstant rating of pereived exertion (RPE: 15 Hard). Condition 2 (Submaximal Externally paed; SubEXT) required the partiipants to perform 5000 m at a onstant power output equivalent to the mean power output attained in the SubRPE ondition. No visual feedbak was provided in the SubRPE ondition to ensure subjets self-paed, while only stroke-to-stroke power output was visible during the SubEXT ondition, and partiipants reeived ontinual reinforement to ensure the required power output was attained. A further experimental ondition (Maximal Time Trial; MaxTT) was inluded to disguise the importane of the two submaximal onditions and to ompare intensity of efforts, while also failitating an element of randomisation in the test sequene. In the MaxTT ondition, partiipants were instruted to perform 5000 m as fast as possible, while stroke-to-stroke power output was visible at all times (fig 1). In the SubEXT ondition, partiipants were deeived to believe that the required exerise intensity was based on a onstant power output equivalent to that of ventilatory threshold attained in the baseline VO 2 max test. This deeption was onduted in an attempt to avoid partiipants realising that the two submaximal efforts in the three test series were mathed for mean intensity and thus paing the SubEXT trial on their previous SubRPE efforts. The exerise trials were held at the same time of the day on eah of the three oasions to avoid diurnal variations in body temperature, and were eah separated by approximately 1 week. The partiipants were instruted to refrain from additional organised physial ativity during the testing period and to maintain habitual exerise routines. The laboratory temperature was standardised at 18uC aross all tests while relative humidity remained onsistent (35 45%). All partiipants onsumed a beverage of water 2 h before the start of the test (5 ml of water per kilogram body mass) to ensure omparable euhydration between partiipants and trials. 18 Oxygen uptake, heart rate and power output measurement Gas exhange and minute ventilation were ontinuously reorded breath by breath (Cortex MetaMax 3B, Cortex Biophysik, Leipzig, Germany). The system was alibrated before and verified after eah test with standard alibration gases. Volume expired was measured by a volume measuring turbine, whih was alibrated with a 3 l syringe (Hans Rudolph, Kansas City, Missouri). Whole blood apillary samples were drawn from the finger tip prior to exerise and at the immediate essation of time-trial performanes for the analysis of blood latate onentration (Latate Pro, Akray, Kyoto, Japan). Heart rates (HR) were ontinuously reorded (S610i, Polar, Kempele, Finland) throughout all exerise tests, time-aligned and averaged into 30 s intervals. Stroke-to-stroke power output was assessed using the RowPro v2.006 software (Digital Rowing) in onjuntion with the Conept II interfae. After eah trial, power output was time-aligned and averaged into 30 s intervals. Measurement of thermoregulatory fators Core temperature (T ) was measured via telemetry from the intestine using a silion-oated thermometer pill (CorTemp2000, HQ, Palmetto, Florida) whih was swallowed by all partiipants 5 h before exerise to ensure that it would be past the stomah and insensible to swallowed hot or old liquids. 19 The alibration of the ingestible pills was heked prior to ingestion at four different temperatures against a ertified merury thermometer in a water bath at temperatures ranging from 30uC to 42uC. In aordane with our earlier work, a linear regression equation was then used to adjust pill measurements Br J Sports Med 2009;43: doi: /bjsm

3 Table 1 Baseline anthropometri and ardiovasular harateristis of the partiipants Anthropometri harateristis Height (m) Weight (kg) Age (years) VO 2 peak (ml/kg/min) HR at VO 2 peak (b/min) Mean (SD) 1.77 (0.06) (8.11) 29 (6) (2.75) (7.86) Skin temperatures (T sk ) were measured at four sites using stainless steel surfae skin themistors (Grant Logistis, Cambridge). Temperatures were reorded ontinuously throughout the trial using a data logger (SQ400 Squirrel Data logger, Grant Logistis). The mean body skin temperature was alulated using the formula previously desribed by Ramanathan 21 and others Measurements were taken of T and T sk ontinuously throughout the trials in order to assess dynami responses to exertion, time-aligned and then averaged into 30 s intervals for omparison with other dynami exerise data sets. Measurement of surfae iemg Surfae eletrodes (Medi-Trae 230 Foam Eletrode, Kendall Healthare, Mansfield, Massahusetts) were plaed 20 mm apart on the belly of the bieps brahii and vastus lateralis musles, and a referene eletrode was plaed on the lateral aspet of the styloid proess of the radius The skin surfae was leaned and shaved prior to eletrode appliation in order to avoid interferene and to inrease adhesion; all eletrodes were additionally fastened with medial adhesive tape. As rowing involves bilateral ativation of the musles, 26 reordings were taken from only one side (right) of the body. Sores were not standardised against a standard isometri maximal voluntary ontration (MVC), as the dynami nature of the movement pattern involved in rowing has been previously shown to eliit higher peak musle semg in rowing than in the manoeuvre used to produe isolated maximal voluntary ontrations. 26 In addition, the plane of movement in rowing is diffiult to repliate in MVC onditions, and pretrial evaluations did not support the use of that tehnique. iemg was reorded during the final 100 m of eah 1000 m period using the Power Lab data aquisition system (Power Lab AD Instruments, NSW, Australia). Raw sores were digitally filtered (band pass filter; 20 Hz to 480 Hz), digitised (1 khz sampling rate) and stored (Chart 5 v5.5.5, AD Instruments, NSW, Australia). Eah stroke was visually identified and quantified using the root mean square (rms) method, and the mean of three strokes at the end of eah 1000 m interval aross the three trials was then bathed for the purposes of statistial omparisons. Data analysis Dynami variations attributable to paing were assessed by the measurement of oxygen uptake, heart rate and power output gained from the 30 s time-aligned data series of eah outome measurement. A simple and effetive means of determining time-domain variability is to alulate the standard deviation (SD) of eah data point (ie, eah 30 s time-aligned interval) as a series. Sine variane is mathematially equal to the total power of spetral analysis, the SD of the data series reflets all the yli omponents responsible for variability in the period of reording, in this ase the time trial. This method of analysis is frequently used in the study of heart rate variability. The standard deviation for eah data series was therefore used to provide an overall omparative measure of dynami time trial variability (ttv) between test onditions using the following outome measurements: (1) oxygen uptake (VO 2ttv ), (2) heart rate (HR ttv ) and (3) power output (PO ttv ). Statistial analysis The statistial software pakage SPSS (version 11.0, SPSS, Chiago) was used for all statistial analysis. Parametri results were statistially ompared using one-way repeated-measures analyses of variane (ANOVA) and post-ho Tukey tests of Figure 1 Experiment protool for the three test sequene of rowing ergometer trials. Br J Sports Med 2009;43: doi: /bjsm

4 Honest Signifiant Differenes as appropriate. Other omparisons were made using paired Student t tests. Non-parametri data were assessed using the Friedman analysis of variane and Mann Whitney U tests. Probability values of less than 0.05 were onsidered signifiant. All results are expressed as mean (SD). RESULTS The fastest mean 5000 m performane time was observed in the MaxTT ondition, and this was shorter in duration than both SubRPE (p,0.01) and SubEXT (p,0.01) (table 2). There was no differene in the performane times of the two submaximal mathed-intensity trials (SubRPE and SubEXT). The mean performane harateristis of VO 2, HR, and power output were not different between both submaximal onditions, but these were all signifiantly elevated in MaxTT (tables 2, 3). There were no differenes in mean or dynamis of oxygen uptake or heart rate between the submaximal onditions (table 3). However, the dynamis of power output (PO ttv ) aross the time-trials showed signifiantly greater variability in SubRPE ompared with SubEXT (p,0.01) (table 2). The variability of power output was further elevated in MaxTT ondition ompared with both the submaximal trials (p,0.01) (table 2). Mean T sk was similar aross all three (submaximal and maximal) onditions (fig 2). The mean T was signifiantly lower in SubRPE than in both SubEXT (p,0.05) and MaxTT (p,0.01). There was no differene in mean T between either SubEXT or MaxTT. BLa onentrations taken immediately post-exerise were signifiantly elevated in SubEXT (6.2 (SD 2.5) mmol/l) ompared with SubRPE (5.2 (2.2) mmol/l) (p,0.05). Both submaximal blood latate responses were signifiantly lower when ompared with the maximal trial (10.9 (2.4) mmol/l) (p,0.01). Mean iemg ativity measured at both the vastus lateralis and bieps brahii was greater at eah 1000 m interval in SubEXT when ompared with SubRPE (p,0.05). The mean iemg ativity of MaxTT was signifiantly higher than both the submaximal onditions at eah 1000 m (fig 3). Core temperature was not orrelated with power output in any exerise ondition, while T sk was orrelated with power output in both submaximal trials SubRPE (r = 0.67; p,0.01), SubEXT (r = 0.54, p,0.01) but not with MaxEXT. Immediate post-test evaluation of RPE in the SubEXT ondition demonstrated a tendeny for subjets to pereive that ondition (RPE: 16 (1.9); p = 0.08) to be more hallenging than that of the presribed RPE of 15 in the SubRPE ondition. All subjets rated the MaxTT ondition to be of maximal pereived effort (RPE: 20 (0)) on the RPE sale. Salar evaluation of 30 s power output data identified that all partiipants demonstrated a spurt of power (identified as a visible upward alteration in the trajetory of power output) at a Table 2 Mean and dynami responses of performane time and power output in the three experimental onditions Performane outome measurements Performane time Power output (s) (W) (ttv) SubRPE (77.53){ (26.63){ (3.60){{ SubEXT (71.59){ (26.51){ (3.65)*{ MaxTT (53.92)*{ (27.12) (13.66)*{ Signifiantly different from Submaximal Self-paed using RPE sale p,0.01; *Submaximal Externally paed { p,0.01; Maximal Time Trial p,0.01{. ttv, Time trial variability. Table 3 Mean and dynami responses of oxygen uptake and heart rate in the three experimental onditions Physiologial (oxygen uptake and heart rate) responses Oxygen uptake Heart rate (ml/kg/min) (ttv) (bpm) (ttv) SubRPE (4.96){ 4.68 (0.72){ (15.17){ (1.49) SubEXT (5.62){ 4.30 (0.82) (17.42){ (1.56) MaxTT (4.16) 5.12 (0.65)* (11.60) (2.36) Signifiantly different from Submaximal Self-paed using RPE sale p,0.01; *Submaximal Externally paed { p,0.01; {Maximal Time Trial p,0.01. {ttv, Time trial variability. similar stage of their maximal trial (MaxTT) (89 (5)% trial; range: 81 95% of trial duration). DISCUSSION The main finding from this study was that, in submaximal exerise, the enfored onstant paed ondition (SubEXT) posed signifiantly greater physiologial and thermoregulatory hallenges to homeostasis than the mathed-intensity self-paed trial despite there being no differene in performane. Speifially, the SubEXT ondition resulted in elevated mean ore temperatures (p,0.01), greater post-test blood latate onentrations (p,0.05) and elevated iemg ativity at both bieps brahii (p,0.05) and vastus lateralis (p,0.01). The most likely explanation for this appears to be that self-paed exerise failitates the opportunity for individuals to ontinually modify effort via feedbak and feedforward mehanisms in response to frequent homeostati hallenges. Thus, the greater time trial variability of power output (PO ttv ) observed in the SubRPE ondition ompared with SubEXT (p,0.01) may indiate the presene of a entral regulatory mehanism. The greater variation of PO ttv in the SubRPE ondition ompared with SubEXT was not aompanied by greater ondition-speifi variability in either VO 2 or HR. However, the similarity of HR and VO 2 between SubRPE and SubEXT is logial, as power output is the variable manipulated as a behavioural response (to transient sensations of fatigue), and alterations in both VO 2 and HR are therefore onsequent with that behaviour, that is they are both responses to that hange in power output. This delay in physiologial response an also be explained via ommon system response times. For example, it is well known that the tau of oxygen uptake in response to dynami hanges in work is approximately s among well trained partiipants, 29 while the tau of heart rate is appreiably slower. 30 Consequently, in self-paed exerise, dynami variations in power output are probably too small and frequent for either VO 2 or heart rate to disreetly follow eah alteration. As noted by other authors, 6 the importane of suh dynami responses have often been overlooked, probably due to the relatively reent emergene of fast-response tehnology. Nevertheless, suh minor alterations in power output probably infer a mehanism by whih voluntary behaviour (up- or downregulation of effort) maintains a onstant metaboli hallenge at a sustainable level throughout the bout. 3 Behavioural hange (paing) therefore ats to defend homeostasis (e.g by defending ore temperature and blood ph) and this proess is ompromised where self-paing is not failitated. The greater variability of power output observed in the maximal trial (p,0.01) ompared with the submaximal trials was a likely onsequene of the greater freedom to alter pae in that ondition in omparison with the restrited onditions (fixed RPE or fixed power output) of the submaximal trials. It is therefore 792 Br J Sports Med 2009;43: doi: /bjsm

5 Figure 2 Mean thermoregulatory responses to the mathed intensity submaximal exerise trials (SubRPE, SubEXT) and the maximal ondition (MaxTT). preditable that the maximal trial would demonstrate greater variability than the two submaximal onditions. Evaluation of the post-test RPE sores demonstrated that subjets tended to pereive the SubEXT ondition (RPE: 16 (1).9) to be more hallenging than the SubRPE ondition (seven of the nine subjets rated it higher than SubRPE ondition) but this did not reah statistial signifiane (p = 0.08). Two of the subjets did not rate the SubEXT ondition to be more hallenging than SubRPE although their metaboli responses (blood latate onentrations and T ) were elevated in the SubEXT ondition. Our onlusion from this observation was that these two subjets were not able to verbally express their pereptions of effort as finely as their bodies were able to distinguish between the two submaximal onditions. The MaxTT ondition produed a lear (and more obvious) distintion in RPE evaluation from the two submaximal onditions (p,0.01) whereby all subjets rated their efforts as being at the top of the sale. The maximal trial was inluded in this study for several omparative purposes but most usefully to identify whether partiipants were able to distinguish between working at different levels of exertion in response to a 5000 m rowing exerise test. Partiipants in this experiment were learly able to aomplish this task. Previous studies have identified that an end spurt in performane tends to our at 90% of task ompletion, and the maximal ondition in this study was onsistent with those observations. This appears to onfirm that the inreased final Figure 3 Mean iemg bar hart for eah of three experimental onditions. BB, bieps brahii; VL, vastus lateralis. Br J Sports Med 2009;43: doi: /bjsm

6 What is already known on this topi It is widely aknowledged that the ability to aurately self pae is an important feature of rae and time trial performanes. Non-random flutuations of power output have reently been observed during self-paed exerise, whih may be assoiated with system ontrol mehanisms. A entral (brain) governor model has been proposed to regulate both afferent and efferent physiologial mehanisms to defend homeostasis via paing during exerise and onsequently avoid the development of bodily harm. What this study adds This study demonstrates that self-paing exerise poses a redued metaboli hallenge when ompared with mathedintensity enfored onstant paed submaximal exerise. The greater voluntary time trial variability of power output observed in a self-paed ondition appears to be a behavioural response to metaboli disturbane, thereby resulting in greater up- and downregulation of effort with no net loss in performane ompared with a mathed intensity enfored onstant paed submaximal ondition. Use of modern fast-response tehnologies allows a higher apture rate of both dynami physiologial and thermoregulatory responses to exerise whih are fundamental to the investigation of paing during an exerise bout. effort in maximal trials is representative of salar rather than absolute paing 5 and provides little support for the onept that maximal intensity exerise progressively indues dereases in fore prodution toward a terminal endpoint of fatigue. No differenes were seen in T sk aross submaximal and maximal trials (fig 2), although the T sk of both SubRPE and SubEXT were orrelated with power output (SubRPE: r = 0.67, p,0.01; SubEXT: r = 0.54, p,0.01). As the trajetories of T sk for both submaximal trials were similar (and also distint from MaxTT), it is possible that T sk may have ated in some way as a regulatory mehanism in the submaximal onditions, but this was ineffetual for the maximal ondition, whereby an optimal temperature (plateau) was reahed at an earlier stage due to the greater physial ost of working in that ondition. The most likely explanation for the similarity of skin temperatures between all onditions is probably that T sk progresses towards an optimal level during laboratory exerise and in the absene of further opportunities for onvetive ooling beomes ineffetual as a regulatory system when a steady state is attained. Further researh needs to be onduted to onfirm this observation. It was antiipated that the MaxTT ondition would result in signifiantly faster performanes ompared with the submaximal trials, but the similarity of physiologial responses between SubEXT and MaxTT provides further evidene of the greater metaboli hallenge of externally paed submaximal work in omparison with self-paed exerise. Indeed, where the ability to self pae is denied, the metaboli hallenge progresses toward a similar level to that of maximal exerise. CONCLUSIONS This study demonstrates that self-paing exerise poses a redued metaboli hallenge when ompared with mathedintensity enfored onstant paed submaximal exerise. It is likely that this is attributable to the ability to voluntarily flutuate power output in aordane with transient sensations of fatigue during the exerise bout. The voluntary behavioural hange to flutuate pae is therefore a onsious deision based on subonsious physiologial feedbak from an array of peripheral reeptors. Externally paed (enfored paing) submaximal exerise thereby fores an individual to abandon their own paing plan and minimises opportunities for self-managing the onsious signs of fatigue. This suggests that paing is an important physiologial mehanism to minimise the adverse onsious sensations of fatigue experiened during exerise whih enables homeostasis to be defended during exerise. To our knowledge, this study is the first to thoroughly examine both the ardiorespiratory and thermoregulatory responses to rowing performane in relation to mathedintensity self- and externally paed onditions. Further work is now required to establish whether this effet is onsistent aross more dynami exerise hallenges. Competing interests: None. Ethis approval: Ethis approval was provided by Central Regional Ethis Committe of New Zealand. Patient onsent: Obtained. Provenane and peer review: Not ommissioned; externally peer reviewed. REFERENCES 1. Abbiss CR, Laursen PB. 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