English
中文Introduction
Field-based team sports, such as field hockey, rugby union, rugby league and soccer are hugely popular sports worldwide. The majority of athletes incorporate some form of warm-up into all practice and competition situations, little however is understood about the advantages of this action. A number of alternative warm-ups have been suggested in recent years. Many team games involve repeated sprints of short duration, interspersed with periods of low intensity recovery. During the course of a field hockey or soccer match it has been recorded that a sprint or high intensity run occurs once every 30 s (Duthie et al., 2003; Reilly and Gilbourne, 2003). These sprints tend to be short in duration (<4 seconds). Therefore it was decided that there would be large practical implications for games players if a study was completed to determine the effects of warm-up on repetitive sprints. The aim of this study is to determine the effects of a passive warm-up on repeated sprint performance.
Many studies have been completed to determine fatigue and recovery of peak power output (PPO) on repetitive sprint exercise (Bogdanis et al. 1995, 1996, 1998; Casey et al., 1996; Gaitanos et al., 1993; McCartney et al., 1986). Research has also been undertaken to determine the effects of heating muscle on either a single bout of maximal activity (Febbraio et al., 1996; Sargeant, 1987) or on exhausting work (Craig & Froehlich, 1968; Martin et al. 1975). With the exception of papers by Gray & Nimmo (2001) little research has been completed which combines these two areas.
Pre-heating a muscle has consistently been reported to increase performance. Previous research has indicated that pre-heating will elicit an increase in PPO from ~8-33% (Sargeant, 1987). The mechanisms which led to these increases are still being debated, however the increased blood flow which results from passive heating is thought to be a major factor (Sargeant, 1987).
Very few studies have compared passive and active heating. When these warm-ups were compared no performance differences were reported (Gray & Nimmo, 2001). It is therefore hoped that a passive warm-up will allow the benefits of warm-up, like increased skin bloodflow, to be achieved without the subsequent decease in PCr stores which occurs with an active warm-up. Although an elevated muscle temperature is expected to promote sprint performance, power output during repeated sprints was reduced by hyperthermia in a study conducted by Drust and colleagues (2005). The impaired performance was not related to the accumulation of metabolic fatigue agents. It was therefore suggested that it may relate to the impact of high core temperature on the function of the central nervous system. However, this study involved only seven men completing 40 minutes of intermittent work on a cycle ergometer instead of a treadmill. This lack of specificity regarding exercise modality may be considered as a limitation to this methodology. A study conducted by Mohr and colleagues (2004), assessed the impact of muscle temperature on repeated sprint performance during a soccer match. It was concluded that in soccer, the decline in muscle temperature and core temperature during half-time was associated with a lowered sprint capacity at the onset of the second half, however sprint performance was maintained when low-intensity activities maintained muscle temperature. It would therefore seem prudent to assume that a passive re-warm up at half time would be advisable.
A common finding during repetitive sprints is a decrease in performance in the later sprints due to muscular fatigue (Bogdanis et al. 1998). Sargeant (1994) defines fatigue as "the failure to generate or maintain the required or expected force or power output, resulting from muscle activity and reversible by rest." For this reason fatigue is seen as a negative occurrence. Whether a passive warm-up will prolong the initiation of fatigue is debatable as the literature indicates a larger fatigue index for PPO in a pre-heated muscle (Sargeant, 1987).
In light of previous research, it is hypothesised that by introducing a passive warm-up to directly heat the muscle, PPO will be increased. Fatigue is expected to occur regardless of whether the muscle is pre-heated.
Methods
Subjects
Both males and females are required to participate. All subjects will have at least one years experience of playing in a team sport.
Equipment
All trials will be carried out on a friction-loaded cycle ergometer (Monarch, 720) interfaced with a BBC microcomputer. The cycle ergometer will be calibrated and the saddle height adjusted for each subject. The power output will be calculated from friction load and flywheel velocity, after correction for flywheel acceleration (Lakomy, 1986). The test load will be body-mass dependent at 75 g.kg-1BM. Heart rate will monitored throughout the trials using a Polar heart rate monitor.
Protocol
All subjects will be required to make five visits to the laboratory, each separated by one week, to allow adequate recovery between trials. The order of the trials will be a randomised cross-over design to control for any trial order effect that may be encountered. Each trial will be performed at the same time of day in the same ambient conditions to ensure any changes in circadian rhythm are controlled. In addition each subject will perform a 30s familiarisation sprint one week before data collection began. This is to ensure there is a minimal learning effect.
Each trial will consist of a standardised warm-up of 2 x 30s at 80rev.min-1 and 110rev.min-1 with a resistive mass of 1.5kg, separated by a 30s rest. Duplicated 20ml samples of blood will be taken from a pre-warmed thumb one-minute post warm-up. After a 5 minute recovery period the subject will be required to perform a series of 6 x 6s sprints with 24s recovery (Figure 1). Data logging by the computer will be initiated 3s before the first sprint, and the subject will be given a 3s countdown before subsequent sprints, this prevented timing errors from the subject starting too early. Expired air samples will be collected every 30s (6s sprint and 24s recovery) using both two-way and single valve Douglas bags. Duplicate 20ml thumb prick blood samples will be taken again immediately after the final sprint. In the heated trial the subject will preceded the active warm-up with a passive warm-up which will consist of 25 minutes in a water bath (39±1 oC) followed by one minute drying and shoe time. The oral temperature will be taken every 5 min during heating.
Peak power output (PPO), mean power output (MPO) and peak pedal revolutions will be obtained for each sprint. The fatigue index (FI) for power output will be calculated as the percentage decline from peak power output to the final power output for each subject. The samples of expired air will be analysed for composition of O2 and CO2 using a Servomex gas analyser.
Statistical analysis
Data will be checked for normal distribution and Sphericity (Mauchley’s correction). Two-way analysis of variance (ANOVA) for repeated measures will be used to determine differences between trials for PPO, MPO, peak pedal revolutions, O2 uptake and heart rate (HR). Where a significant difference is found a post-hoc Tukey test will be used. A paired t-test will be used to analyse percentage decline in PPO and MPO and temperature data. Pearson product moment correlation will be used to determine any relationships present. Significance will be determined as P<0.05. The results will be presented as the mean ± standard error (SE).
Limitations
There are a few limitations to this study. The major limitation is that the training status of the subjects is unknown. Therefore, no indication of the proportion of type I and type II fibres is able to be inferred from the type of activity completed by the subjects. The different fibre types work at different contractile speeds and show different velocity-power relationships. This will have large implications on the magnitude of the PPO at the given pedal rate velocity (Bogdanis et al. 1998; Sargeant, 1994). It is not possible to obtain values for muscle lactate as a muscle biopsy is too intrusive, so blood lactate values will be taken instead. Although this does give a good indication of the conditions prevailing it does not give an accurate measure of muscle lactate level or muscle pH values which would give a clearer indication of the metabolic pathways occurring. Temperature will be gauged using a measure of aural body temperature. More distinct evidence of the effect of a passive warm-up would be provided if muscle temperature could be measured as the heating provided is aimed at heating solely the working muscle.
