The Physiology and Bioenergetics of Ultraendurance Mountain Bike Racing

Abstract

Ultraendurance mountain bike racing is a relatively new sport and has received scant research attention. The practical difficulty of field-testing during competition has played a role in this dearth of knowledge. The purpose of this thesis was to investigate the physiology and bioenergetics of cross-country marathon (XCM) and 24 hour team relay (24XCT) mountain bike racing.

Study One analysed the physiological characteristics of XCM competitors and compared them to data from studies in the literature for Olympic-distance cross-country (XCO) mountain bike competitors. The XCM participants had lower mean peak aerobic capacity (58.4 ± 6.3 mL•kg-1•min-1), greater body mass (72.8 ± 6.7 kg) and estimated percentage body fat (10.4 ± 2.4%) compared to values reported for XCO competitors in the literature. Stature (1.77 ± 6.0 m) and normalised peak power output (5.5 ± 0.7 W•kg-1) were comparable. These data suggest that specific physiological characteristics of XCM competitors differ from those of XCO competitors.

Study Two quantified and described the exercise intensity during a XCM race by monitoring heart rate responses. The mean heart rate (150 ± 10 beats•min-1) for the duration of the race equated to 82 percent of maximum heart rate and did not differ significantly throughout the race (p = 0.33). The data indicated that the XCM race was of a high aerobic intensity. Prior to the competition the relationship between heart rate and O2peak for each participant was established during an incremental laboratory test. Energy expenditure was estimated by assigning 20.2 kJ to each litre of oxygen consumed. The mean rate of energy expenditure during the race was estimated to be 59.9 kJ•min-1. Furthermore, no anthropometric or physiological measures were correlated to race speed, indicating that other factors contribute to race performance.

The third study was a laboratory-based investigation to determine whether physiological factors relevant to 24XCT racing change with time of day. On separate days participants cycled on an ergometer for 20 min at 82 percent of maximum heart rate at 06:00, 12:00, 18:00, and 00:00 h. Significant differences (p < 0.05) were observed for several physiological responses (heart rate, oxygen uptake, salivary cortisol concentrations and intra-aural temperature) but not for performance variables (power output and self-selected cadence). It was concluded that the laboratory protocol lacked ecological validity and that it was necessary to test within a race using authentic 24XCT competitors.

In order to measure in-race performance, Study Four examined the agreement between a bottom-bracket ambulatory ergometer (Ergomo®Pro) and the criterion SRM power meter in a field-based setting. Analysis of absolute limits of agreement found that the Ergomo®Pro had a systematic bias (± random error) of 4.9 W (± 6.12). Based on tolerances recommended in the literature the unit was considered fit for purpose for measuring power output during 24XCT racing.

Study Five was a multiple case-study design that examined the physiological and performance parameters of a team during a 24XCT race. It was reported that mean work-shift speed (18.3 ± 2.6 km•h-1), power output (219 ± 50.9 W) and cadence (64.1 ± 9.3 rpm) were variable between participants and between work-shifts. A commonality amongst the participants was an increase in speed during the final work-shift compared to the penultimate one. A decline in work-shift heart rate was observed throughout the race. For the majority of participants an increase in gross efficiency (1.7 ± 1.4 %) was reported from the penultimate to the final work-shift. It was concluded that pacing strategies were employed and that the improved efficiency was caused, in part, by an increased familiarity with the course during the race.

Study Six examined the nutritional practices and energy expenditure of the same team during the same 24XCT race. Energy expenditure during the work-shifts was estimated in accordance with Study Two. Resting energy expenditure during the recovery periods was estimated using the Harris and Benedict formula (1919). Food and fluid consumption were determined via food diaries and hydration status was assessed by measuring the refractive index of urine. Energy consumption (17.3 ± 2.2 MJ) was considerably less than energy expenditure (30.4 ± 6.1 MJ) with the former accounting for only 57 percent of the latter. The energy cost during the work-shifts was estimated to be 74.5 kJ•min-1. Mean fluid intake (6.3 ± 0.9 L) for the 24 h was sufficient to maintain hydration status.

Based on these studies an integrated model of the factors that influence ultraendurance mountain bike performance was developed. The domains that influence race speed are physiological factors, technical and tactical factors, and nutritional strategies. The sub domain that influences these is environmental factors. Collectively this information is of practical importance to sport scientists, coaches and athletes involved with designing nutritional and tactical preparation strategies and training programmes for this sport.