Paul W. Macdermid

Mechanical work and physiological responses to simulated cross country mountain bike racing

Abstract The purpose was to assess the mechanical work and physiological responses to cross country mountain bike racing. Participants (n = 7) cycled on a cross country track at race speed whilst O2, power, cadence, speed, and geographical position were recorded. Mean power during the designated start section (68.5 ± 5.5 s) was 481 ± 122 W, incurring an O2 deficit of 1.58 ± 0.67 L – min−1 highlighting a significant initial anaerobic (32.4 ± 10.2%) contribution. Complete lap data produced mean (243 ± 12 W) and normalised (279 ± 15 W) power outputs with 13.3 ± 6.1 and 20.7 ± 8.3% of time spent in high force-high velocity and high force-low velocity, respectively. This equated to, physiological measures for % O2max (77 ± 5%) and % HRmax (93 ± 2%). Terrain (uphill vs downhill) significantly (P < 0.05) influenced power output (70.9 ± 7.5 vs 41.0 ± 9.2% Wmax),the distribution of low velocity force production, O2 (80 ± 1.7 vs 72 ± 3.7%) and cadence (76 + 2 vs 55 ± 4 rpm) but not heart rate (93.8 ± 2.3 vs 91.3 ± 0.6% HRmax) and led to a significant difference between anaerobic contribution and terrain (uphill, 6.4 ± 3.0 vs downhill, 3.2 ± 1.8%, respectively) but not aerobic energy contribution. Both power and cadence were highly variable through all sections resulting in one power surge every 32 s and a supra-maximal effort every 106 s. The results show that cross country mountain bike racing consists of predominantly low velocity pedalling with a large high force component and when combined with a high oscillating work rate, necessitates high aerobic energy provision, with intermittent anaerobic contribution. Additional physical stress during downhill sections affords less recovery emphasised by physiological variables remaining high throughout.

Transference of 3D accelerations during cross country mountain biking

Investigations into the work demands of Olympic format cross country mountain biking suggest an incongruent relationship between work done and physiological strain experienced by participants. A likely but unsubstantiated cause is the extra work demand of muscle damping of terrain/surface induced vibrations. The purpose of this study was to describe the relationship between vibration mechanics and their interaction with terrain, bicycle and rider during a race pace effort on a cross country mountain bike track, on both 26″ and 29″ wheels. Participants completed one lap of a cross country track using 26″ and 29″ wheels, at race pace. Power, cadence, speed, heart rate and geographical position were sampled and logged every second for control purposes. Tri-axial accelerometers located on the bicycle and rider, recorded accelerations (128Hz) and were used to quantify vibrations experienced during the whole lap and over terrain sections (uphill and downhill). While there were no differences in power output (p=0.3062) and heart rate (p=0.8423), time to complete the lap was significantly (p=0.0061) faster on the 29″ wheels despite increased vibrations in the larger wheels (p=0.0020). Overall accelerometer data (RMS) showed location differences (p<0.0001), specifically between the point of interface of bike-body compared to those experienced at the lower back and head. The reduction in accelerations at both the lower back and head are imperative for injury prevention and demonstrates an additional non-propulsive, muscular, challenge to riding. Stress was greatest during downhill sections as acceleration differences between locations were greater when compared to uphill sections, and thus possibly prevent the recovery processes that may occur during non-propulsive load.

The influence of tyre characteristics on measures of rolling performance during cross-country mountain biking

Abstract This investigation sets out to assess the effect of five different models of mountain bike tyre on rolling performance over hard-pack mud. Independent characteristics included total weight, volume, tread surface area and tread depth. One male cyclist performed multiple (30) trials of a deceleration field test to assess reliability. Further tests performed on a separate occasion included multiple (15) trials of the deceleration test and six fixed power output hill climb tests for each tyre. The deceleration test proved to be reliable as a means of assessing rolling performance via differences in initial and final speed (coefficient of variation (CV) = 4.52%). Overall differences between tyre performance for both deceleration test (P = 0.014) and hill climb (P = 0.032) were found, enabling significant (P < 0.0001 and P = 0.049) models to be generated, allowing tyre performance prediction based on tyre characteristics. The ideal tyre for rolling and climbing performance on hard-pack surfaces would be to decrease tyre weight by way of reductions in tread surface area and tread depth while keeping volume high.

A longitudinal analysis of start position and the outcome of World Cup cross-country mountain bike racing

Abstract For any athlete competing at the highest level it is vital to understand the components that lead to successful performance. World cup cross-country mountain biking is a complex sport involving large numbers of athletes (100–200) competing for positional advantage over varied off-road terrain. The start has been deemed a major part of performance outcome in such races. The purpose of the present study was to establish the relationship between start and finish position in cross-country mountain bike World Cup events over a 10 year (1997–2007) period and to make comparisons with a model manipulating start position based on predicted athletic capabilities. Data collection and comparisons included results from World Cup events from 1997 to 2007 (males and females), and modelled race data based on potential performance capabilities over the same period. Analyses involved the association of annual plus pooled start and finish position (Kendalls tau) along with banded mean, standard deviation for number of changes in position, while non-constrained linear regression enabled comparison between seasons. Actual race data showed significant positive correlations between starting position and finishing position (P < 0.01) in all cases but less than the model. A mean 57.4% (s = 5.6) of males changed < 15 positions, while 62.9% (s = 9.1) of females changed < 10 positions compared with modelled data (83.6%, s = 0.8 and 91.6%, s = 1.5 for males and females respectively). Individual season comparisons show general patterns to be identical (P > 0.05) for both males and females. In conclusion, finishing position is highly dependent on start position and strategies need to be devised for competing athletes to progress in the sport.

The Effects of Vibrations Experienced during Road vs. Off-road Cycling.

The purpose of this investigation was to compare the effects of vibrations experienced during off-road and road cycling. It was hypothesised that additional damping will be expressed through a greater work demand and increased physiological markers when travelling at the same speed over an identical terrain profile. Participants ascended a tar-sealed road climb and a single-track off-road climb at a predetermined speed. Time, speed, power, cadence, heart rate and V̇ O2 were sampled and logged every second while tri-axial accelerometers recorded accelerations (128 Hz) to quantify vibrations experienced. Statistical analysis indicated accelerations to be greater during the off-road condition (p<0.0001) with post-hoc analysis exposing differences (p<0.001) for handlebar, arm, leg and seat post but not the lower back or head. The increased accelerations during off-road riding are associated with the increased vibrations and rolling resistance experienced. This led to increases in the work done (road: 280±69 vs. off-road: 312±74 W, p=0.0003) and, consequentially, a significant increase in the physiological markers V̇ O2 (road: 48.5±7.5 off-road 51.4±7.3 ml·kg(-1)·min(-1), p=0.0033) and heart rate (road: 161±10 off-road 170±10 bpm, p=0.0001) during the off-road condition. Such physiological differences and their causes are important to understand in order to provide suitable training recommendations or technological interventions for improving competitive performance or recreational enjoyment.

Performance and physiological effects of different descending strategies for cross-country mountain biking

Abstract This study investigated the performance-related feasibility and physiological benefits of purposefully eliminating propulsive work while descending in mountain biking and compared values to those measured during road descending. Participants cycled uphill on a road at race pace before descending over three conditions (off-road pedalling; off-road coasting; road coasting). Relatively low power output during off-road pedalling was associated with a greater oxygen uptake (p < .01) when compared with off-road coasting despite no difference in vibration exposure (p > .05). Importantly, pedalling did not invoke a performance benefit (p > .05) on the descent used in this study. Significantly greater heart rate and oxygen uptake (both p < .01) were observed between road and off-road descending, likely caused by the increase in terrain-induced vibrations (p < .01) experienced between the bicycle and rider. Results indicate that reducing propulsive work during descending can improve recovery without being disadvantageous to performance. Similarly, the vibrations experienced during road descending are relatively low, and further reduce oxygen cost. In an effort to increase efficiency, it is recommended that mountain bike athletes focus on skills to increase descending speed without the addition of pedalling, and that equipment be used to decrease vibrations nearer to those seen on the road.

Validity of a device designed to measure braking power in bicycle disc brakes.

Abstract Real-world cycling performance depends not only on exercise capacities, but also on efficiently traversing the bicycle through the terrain. The aim of this study was to determine if it was possible to quantify the braking done by a cyclist in the field. One cyclist performed 408 braking trials (348 on a flat road; 60 on a flat dirt path) over 5 days on a bicycle fitted with brake torque and angular velocity sensors to measure brake power. Based on Newtonian physics, the sum of brake work, aerodynamic drag and rolling resistance was compared with the change in kinetic energy in each braking event. Strong linear relationships between the total energy removed from the bicycle-rider system through braking and the change in kinetic energy were observed on the tar-sealed road (r2 = 0.989; p < 0.0001) and the dirt path (r2 = 0.952; p < 0.0001). T-tests revealed no difference between the total energy removed and the change in kinetic energy on the road (p = 0.715) or dirt (p = 0.128). This study highlights that brake torque and angular velocity sensors are valid for calculating brake power on the disc brakes of a bicycle in field conditions. Such a device may be useful for investigating cyclists’ ability to traverse through various terrains.

The impact of uphill cycling and bicycle suspension on downhill performance during cross-country mountain biking

ABSTRACT Non-propulsive work demand has been linked to reduced energetic economy of cross-country mountain biking. The purpose of this study was to determine mechanical, physiological and performance differences and observe economy while riding a downhill section of a cross-country course prior to and following the metabolic “load” of a climb at race pace under two conditions (hardtail and full suspension) expected to alter vibration damping mechanics. Participants completed 1 lap of the track incorporating the same downhill section twice, under two conditions (hardtail and full suspension). Performance was determined by time to complete overall lap and specific terrain sections. Power, cadence, heart rate and oxygen consumption were sampled and logged every second while triaxial accelerometers recorded accelerations (128 Hz) to quantify vibration. No differences between performance times (P = 0.65) or power outputs (P = 0.61) were observed while physiological demand of loaded downhill riding was significantly greater (P < 0.0001) than unloaded. Full suspension decreased total vibrations experienced (P < 0.01) but had no effect on performance (P = 0.97) or physiological (P > 0.05) measures. This study showed minimal advantage of a full suspension bike in our trial, with further investigations over a full race distance warranted.

Agreement between Powertap, Quarq and Stages power meters for cross-country mountain biking

Abstract Advances in technology have made the use of a variety of power meters ubiquitous in road cycling along with an ever-increasing popularity during mountain biking. This study compared data from one bicycle using three power meters: Stages (non-driveside crank arm); Quarq (chainring spider); and Powertap (rear-wheel hub). While no differences (p > .05) between power meters were present during treadmill riding at high or low cadences, dissimilarities for both power (W) and cadence (rpm) were apparent during actual cross-country mountain bike riding. Frequency distribution and analysis of coasting indicate that the Stages records more time (p < .001) at zero watts (6.9 ± 3.3 s) and zero cadence (6.9 ± 3.3 s) compared with Quarq (W = 3.3 ± 1.5 s, rpm = .8 ± .7 s) and Powertap (W = 1.1 ± .8 s, rpm = 3.0 ± 1.2 s). Consequently, significant interactions (power meter × terrain, p = .0351) and main effects (power meter p < .0001, and terrain p < .0001) for power output were present and included: uphill (317.5 ± 50.7, 340.8 ± 52.6, 327.3 ± 48.6 W); downhill (127.6 ± 12.3, 147.4 ± 23.8, 160.1 ± 24.0 W); and flat (201.1 ± 21.6, 225.2 ± 27.2, 224.0 ± 29.6 W) for the Stages, Quarq and Powertap, respectively. It is likely that accelerometry (Stages) compared with reed switch (Powertap and Quarq) technology to determine cadence, resulted in the discrepancies between power meters. However, while the reliability of the different methods appears acceptable for intermittent exercise such as cross-country mountain biking, the validity of each in such a situation requires confirming.

Tyre Volume and Pressure Effects on Impact Attenuation during Mountain Bike Riding

Exposure to impacts and vibrations has been shown to be detrimental to cross country mountain bike performance and health. Therefore, any strategy aimed at attenuating such exposure is useful to participants and/or industry. The purpose of this study was to assess the influence of tyre size and tyre inflation pressure on exposure to impacts. Participants completed nine trials of a technical section (controlled for initial speed and route taken) including nine separate conditions involving three tyre sizes and three tyre inflation pressures normalised per tyre. Performance was determined by time to negotiate the technical section while triaxial accelerometers recorded accelerations (128 Hz) to quantify impact exposure and the subsequent effects on soft tissue response. Increases in tyre size within the range used improved performance while changes to tyre inflation pressure had no effect on performance. Larger tyre sizes and lower tyre inflation pressures significantly reduced exposure to impacts which could be augmented or negated due to an interaction between tyre size and inflation pressure . It is recommended that mountain bikers use larger tyres, inflated to the moderate pressures used within this study, in order to increase performance and reduce the risk of overuse injuries.