Bike fit research

Aerodynamics isn’t everything: position also has an impact on metabolic cost. Jed Campbell-Williams describes his undergraduate research on the effect of bike set-up on cardiorespiratory function.

In bike fit, back angle or torso orientation is an important consideration, as it’s often the difference between an aggressive and a relaxed position. Lowering the back angle can help to decrease the frontal area of the rider, which in turn helps to reduce aerodynamic drag. And as we all know, lower drag means more speed for the same effort.

However, with most riders it simply isn’t appropriate to fit the longest stem in the shop, slam it as low as it will go and expect them to ride faster. Other factors must be considered such as the rider’s comfort, biomechanics and physiology.

The study

This article is based on the results of an undergraduate sports science dissertation. There were two questions being asked by the researcher; the first was looking into whether decreasing torso orientation at a set power output increased the load on the heart and lungs. The second looked at how different power outputs changed how torso orientation impacted the body.

The study consisted of two separate visits for each participant. Visit one included a ramp test to identify ventilatory thresholds. The threshold analysis gives two key values — the ventilatory threshold (Tvent) and the respiratory compensation point (RCP). These are key physiological markers. Tvent can be closely linked with the lactate threshold, and RCP is at a similar intensity to maximal lactate steady state (MLSS) (the maximum intensity that can be sustained in a steady state, similar to functional threshold power or FTP).

After these physiological markers were identified, visit two involved six, 6-minute efforts, consisting of two intensities and three positions. The intensities were: a) just under the Tvent value, and b) halfway between Tvent and RCP. This allowed an examination of moderate and hard intensity riding. The three positions were riding on the hoods, the drops, and using clip on TT extensions. These gave back and shoulder angles of:

These angles were measured statically using a handheld goniometer before starting the trials. The same saddle height and setback were used in all the trials. Torso angle was measured from a horizontal plane at the greater trochanter to the acromion process in all positions. In the hoods and the dropped positions shoulder angle was measured at the acromion process (shoulder) between the greater trochanter (hip) and styloid process of the ulna (wrist). In the TT position, as the elbow is the first point of contact with the upper body and the handlebar, shoulder angle was measured at the acromion process between the greater trochanter and the olecranon process (elbow).

Results

As expected, there was a significant increase in the magnitude of physiological responses following an increase in workload. This was across all variables measured and was present regardless of position. This is in line with previous studies that have manipulated both workload and position.

There was significance found across two variables: V ̇ O2 and RQ. The differences in V ̇ O2 can be seen in the graph below, which shows a significantly lower V ̇ O2 in the upright position at the lower workload, but when looking at the higher workload there was no significant difference in VO2 across any of the positions.

Interaction effect between workload and position for rate of oxygen consumption, mean observed value with error bars at extremes set at 1 SD (mL·min -1 ). UP=upright position, DP=dropped position, AP=aero posture.

DISCUSSION

It has been suggested that the decrease in metabolic work required in upright postures results from the reduced upper body muscle activation required to maintain position.

When trunk position reaches the lower angles of flexion, the diaphragm is unable to work in the optimal region of its length tension curve. This requires more work to maintain the same and could explain why V ̇ O2 is increased in DP and AP.

Respiratory quotient was not shown to be significantly affected by position; however, there was a significant interaction effect found between position and workload. Respiratory quotient in AP is more affected by workload than either UP or DP. Respiratory quotient increasing indicates that the amount of CO2 produced (V ̇ CO2) is increased. V ̇ CO2 increasing at higher intensities is associated with an increased blood lactate. This suggests that while position does not significantly affect blood lactate accumulation, in the aerodynamic tuck blood lactate increases at a faster rate when workload is increased when compared to the other position.

This research suggests that as trunk orientation decreases, the rate of oxygen consumption increases when working at lower power outputs. When looking at the effect of RQ we can see that there is the potential for a faster accumulation of lactate within the blood. This suggests that over longer distances at higher workloads, the rider may succumb to fatigue faster in more aggressive positions. The important trade-off to consider with these results is how much aerodynamic improvement can be obtained without compromising metabolic cost. That is likely to be different from rider to rider and may depend on what type of riding they’re used to and spend most of their time doing.

This study was carried out on untrained participants inline with an undergraduate dissertation.