# An Optical Intervention to Improve Cycling Time Trials: A Feasibility Study
###### Abstract  
(1) Background: Many professional and recreational cyclists experience that neck extension in time trial position negatively impacts either speed, comfort, or power production--especially at high cycling speeds or for long distances. We conducted a feasibility study with one subject to assess whether redirecting the sight of a cyclist while in time trial position could reduce aerodynamic drag and neck strain by maintaining a more neutral neck position. (2) Methods: A physical immersive exercise bike was developed (called a FAAST-trainer) that emulates posture, velocity, and power to be delivered by the user through an adaptable power load adjusted in real time. As an optical intervention, we used prism glasses to redirect the cyclist's sight. The subject trained his perceptive-muscular system while cycling on the FAAST-trainer to get used to wearing prism glasses. He feels confident that the glasses are safe to test for future experiments in a velodrome. (3) Results: A consistent reduction in drag was found (\(p<001\)) when wearing prism glasses with the FAAST-trainer, ranging from 3.5% to 4.7%. Accordingly, the cyclist could thus save between 9.7 watts and 13.0 watts cycling at 45 km/h, compared to having his head in an upright position. (4) Conclusions: Our experiment on the FAAST-trainer indicates that an optical intervention to reduce neck extension by redirecting sight might be safe to use for outdoor cycling. However, no vestibular effects, neither auditive nor complex combinations, were assessed, so we recommend additional research and development of a dedicated design for the prism glasses. Outdoor experiments should be conducted to confirm this reduction in aerodynamic drag and further asses the safety when wearing prism glasses. cycling aerodynamics; prism glasses; biomechanical efficiency; immersive exercise bike +
Footnote †: journal: Medical Engineering  
Here _rho_ is the density [of the ambient air]; Cd is a dimensionless constant that represents the shape of the object; and v[air] is the speed of the ambient air, relative to the cyclist and their bike. Given that power is force multiplied by the speed of the cyclist, the power needed to overcome the air friction is calculated with:  
\[\text{P}=1/2\;\textit{rho}\;\text{[air]}\times\text{Cd}\times\text{A[frontal]}\times\text{v[air]}\times\text{v[air]}\times\text{v[ ground]} \tag{3}\]  
The authors of [3] discuss an extended classical treatise on aerodynamics and the above-mentioned formulas. In cycling aerodynamic assessment, it is common to retrieve only one free parameter from the above-mentioned formula, namely Cd \(\times\) A or CdA, allowing for a clear quantification of a cyclist's aerodynamic drag. The CdA of a cyclist ranges from 0.17 m\({}^{2}\) to 0.34 m\({}^{2}\), depending on the measurement technique and the cyclist's posture [4]. Considering the variation in Cd and in frontal surface area A separately, the drag coefficient of a cyclist ranges from Cd = 0.6 for an exceptionally streamlined time trial position to Cd = 0.8 for a non-aerodynamic upright position [5; 6], while the frontal surface area triples in that range. Therefore, when changing cycling positions, the frontal surface area of a cyclist and bike has, on the whole, a greater effect on total drag than the drag coefficient. When cycling at high speeds in time trial position, riding with the head up, looking at the road ahead, the cervical spine is extended to watch the road. This might increase the frontal surface area, thereby increasing aerodynamic drag. In addition, there is more tension in the anterior neck muscles, caused by the arms and shoulders, to counterbalance the forces exerted by the legs. From an ergonomic point of view, this position in the time trial bars with a very small torso angle--that is almost parallel to the ground in some cases--creates this tension in the posterior neck muscles due to the head flexion needed to look at the road ahead. This is extremely undesirable and the most common overuse injury in cycling [7].