Triathlon is one of the fastest growing sports in the UK. The three-sports-in one format appeals to many. It also throws up some interesting issues for the optical practitioner who has triathlete clients. In the second article in this series, we look at the visual demands of the cycling discipline.
Riding posture
To understand the visual demands of cycling in triathlon, it is necessary to understand how cycling as part of a triathlon may differ from regular cycling. There are two main types of bicycles commonly seen on UK roads: those with flat handlebars (hybrid or mountain bikes) and those with dropped handlebars (road or racing bikes). The first category allows the rider an upright, vertical position and thus a head posture similar to that of walking. The second category requires the rider to lean forward more and thus requires the head to be tilted back slightly.
In triathlon, it is very common (and the norm for professional triathletes) to see a third category of bicycle. This is a machine built almost exclusively with speed in mind rather than comfort. It can be difficult (and sometimes uncomfortable) to ride, so is usually only seen in races or training. This triathlon bicycle (or time-trial machine as it is often called) uses a smaller frame and an extended handlebar, on to which the rider leans forward to achieve a very low, aerodynamic, position (Figure 1). Using this low, stretched position is effective in cutting wind resistance, thus allowing the bike and rider to travel faster for the same amount of effort.
The prone position requires the rider to tilt his neck and head to an extreme angle to see where he is going. From an ocular point of view, this requires the eyes to be very elevated, giving a similar ocular muscle action to that achieved by extreme vertical gaze. This type of viewing angle is rarely used in everyday life. We know that most daily tasks require horizontal viewing or depression rather than elevation. Even in sport it is unusual for elevation to be the primary visual position. Sports such as hockey, basketball or bowls require some viewing in elevation but it is intermittent. In triathlon cycling, the position has to be held constantly for long periods – the bike leg of an Ironman is over 100 miles! The position can be so extreme that it may cause binocular vision issues in some patients, for example those with decompensating exophoria or a ‘V pattern’ motility defect.
Vision correction
Contact lenses
As mentioned in Part 1 in this series (21.03.14), when correcting vision for sport, contact lenses are often the first choice. This is due to convenience, comfort and the unobstructed field of view they offer. In most sports, a soft contact lens would be preferable to a rigid lens due to its complete corneal coverage and the fact that it is unlikely to become dislodged during activity.
In many scenarios, soft contact lenses would cope admirably well with the visual requirements of a triathlon bike leg. They would give the rider clear, stable vision and a good peripheral view. A disposable lens would be preferable due to the swimming discipline and the potential for deposition from small particles coming into the eye while riding.
However, soft contact lenses designed to correct astigmatism may not perform well in triathlon cycling. Modern soft toric designs use methods of stabilisation that usually require some interaction between lens and eyelids. Common examples include:
? A prism ballast optic zone, where the upper lid forces during blinking maintain the lens in position
? Thick and thin zones, where a thicker horizontal section of the lens is ‘squeezed’ between the two lids.
As we have already mentioned, during triathlon cycling the athlete adopts a ‘heads down, eyes up’ posture. This will inevitably cause a significant portion of the lens to be under the top eyelid. It will also significantly alter the forces of the eyelids on the contact lens. In reality, this leads to lens rotation and a resultant loss of focus. Therefore, wearers of toric soft contact lenses (even if they are ordinarily very happy with their contact lens vision) cannot be guaranteed to achieve clear vision in this extreme triathlon position. In cases where astigmatism is low, the wearer may prefer to forgo the toric correction in preference for the more stable vision afforded by a spherical lens.
In cases where the astigmatism is higher, alternatives are few. A rigid lens may give good vision on the bike if fitted in a lid attachment manner, but this format is wholly undesirable for swimming.
Another alternative may be to use a hybrid RGP/soft lens such as the Duette lens from No7, as this does not require stability of movement to maintain clarity of vision. Unfortunately, it is not available as a disposable lens and so is not ideal for triathlon.
Spectacle frames
If athletes are emmetropic or contact lens wearers, they should still be advised to wear suitable frames when cycling to protect their eyes. These frames should contain impact resistant lenses (polycarbonate or similar) to protect the eyes, both from airborne particles and in the event of an accident (Figure 2). They will also prevent wind from directly hitting the cornea, which can result in epiphora and blurred vision, especially in contact lens wearers. This will help maintain clarity of vision. The eyewear will also help guard against UV damage to the eyes – a necessary consideration for a group of people likely to spend so much time outdoors. Thankfully, it is now commonplace for triathletes to wear frames when cycling.
[CaptionComponent="200"]Frame style
The head down position and elevated eye position of a triathlete can lead to the upper portion of the frame obscuring the rider’s view. In fact, if triathletes are studied closely, it is not uncommon to see them looking over the top of their frames. This is not ideal, especially when the frames are needed to correct vision.
The manufacturers of sports eyewear are aware of these issues and have offered several potential solutions. A popular choice is to use a frameless design, with larger ‘mask’ style lenses. These give a wider vertical field of view by removing the frame brow bar (Figure 3).
A second option is to ‘extend’ the vertical aspect of the lens, so that there is more usable space in the superior portion of the eyewear. Perhaps the most well known example of this solution is the Oakley Radar XL. This is almost identical in design to the regular Radar model but with a higher lens above the nose bridge.
This frame is said to have been designed following consultation with the now disgraced US professional cyclist Lance Armstrong. He was frustrated at having to remove his sunglasses for time trials due to the frame obscuring his view.
A third, and increasingly popular solution, is for manufacturers to include an integral ‘visor’ in the helmet design. This allows for a much larger ‘lens’ but does not give a prescription correction option.
As with the swimming discipline of triathlon, some competitors will choose not to wear contact lenses, but opt for an alternative solution to correct their ametropia. Unfortunately, none of the above solutions for frame design lend themselves well to a prescription format. Single ‘mask’ style lenses are not readily glazed with prescription lenses. Instead, the dispensing optician must rely on conventional, two lens designs. Lens choice will be discussed in more detail in the third part of this series. Here we will concentrate more on the dispensing requirements for cycling.
[CaptionComponent="201"]Lens centration
When dispensing lenses for triathlon cycling, due consideration should be given to the lens centration. As discussed above, the user will predominantly be looking through the upper section of the lens. The clearest view will be achieved if the lens centres are raised proportionally. This is particularly important for wrap or 8-base lenses, where the lens power can vary considerably from the centre to the edge of the lens.
It is therefore necessary to measure a vertical fitting height for the lenses, even if they are for single vision correction. This can be done by asking patients to adopt their cycling head posture. Lens fitting height can then be marked.
However, we must still remember the multi-discipline nature of triathlon. If the patient wishes the spectacles to have some additional functionality, it may be pertinent to set the heights slightly lower than measured. Most people would like to wear their frames for other activities as well as cycling, but triathletes particularly are likely to use the same eyewear for running. In reality the practitioner will likely choose to raise the centres compared to normal but not to the full extent of the cycling position. A suggestion is to place the optical centres equidistant between that found in normal viewing and cycling positions. Additionally, this will assist with lens manufacture and improve lens appearance by minimising blank size and lens edge thickness.
Frame fitting
When an athlete assumes the head down cycling position, it is easy to see how the frame can obscure the line of vision. One way to counteract this is to adjust the frame accordingly. If the frame has nose pads, the pad arms can be brought together to move the whole frame upwards on the face. The pantoscopic tilt can also be adjusted to move the inferior lens further from the face. This decrease in pantoscopic angle will bring the lens more perpendicular to the line of sight, and help reduce distortions.
Some frames will allow the pantoscopic tilt to be readily adjusted for different disciplines, by using an adjustable hinge design. This can be done quickly and easily by the athlete.
Prescription lens inserts
A popular method of correcting ametropia in sports frames is to use a secondary lens insert, which sits behind the main lens of the glasses. The wrap and tint are created by the exterior lens or lenses and the prescription is corrected by the insert lens. This allows the insert to be of a lower base curve and smaller in size than the main lens, thus making dispensing easier.
The downside of this design for triathlon cycling is that the tilted head position almost always leads to the athlete looking over the top of the prescription lens. Consequently, lens inserts are often not the first choice for triathletes, although they remain a good option for other forms of cycling.
[CaptionComponent="202"]Helmets and eyewear
If someone wishes to compete in a British Triathlon sanctioned event they must wear a helmet during the cycle leg. As you would expect, the size, design and shape of helmets can vary considerably and it is always a good idea to check that the helmet shell and straps are compatible with the chosen eyewear prior to dispensing (Figure 4). If the sides of the frame come into regular contact with the helmet or straps, it can be annoying but also (more importantly) can cause the frames to be dislodged and therefore compromise safety.
Summary
Vision correction in cycling is a huge topic with many considerations. Cycling is one of the few sports where in a competitive environment it is more common to see athletes wearing eyewear than not. When dispensing eyewear for cycling, the practitioner must weigh up various factors including clarity of vision, comfort and safety.
Cycling as part of a triathlon has its own individual nuances which will have an impact on the dispensing process. When dispensing for ametropia, there does not currently appear to be a perfect solution to the triathlon conundrum, but there are still several areas where the practitioner can give helpful guidance.
Read more
Vision in triathlon: The challenges facing runners
Vision in triathlon: The challenges facing swimmers
Colin Moulson is a sports vision optometrist working in private practice
