Highlighting recent research, Andrew Hamilton explains why cyclists suffering from chronic low-back pain should look inwards before blaming their bike MORE
Pedaling performance: don’t let it become a pain in the foot!
Foot injury as a result of the foot/pedal interface is surprisingly common and under-reported in cyclists. PP explains why and provides evidence-based suggestions to keep cycling pain free
Cycling injury research can be broadly collated into two categories: trauma-related, which occurs as a result of a collision or fall, and non-trauma related, which involves overuse type injuries. The epidemiology of collision/impact injuries in cycling has attracted a relatively large amount attention from researchers, presumably because of their life-threatening potential. However, the data suggests that despite the acute risks of collisions and falls, cyclists are more likely to suffer from injuries resulting from the action of cycling itself. A 2006 study on cycling injuries concluded that the prevalence of non-traumatic injury was as high as 85%(1).
Although coaches, physiotherapists and trainers can’t do much about the former, overuse type injuries are amenable to treatment and prevention. A survey of the literature on non-traumatic cycling injuries shows that much of the research to date has been focussed on injuries of the back, neck, arms, hands, buttocks, perineum and knees(2). This is perhaps understandable; cyclists interface with the bike at three different points – the handlebars, the saddle and the pedals. Incorrect handlebar shape/settings, saddle design/height or frame geometry can result in excessive loading on joints and muscles and undesirable movement biomechanics, significantly increasing the risk of injury.
Points of contact and cycling injury potential
Regardless of the bike type, frame geometry and riding position, all riders make contact with their bike at three different points: the pedals, the saddle and the handlebars. At each of these points, forces are generated at the interface of the bike and rider (see figure 1). The greatest forces are those at the pedals, where large forces have to be applied to drive the transmission, and the saddle, where a significant portion of the rider’s weight is perched on a relatively small surface area, resulting in large amounts of force per unit area. Saddle designs that reduce pressure on the perineum region in particular have received lots of attention from manufacturers in recent years, including ‘cut out’ and contoured designs.
Figure 1: The three points of contact
The rider interfaces with the bike at the saddle, handlebars and pedals, and it’s the latter where the greatest forces are generated.
Compared to those at the pedals and saddle, the forces experienced by riders at the handlebar interface are significantly less, although they will vary according to the handlebar design, the rider’s position (ie how far forward he or she is leaning), the amount of cushioning between the hands and the bars (padded tape, gloves etc) and the stiffness of the bars (stiff bars transmit more road vibration to the rider’s hands).
One area that may have been overlooked however is the pedal interface, especially since it’s the foot/pedal interface that typically experiences the greatest forces during cycling. Unlike other overuse cycling injuries mentioned above, there’s very little peer-reviewed research available on foot injuries, with those foot injuries that are reported in the literature being mainly descriptions of foot numbness, metatarsalgia, Achilles tendonitis and plantar fasciitis(3-5).
A study by Dettori and Novell (2006) reported on the prevalence and incidence of lower leg/foot cycling injuries collated from a meta-review of non-traumatic bicycling injuries(1). The data were presented as self-reported levels of pain collected from cyclists participating in tour rides, ranging from the shortest distance of 545km over a six day event to the longest distance of 7242km over an 80-day event. The prevalence of lower leg/foot injuries was reported to be 7%, 13% and 22% respectively, with an incidence rate of 24%.
However this data considered both the lower leg and the foot to be one homogeneous unit rather than separate anatomical regions, which therefore negates its usefulness as meaningful foot pain data for the wider cycling population. In short, there is very little research regarding the frequency, etiology and/or management of foot pain in cycling available to guide the clinician. The small amount of available literature that is available tends to be descriptive non-systematic literature reviews or opinion. Moreover, where data is reported, participants have not been sampled robustly and studies have tended to focus only on elite cyclists rather than those engaged in club, recreational and fitness cycling (the vast majority of cyclists).
Foot pain research: how common?
Given the abundance of anecdotal evidence suggesting that foot injuries in cyclists are more common than is often reported, a 2012 study set out to try and identify the typical kinds of foot injuries suffered by cyclists and just how common these are(6). In the study, Australian researchers set out to answer the following questions about foot injuries in cyclists:
- *What is the distribution of age, gender, foot/pedal interface use and distances cycled amongst cyclists who experience foot pain?
- *What type of pain and in what region of the foot do cyclists typically experience pain?
- *What techniques do cyclists use to try and cope with/overcome foot pain caused by cycling?
- *Are there key groups of cyclists at greater risk of foot pain than others?
To do this, an electronic survey was used to collect information from cyclists within South Australia, during December 2010. Cyclists were invited to take part and complete the survey if they were riding a non-stationary, upright bicycle at least once per week for a minimum of one continuous hour, and were at least 18 years of age. The sample of cyclists studied numbered 397, drawn from Bike SA (the main representative body for South Australian cyclists) as well as Mega Bike (a large bicycle shop in Adelaide) and staff and students of the School of Health Sciences at the University of South Australia. The cyclists were asked to provide information about their level of cycling participation, the pedal interface used (clipless/toe straps etc) and the types of foot pain suffered.
A number of key findings became apparent, the first of which was that over half of the cyclists (53.9%) reported experiencing foot pain whilst cycling. Secondly, it was the forefoot region of the foot that was most likely to be affected by pain (accounting for 61% of foot pain reports), with the participants reporting that the toenails, toes and ball of the foot were particular problem areas. The cyclists typically described the pain as ‘burning’ and/or ‘numbness’.
The most common methods of dealing with this kind of pain included stopping for a period of time during the ride, shoe removal, walking around and massaging/stretching the foot. In terms of risk, cyclists who rode with an attached foot-pedal interface (ie clipless and toe straps/cage) were 2.6 times more likely to suffer foot pain than those who did not and there was also a correlation with age – cyclist under the age of 26 years were less likely to suffer foot pain than those over 26 (the majority).
This study reveals just how common foot pain can be among recreational cyclists. The fact that ‘cleated in’ shoes increased the risk of pain is perhaps no surprise; studies have already shown that these types of shoe tend to localize plantar pressures, which in turn can be detrimental to nerve and blood supply integrity in that region(2,7). Another implication that follows is that cycling shoe design and selection could just as important as saddle and handlebar options for pain-free cycling.
Analysis of the foot-pedal interface
The foot-pedal interface is the only direct site for energy transfer from the cyclist to the bicycle. In a ‘cleated-in’ pedal interface (ie one using clipless pedals and shoes or toe clips and straps), all of the body’s force to make the bicycle move forward is transferred to the pedal through a very small contact area (typically around 60mm2), and there is consistent anecdotal evidence that forefoot pain at this point of energy transfer is common(2,7). The most obvious question this raises therefore is whether the use of certain types of cycling shoe construction, insole materials or contoured surfaces can reduce the occurrence of pressure hot spots by spreading the load more evenly. Indeed, this approach is embodied by certain cycling shoe manufacturers such as Specialised, who use a combination of specific material types and contoured surfaces to try and minimize forefoot discomfort.
Looking at shoe construction first, a recent trend has been the move towards stiffer and stiffer cycling shoe construction and particularly the use of carbon fiber. The reasoning is that greater shoe stiffness results in less shoe flex and therefore less energy wastage during the power transfer. A legitimate question to ask however is whether this trend contributes to an increased risk of pressure, leading to foot pain? To look into this exact question, US scientists conducted a study into cycling shoe stiffness on forefoot pressure(7).
Plantar pressure data were recorded in two different shoe types to determine the effect of cycling shoe stiffness on peak plantar forefoot pressure in cyclists. Two pairs of shoes of the same size and manufacturer, identical except for outsole material and stiffness, were tested. Shoe stiffness measurements were collected under controlled conditions and in two different configurations using a dynamic hydraulic tensile testing machine. Measurements of plantar pressure were gathered using ‘Pedar’ capacitive-based sensor insoles while subjects pedalled in a seated position at a controlled power output of 400 watts (hard!). The pressure distribution in carbon fiber composite shoes during cycling was compared to that of cycling shoes made with (cheaper) plastic soles.
In terms of stiffness, the carbon fiber shoes were certainly stiffer – 42% and 550% stiffer than plastic shoes when subjected to longitudinal bending and three-point bending, respectively. During pedaling, the carbon fiber shoes produced peak plantar pressures 18% higher than those of plastic design, and the authors concluded that competitive cyclists suffering from metatarsalgia or ischemia should be especially careful when using carbon cycling shoes because the shoes increase peak plantar pressure, which may aggravate these foot conditions.
Jumping to conclusions
However, some more recent research suggests that assessing a shoe’s potential to cause foot pain based purely on its construction may lead to erroneous conclusions. This arises from an interesting study by German scientists, who compared the effects of carbon fiber foot orthoses with standard cycling shoe inserts on plantar pressure distribution during cycling(8). In the study, 11 pain-free triathletes were tested on a cycle ergometer at two different cadences (60 and 90rpm) and at two different workloads – 200 and 300 watts. All the subjects performed two trials in a randomized order. These were:
- *A cycling shoe with its standard insole (control condition);
- *The same shoe but with carbon fiber foot orthoses.
The use of carbon fiber for the construction of the orthoses was important; because of its inherent stiffness and ability to be worked into complex shapes, it’s possible to make carbon fiber orthoses that do not yield, even under the highest loads. This in turn means that the foot is held more securely in its desired position/orientation regardless of loading. During the trials, the researchers recorded the mean peak pressure, both for the total foot area and for specific foot regions (rear, mid, fore foot (medial, central, lateral) and toe region).
As figure 2 shows, peak pressures recorded in the total foot area ranged from 70-75kPa at 200 watts power output, and from 85-110kPa at 300 watts. Despite being stiffer, the carbon fiber foot orthoses reduced peak pressures by around 4.1% compared to the standard insole. Within separate foot regions, rearfoot peak pressure was reduced by 16.6 %, midfoot pressure by 20.0% and forefoot pressure by 5.9 %. It was noted however that in the toe region, peak pressure with the carbon orthoses was increased by around 16%. Regardless, of which insert was used, when it came to the forefoot generally, it was the lateral (outer) forefoot that showed the highest peak pressures when compared to medial (inner) and central forefoot (34 % and 59 % higher for the carbon and standard inserts respectively).
Figure 2: Peak plantar pressures at 60/90rpm and 200/300 watts(8)
The authors concluded that carbon fiber can serve as a suitable material for foot orthoses in cycling partly because plantar pressures do not increase as a result of the stiffness of the carbon and also because with individual customization, there’s the potential to further peak pressure in certain foot areas. Certainly, it seems that when it comes to pressure reduction, shape/contouring is more important than orthotic stiffness. This might also explain why the carbon-constructed shoes in the US study produced higher peak pressures; the unyielding flat surface of the carbon sole was in no way contoured to reduce plantar pressure and the standard (non-orthotic) inserts would have offered little help in this respect. By the same token, simply substituting padded insoles in an already uncomfortable cycling shoe is unlikely to result in an automatic remedy for foot pain.
Implications and recommendations for cyclists and coaches
The US researchers mentioned above concluded from their study that the plantar pressures recorded during seated cycling were within the range recorded for normal walking, despite cycling being considered a very low impact sport(7). If a cyclist presents with foot pain therefore, the issue of plantar pressures in cycling should not be dismissed as the possible cause. This is especially true for competitive cyclists who will potentially ‘push’ up to 40 million pedal cycles during their career!
Of course, excessive plantar pressure is not something that occurs in isolation and more research is needed about the role of other factors in precipitating foot pain during cycling. These include bicycle set-up (saddle height, saddle distance, cleat position), type of cycling shoe (type of cleat, material of sole), shoe fit (too tight, too narrow, attached too tightly), foot type of the cyclist (flat foot or high arched), presence of any lower limb biomechanical or structural deformities, differences in body mass, the typical cycling terrain (hills or flat terrain) the preferred pedaling cadence and use of low/high gearing, as well as the experience and fitness of the cyclist. For now however, coaches, physiotherapists and trainers should be aware that foot pain during cycling is surprisingly common and most likely to be related to plantar pressure issues.
Below is a 7-point plan of practical recommendations for cyclists presenting with foot pain and for coaches and trainers with cyclists in their care :
- Cycling shoes should be selected to afford adequate width and support, especially in the forefoot area (bear in mind that the foot will swell significantly when cycling in warm/hot conditions).
- A stiffer soled shoe is not necessarily harmful provided that the footbed/insert is properly structured and contoured to help relieve plantar pressure.
- Simply using a flat padded insole in a shoe with a very stiff sole is unlikely to reduce foot pain – indeed, if the shoe is already a snug fit, it could increase it.
- When adjusting the upper straps of a cycling shoe, care should be taken to ensure that over-tightening doesn’t occur.
- Cyclists who use clipless pedals should experiment with cleat positioning and also ensure that a fair degree of rotational freedom at the heel is allowed during the pedal motion.
- Cyclists who suffer with foot pain should be encouraged to take frequent breaks, choose flatter routes and use slightly higher gears (ie ‘spin’ more).
- In chronic and persistent cases of cycling-induced foot pain, a biomechanical assessment of the foot is recommended. Depending on the results, this might point to a number of further options including stretching/strengthening routines, cycling shoe orthoses and well as treatments to reduce chronic pain and inflammation.
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