Don’t be pressured: why hamstrings matter for cycling comfort

One of the great appeals of cycling as an endurance sport is that the risk of injury is relatively low, especially when compared to sports such as running. Research on triathletes (who engage in both disciplines) has found that running injuries are 3-5 times more frequent as a result of running compared to cycling⁽¹⁾. This is perhaps unsurprising because the smooth, impact-free and supported nature of the pedaling action makes cyclist less vulnerable to impact injuries or those cause by faulty biomechanics. Having said that, cyclists can and do suffer injuries from their sport.

Interface and injury

When a significant injury does occur in cyclists, the most common cause is likely the result of a collision or fall from the bike. Research on road cyclists shows that abrasions, lacerations and bruises as a result of a fall/collision accounted for around half of the total injuries recorded. Meanwhile, fractures (6-15%) were the second most frequent type of injury, with knee pain being the number one overuse diagnosis. The riding posture maintained for long periods of time can also result in back pain, especially when the handlebars are set low (see this article).

What’s often overlooked when considering injury however is the bike-rider interface. With the body weight supported on a narrow saddle and (to a lesser extent) the handlebars, plus constant force being applied through the pedals, these three contact points have the potential to produce injuries, especially when training volumes are high. And of those three interfaces, the greatest sustained forces are experienced in the saddle region, between the surface of the saddle and the groin.

Pain in the perineum

Although cyclists tend to talk of a ‘sore bum’ after hours in the saddle, it’s the perineum (the narrow area running forwards from the anus to the scrotum) that usually falls victim to pressure-induce trauma. The perineum lies just below a sheet of muscles called the pelvic floor muscles, which support the bladder and bowel. The perineum region is sensitive and vulnerable to injury because it contains blood vessels and nerves, which supply the urinary tract and genitals with blood and nerve signals.

Compared to sitting in an ordinary seat, bicycle saddles are much narrower and smaller – an inevitable consequence of the need to allow an efficient pedaling action – ie with the feet aligned roughly under the hips. The smaller area means cyclists will experience more force per unit area (pressure), while the narrow design tends to transfer that pressure to the perineum area. To make matters worse, saddles are relatively bereft of cushioning, which means that jolts, impacts and general ‘road buzz’ are easily transferred from the road, through the bike to the rider, increasing the degree of trauma experienced.

All of these factors can result in cumulative injury to the blood vessels, nerves, and muscles in the perineum, leading to various urogenital complications such as bladder control and sexual problems and erectile dysfunction. The evidence suggests that this problem is widespread, and studies indicate that up to 91% of cyclists have experienced nerve entrapment syndromes at some point, resulting in symptoms of genitalia numbness⁽²⁾. In around 20% of male cyclists, this was also followed by erectile dysfunction. Although these symptoms tend to be temporary and reversible in the early stages, they can become chronic, and lead to more serious conditions such as infertility and prostatitis (inflammation of the prostate gland).

Pelvic posture and saddle pressure

Given that the greatest forces at the three contact points are applied through the saddle, it’s not surprising that saddle design has been well researched by sports scientists and bike manufacturers alike. Design features such as saddle width, cushioning and whether or not it has a groove or cut out under the perineal area have a big impact on the forces experienced by cyclists, and can make all the difference between a saddle that is agony to ride after 30 minutes and one that can be ridden for hours on end with no ill effects (see this article). However, the actual loading experienced at the saddle-bum interface also depends on the position of the rider’s pelvis – in particular the degree of forwards-backwards tilt⁽³⁾.

It’s known that the flexibility of the hamstring muscles affects pelvic posture in terms of backwards-forwards tilt. The hamstring muscles attach to the base of the pelvis (via the ischial tuberosity), and looking at figure 1, we can see that shortened hamstring muscles of the rear thigh will act to pull on and rotate the pelvis in a backwards direction (B in the figure). This effect has been confirmed in previous studies, which have shown that reduced hamstring flexibility rotates the pelvis posteriorly^(4,5). This can reduce the pelvis’s ability to move properly during lifting movements, thus hampering the lower spine in its effect to adopt its natural curve, which increases the forces on spinal ligaments and discs, thus increasing the risk of back injury.

If the pelvis tilts in response to hamstring flexibility/tightness and the angle of the pelvis influences how a cyclist sits on the saddle, a good question to ask is whether pelvic tilt can influence the distribution of saddle pressures? In particular, what are the effects of tight hamstrings and/or lower back muscles on the pressure distribution while seated on the saddle, and could hamstring flexibility play a role in determining whether cyclists suffer from saddle soreness?

Figure 1: Anterior and posterior pelvic tilt