BRINGING SCIENCE TO TREATMENT

Perineal injury: release the pressure

Andrew Hamilton investigates the causes of perineal injury in cyclists and what the science says about reducing injury risk.

Compared to many sports, cycling carries a relatively low risk of injury, thanks largely to the smooth, impact-free, and supported nature of the pedaling action. However, the interface between the cyclist and the bike (at the handlebars, the pedals, and the saddle) has the potential to produce injuries. The greatest sustained forces occur 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 – see figure 1) that usually falls victim to pressure-induced 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.

Figure 1: Anatomy of the perineum

Graphic credit: D. Factor, Mayo Clinic, 2002


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 compact area means cyclists will experience more force per unit area (pressure), while the narrow design tends to transfer that pressure to the perineal area. Furthermore, 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 contribute to cumulative injury to the blood vessels, nerves, and muscles in the perineum. Damage to this sensitive area leads to various urogenital complications such as bladder control, sexual problems, and erectile dysfunction (see box 1). Studies indicate that up to 91% of cyclists experience nerve entrapment syndromes with accompanying genital numbness(1). Approximately 20% of these cyclists went on to experience 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).

Saddle design

Padding and width – While the anatomy of the groin remains constant, the way it interfaces with the bike can change by choosing an appropriate saddle design. However, the research is far from clear cut as to what design works best – not least because of the large inter-individual variations in anatomy. In one study, researchers investigated the amount of perineal pressure generated in men who cycled while using four different bike saddle designs(2):

  1. A narrow heavily padded seat
  2. A narrow seat with medium padding and a V-shaped groove in the saddle nose (see figure 2)
  3. A wider unpadded leather seat
  4. A women’s special wide seat with medium padding and no saddle nose

Despite having more padding, saddle #1 produced higher perineal pressure than saddle #2. And although it was unpadded, saddle #3 created less perineal pressure than #1 or #2. However, it was saddle #4 (the wide design) that produced the lowest amount of perineal pressure, indicating that adding padding is less effective at reducing perineal stress than increasing saddle width.

Figure 2: Saddle with V-groove


Nose length– Saddle nose length also seems to be a factor. In a 2014 study, researchers found that longer saddle nose lengths produced greater subjective feelings of perineal discomfort and that lengths of 0-3cms were the most comfortable(3). Unfortunately, these very short nose lengths had the downside of reducing the riders’ stability, especially when riding on the drops. With a nose length of 6cms however, this instability was eliminated yet the riders still reported high levels of comfort.

Cut-out design– A wide saddle and short nose reduces perineal pressure, but for serious cyclists, these designs are less appropriate, being both less aero dynamic and less stable when riding in a low position. In recent years, an extension of the V-grooved model has become popular – the cut-out design (see figure 3) – where the center section of the saddle under the perineum) is abolished completely. With no direct pressure on the perineum, this design should (in theory) be effective at reducing perineal pressure.

Figure 3: Cut-out saddle


A study on female cyclists compared seat pressures and genital nerve function in 48 women cyclists who regularly rode either using traditional (non-cut-out) saddles or cut-out saddles(4).

However, contrary to expectation, the results showed that the cut-out design did NOT reduce saddle pressures over a traditional design. In fact, it slightly increased them, although there were no differences in nerve sensation.

These results are similar to an earlier study, which found that cut-out saddles increased seat pressures compared to a traditional gel-cushioned saddle(5). Why is it that the cut-out saddles seemed to confer no benefits? One explanation is that although there’s no pressure applied to the area directly under the perineum, the cut-out design results in increased forces along the edge of the cut-out, which are then transferred to the perineal area. Also, with less surface area than an equivalent-width non-cut-out saddle, the actual seat pressures are higher overall.

Saddle positioning

Regardless of design, correct saddle positioning is vital. A saddle positioned too high increases perineal pressure, particularly at the bottom of the pedal stroke when the foot is near the six o’clock position. The recommended saddle angle is level and parallel to the ground and the nose of the saddle should never be higher than the seat. However, there’s evidence to suggest that a very slight downward tilt (so the nose is slightly lower than the seat) can help reduce perineal pressure(6).

Seat of the pants

Cycling short inserts claiming to reduce pressure and increase comfort have become commonplace in modern cycling clothing. Many of these products are undoubtedly comfortable, but some research suggests that perceived comfort is not always a good guide to the perineal protection offered. In one study, club cyclists rode for 20 minutes while wearing cycling shorts fitted with three cycling short inserts, each of different design and thickness(7). Surprisingly, the cyclists’ ratings of comfort didn’t tally with the actual perineal pressure readings – ie the most comfortable shorts didn’t necessarily result in the most protection from shock/vibration or the lowest perineal pressure.

One possible reason is that a slightly less comfy short insert encourages the cyclist to shift his/her weight around more and take more weight through the pedals rather than ‘sitting back’ on the saddle. So what are the best designs and materials for cycling short inserts to minimize perineal pressure? The truth is that no other studies have been carried out into this topic, so (despite the claims of manufacturers) we don’t know.

Multi-factorial risks

Although a significant factor, the risk of perineal injury is not solely due to saddle design. In a 2019 study on 2,774 male cyclists, researchers looked at all the contributing factors(8). They found that there was a statistically significant increase in the risk of genital numbness with the following:

  • More years of cycling
  • More frequent weekly cycling
  • Longer cycling distance at each ride
  • Only periodic use of padded shorts
  • Lower handlebar settings

The first three factors suggest a cumulative damage effect, which implies cyclists seeking to increase mileage/training volume need to take maximum precautions against perineal injury risk.

Road conditions may also be a factor in determining perineal injury risk. A study in 2018 simulated the effect of riding on a smooth surface with no vertical oscillations, or with moderate and severe oscillations (mimicking bumpier road surfaces)(9). It found that as the number of oscillations increased, the perineal pressure also increased from 10.3% over baseline to 19.4% over baseline. In addition, there was a strong linear relationship between the amount of oscillations and an increase in pressure. Interestingly, when the cyclists rode with a seatpost shock absorber (Suntour SP12-NCX Suspension Travel Seatpost), the impact of the oscillations was decreased by up to 53%. The study suggests that shock-absorbing seatposts could help cyclists reduce the risk of perineal injury, especially if they ride poor road surfaces!

Summary and practical advice

If you’re a physio with cyclists in your care, what are the key points you should educate them on regarding perineal health? Here are some guidelines:

  • There’s no ‘best’ saddle – just the saddle that’s best for an individual’s anatomy. Cyclists should be prepared to try a few saddles until they’re happy they have one that works (most good local bike shops will have loan saddles cyclists can try for a week or so before going ahead with a purchase).
  • Regardless of the amount of cushioning or presence/absence of V-grooves and cut outs, a wider saddle is likely to spread the load more evenly, reducing perineal pressure. This advice is especially relevant for female cyclists, who have wider spacing requirements for the sit bones.
  • Saddle angle/height matters; a slight downwards tilt of just a couple of degrees can help reduce perineal pressure. Meanwhile, saddle height should not be excessive.
  • Time trial and triathlon cyclists who do a lot of ‘aero’ riding should consider a ‘clipped-nose’ saddle (eg Adamo’s Podium saddle or Specialised’s S-works Power saddle), which reduces perineal pressure when the trunk is at a low angle.
  • Cyclists who ride poor/bumpy road surfaces may benefit from a shock-absorbing seatpost (or switching to smoother tarmac). Experimenting with cycling short design may also be worthwhile. Gel inserts are effective at absorbing vibration from the road, so may confer an advantage.
  • Regardless of saddle comfort, cyclists should be encouraged NOT to sit in the same position for long periods. Getting out of the saddle for 20 seconds or so every few minutes helps relieve pressure on the perineal area and makes for more comfortable riding. This is especially relevant when mileage/training times are increased.

References

  1. Eur Urol. 2005 Mar;47(3):277-86
  2. Eur Urol. 2002 Feb;41(2):139-43
  3. Appl Ergon. 2014 Jul;45(4):1181-6
  4. J Sex Med. 2011 November ; 8(11): 3144–3153
  5. J Sex Med. Oct; 2009 6(10):2728–35
  6. Med Sci Sports Exerc. 2003 Sep;35(9):1620-5
  7. Medicine (Baltimore). 2015 Jul;94(29):e1186
  8. BJU Int. 2019 Aug;124(2):336-341
  9. Sex Med. 2018 Sep;6(3):239-247
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