Cycling and marathon running can both affect male fertility, but the changes are usually temporary and tied to a few specific mechanisms. The main one is heat. Sperm production works best when the testicles stay a few degrees below core body temperature. Activities that raise scrotal temperature, press the area, or keep it hot for hours can temporarily lower sperm count and worsen sperm morphology. A 2013 study in Human Reproduction found that sauna-level heat does exactly that, and the effect reverses once the heat exposure stops (Garolla et al., 2013). Sports like cycling and marathon running rarely reach sauna temperatures, but they can hold the testicles at a higher temperature for much longer.
Heat is the shared mechanism
The scrotum is built around temperature control. It hangs outside the body and adjusts through muscle contraction and blood flow. When a man sits for long periods, the area warms. Jung et al. (2005) measured that sitting raises scrotal temperature compared with standing. Mieusset et al. (2007) reported similar rises with tight clothing and prolonged seated positions. The effect compounds when the thighs trap heat against the groin.
Underwear choice matters too. In a fertility-center cohort, men who reported wearing looser, boxer-style underwear had higher sperm concentration and total count compared with men who wore tighter briefs (Minguez-Alarcon et al., Human Reproduction, 2018). That study was observational, so it shows an association, not proof that underwear alone drives the difference. It does line up with the temperature data.
At the cellular level, heat makes sperm production less efficient. The cells that divide to form sperm work best below core temperature. A 2015 review in Reproductive BioMedicine Online described how heat stress disrupts the support cells in the testicle, leading to fewer and more misshapen sperm (Durairajanayagam et al., 2015). The practical point for athletes is that the process usually recovers once the heat source is removed.
Cycling adds pressure to the heat
Cycling adds a second stressor: sustained saddle pressure. The perineum gets compressed for the duration of a ride. Compression can reduce blood flow through the area, and reduced blood flow makes it harder for the scrotum to shed heat. Tight cycling shorts and a forward riding position increase both heat retention and pressure.
Large trials have not isolated cycling saddle pressure from other seated activities and measured sperm output directly. The mechanism is well established though. Sitting raises scrotal temperature (Jung et al., 2005), and long seated sessions do the same (Mieusset et al., 2007). Cycling stacks mechanical compression on top of that. Duration matters. A 20-minute commute is a different stimulus from a four-hour weekend ride repeated every week for years.
Some men who cycle heavily report numbness in the groin. Numbness is a nerve signal, not a fertility measure, but it indicates pressure high enough to impair sensation. The same pressure can also compress the blood vessels that help cool the testicles.
Marathon running adds fuel and stress pathways
Marathon running also raises body temperature and traps heat in tight shorts, so the heat pathway still applies. Endurance running adds two more factors: energy availability and stress hormones.
Long training blocks burn a lot of energy. If a man does not eat enough to match the workload, the body prioritizes immediate energy needs over reproductive function. The brain's reproductive control center reduces its signaling when energy is scarce. Heavy training also raises cortisol, and sustained high cortisol interferes with the hormonal signals that drive sperm production.
The direct evidence on marathon training and sperm quality is limited and mixed. The mechanisms are plausible: heat from running and tight clothing, energy deficit, and elevated cortisol. Large controlled studies have not consistently measured how much training load is needed to change semen parameters. This is a case where the physiology is clearer than the data.
Reversibility of sport-related changes
The strongest reason not to panic is the reversibility data. In the Garolla et al. (2013) study, sperm parameters recovered after heat exposure stopped. Shefi et al. (2007) in Fertility and Sterility followed men who had been using wet heat in baths or hot tubs. After they stopped, semen quality recovered within months. Neither study involved cycling or running directly, but both show that the testicle can recover its sperm production once the heat stress is removed.
That matters because sports-related stress is rarely constant. Training cycles have rest days, off-seasons, and easier weeks. If a man has a normal baseline and removes the heat or pressure source, recovery is the expected outcome. Sperm production takes roughly two to three months from start to finish, so changes in lifestyle often show up in a semen analysis about two to three months later.
Practical adjustments for cyclists and runners
For cyclists, a few mechanical adjustments can reduce heat and pressure:
- Stand up on the pedals periodically to restore blood flow.
- Choose a saddle that reduces perineal pressure if numbness is a problem.
- Wear breathable shorts rather than thick, non-breathable padding.
- Limit the number of consecutive days with long rides if you and a partner are trying to conceive.
For runners, the main levers are clothing, load, and fuel:
- Wear looser shorts that let air move.
- Eat enough to support the training volume instead of running in a large energy deficit.
- Include rest days or lighter weeks so the reproductive system is not under constant stress.
This does not mean you should stop cycling or running. The dose and the setup matter. A man who rides an hour a few times per week with a comfortable saddle and breathable shorts is in a different situation from a man who logs 15 hours a week in tight gear. The same logic applies to marathon training. Moderate volume with proper recovery is a different stimulus from chronic overreaching.
If you and a partner have been trying to conceive for 12 months without success, or sooner if you have known risk factors, a semen analysis is the standard next step. A doctor can look at your training load in context and help you decide what to adjust. Consulting a doctor earlier is reasonable if you are planning to conceive soon and have concerns about your training volume.
This content is for educational purposes only and is not medical advice. Oakman products are designed for physical comfort and cooling. They make no claims about fertility, sperm quality, or hormone levels. Consult a healthcare professional for personalized advice.