When a semen analysis comes back with low sperm motility, most men hear one number and spiral. That number deserves more context than it gets.
Average sperm count has been falling for decades. A 2023 meta-analysis in Human Reproduction Update covering 53 countries found a large and continuing decline since the 1970s (Levine et al., 2023). Motility gets less attention than total count, but it matters for a simple reason. A man can have a normal count and still have very few sperm swimming forward well enough to reach the egg. A standard semen analysis grades motility as the percentage of sperm moving forward, and the World Health Organization publishes reference ranges for that percentage.
When I went looking for the motility-specific story, I found a paper trail rather than a single villain. Three variables show up again and again:
- Heat from saunas, hot tubs, heated car seats, and long sitting
- Clothing fit and fabric, which change local temperature and skin contact
- The chemical environment, including microplastics and plastics chemistry
None of them acts alone.
The 1971 paper that put scrotal skin on the map
Human absorption studies published in 1971 by Maibach and colleagues in Archives of Environmental Health found that scrotal skin sits near the top of the range for regional skin permeability, far above the forearm and higher than the scalp, forehead, and underarm. The paper was not about fertility. It was a dermatology study. But it gave researchers a concrete reason to care about what touches that small area all day. Thin skin, dense blood supply, and high permeability mean the local environment matters more there than almost anywhere else.
The thermal story developed alongside it. Sperm production requires the testicles to run a few degrees cooler than core body temperature. When that gradient shrinks, the cells that become sperm get stressed. The result shows up in a semen sample.
Heat is the most replicated input in the record
Sauna-level heat temporarily lowers sperm count and can worsen morphology (Garolla et al., Human Reproduction, 2013). The effect reverses once the heat exposure stops, because sperm production needs the testicles a few degrees cooler than core body temperature. That wording matters. Heat does not permanently destroy the production line. It interrupts it.
Recovery after stopping wet heat is also on the record. Shefi and colleagues, writing in Fertility and Sterility in 2007, documented that semen parameters recovered after men stopped regular wet heat exposure. The timeline varies from person to person, but the direction is consistent.
Motility often gets measured alongside concentration and morphology in these studies. The strongest human heat findings are on count and morphology, but the underlying thermal stress affects the same system. A low motility number tends to travel with other heat-related changes rather than appearing in isolation.
Sitting, underwear, and the boxer briefs footnote
Two findings sit side by side. Jung et al. 2005, in Human Reproduction, found that sitting and tighter underwear both raised scrotal temperature compared with standing and looser underwear. That was a physical measurement, not a fertility outcome.
A 2018 fertility-center cohort in Human Reproduction went further. Men who reported wearing looser, boxer-style underwear had higher sperm concentration and total count than men who reported tighter underwear (Minguez-Alarcon et al., 2018). This was an observational study, so it cannot prove the underwear caused the difference. Men who choose boxers may also differ in other ways. But the physical temperature data from Jung et al. gives the finding a mechanism.
A 2026 narrative review in Translational Andrology and Urology tied garment fit, scrotal thermoregulation, and fertility outcomes together (Zleczewski et al., 2026). The review does not add a new experiment. It organizes what the older studies already said.
The plastic layer arrived later
Microplastics are now showing up in the tissues that make sperm. Hu et al. 2024, in Toxicological Sciences, detected microplastics in human and dog testes and reported an association with sperm count. Zhao et al. 2023, in Science of the Total Environment, detected microplastics in human testis and semen using complementary methods. Li et al. 2024, also in Science of the Total Environment, found microplastics in every seminal fluid sample they examined. Zhang et al. 2024, in eBioMedicine, detected microplastics in all semen and urine samples in a multi-site Chinese cohort and linked mixed exposure to poorer sperm parameters in regression models.
Those are presence findings and associations, not a proven cause. No study traces the particles to a specific food, water source, or pair of underwear. At the same time, synthetic garments shed plastic microfibers during ordinary wear, not just in the wash (De Falco et al., 2020, Environmental Science and Technology). That does not mean the particles in testes came from clothing. It means the exposure routes are broad.
Plastics chemistry sits in the same record. Phthalates and bisphenols are studied for effects on male reproductive health (Rodprasert et al., 2021, Frontiers in Endocrinology), and a 2024 meta-analysis in Toxics linked BPA exposure to poorer sperm parameters (Lu et al., 2024). Again, these are associations. The endocrine-disruptor field has a lot of signal and not yet a clean causal map.
What the research gives you to act on
Heat exposure is the most actionable variable. If you use hot tubs, saunas, or heated car seats regularly, the human research says the effect on sperm parameters can reverse once the heat exposure stops (Shefi et al., 2007). Some men choose to reduce or pause regular wet heat while trying to conceive, then reassess with a semen analysis. The return timeline is individual.
Clothing and sitting are next. A relaxed fit that lets air move keeps the area closer to the temperature it prefers than tight, heat-trapping underwear does. The 2018 underwear study is observational, but the physical temperature data from Jung et al. 2005 gives a mechanism. Standing and walking after long sitting changes the local heat equation, which is why breaks matter.
Food-level choices show up in the wider fertility literature. Zinc, for example, is a nutrient the body uses in sperm development, and food sources include oysters, pumpkin seeds, beef, and lentils. Keep this at the food level unless a doctor has identified a specific deficiency. Supplement protocols are a different conversation.
Many men find that the most direct next step is a semen analysis. That is the test that measures motility, concentration, and morphology together. It gives you a number rather than a guess. A doctor can order the test and interpret it in the context of your own numbers.
Where the next decade might point
A future fertility workup might include a continuous scrotal temperature log, the way a cardiologist reads a Holter monitor. The pieces are already partially in place. Home semen testing exists, and wearable temperature sensors are common in sports. A version tuned for this question is plausible within a decade. Phone-based motility testing could sit next to that data, giving men a rolling picture instead of one lab snapshot.
None of that is standard in clinics today. It is the direction the evidence points.
The last fifty years did not produce a single villain. They produced a changed daily environment: more sitting, tighter synthetic clothing, more heat exposure, and more plastic in the body. Sperm motility is one of the readouts of that environment. The research is clear about direction. Heat stress is reversible, observational studies point to fit and sitting, and microplastics are present in the tissues that make sperm. What is not proven is that any single change turns a low motility number around. That part remains individual.
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.
Frequently asked questions
does heat permanently lower sperm motility
No. Heat exposure such as sauna use temporarily lowers sperm count and can worsen morphology, and the effect reverses once the heat exposure stops (Garolla et al., 2013). Recovery after stopping regular wet heat has been documented in follow-up semen analyses (Shefi et al., 2007). The return timeline varies from person to person.
can tight underwear reduce sperm motility
Tight underwear raised scrotal temperature in controlled physical measurements (Jung et al., 2005), and a 2018 observational study linked looser boxer-style underwear to higher sperm concentration and total count (Minguez-Alarcon et al., 2018). That study cannot prove causation, but the temperature data gives the finding a plausible mechanism. Motility itself was not the primary endpoint in that cohort.
are microplastics proven to cause low sperm motility
No. Microplastics have been detected in human testis and semen, and some studies report associations with poorer sperm parameters. Those are presence findings and statistical associations, not proof that microplastics cause low motility. No study has traced the particles to a specific source such as clothing or food.
what should I do first if my sperm motility is low
A semen analysis is the most direct first step. It measures motility, concentration, and morphology together. A doctor can order the test and interpret the results in the context of your own numbers, which is more useful than guessing at a single cause.