While exercise is always a healthy decision, when women and men do it consistently at an extreme level they chip away at their ability to conceive. The exception here appears to be women with PCOS who do benefit from vigorous exercise. Meantime, increased levels of moderate exercise are of real help, especially in women who are having trouble conceiving.
As for very vigorous exercise, in our minds the best study comes from Denmark and gauged a woman’s weekly level of exercise and the length of time it took her to conceive naturally. As you can see in just about every group (except women with a BMI of 25 or higher), each hour of vigorous weekly exercise correlates with a reduction in ability to conceive compared to women in that same age or weight bracket who do no vigorous exercise.
In a much smaller, but prospectively run study, PCOS patients who could not ovulate and had a BMI of 30 - 35 showed a dramatic improvement under an exercise regime (regular biking) that would have qualified in the above study as “vigorous.” Specifically, after 24 weeks this group showed dramatically higher rates of returning to ovulation and conceiving naturally than they did when they started the study (“baseline”) or a similar group of women that underwent a diet-only weight loss routine.
As far as moderate exercise (like walking) goes, it appears increased amounts coincided with a modest improvement across women of varying ages and BMIs, but proved especially helpful in women who had previously tried to conceive for at least three months.
Unfortunately, men who are sedentary are over ten times more likely to suffer from erectile dysfunction. However, even if that’s not an issue, generally speaking they produce less sperm. In the below study, investigators followed almost 200 18 – 22 year olds and charted their semen parameters against self-reported levels of physical activity per week. After correcting for a few variables, the investigators saw a tight correlation between hours of weekly exercise and sperm count.
At the same time, most reproductive urologists believe there is such a thing as too much exercise, and after 2 -3 hours of intense daily exertion he may be doing more harm than good. The underlying premise is that prolonged, vigorous and repeated exertion amplifies heat and pressure in the testicles, which reduces sperm production.
There is also a fair amount of data that biking in particular, even as little as a few hours per week, can negatively impact sperm concentration. Below is an analysis that shows how an increased level of exertion can correlate with lower sperm concentration and worse morphology (the shape of the sperm). However, the analysis suffers from a small sample size (45 participants) who do differing activities (the triathlete group participates in cycling).