Sperm that don't swim well have stiffer outer membranes than sperm that swim well, and this stiffness doesn't go away even when the sperm are supposed to become ready for fertilization.

We analyzed the available evidence and found that sperm with poor motility tend to have stiffer outer membranes compared to sperm with high motility, both before and after capacitation. This difference in membrane stiffness appears consistent across the studies we reviewed, with no evidence suggesting the opposite [1]. Capacitation is a natural process sperm undergo to become capable of fertilizing an egg, and during this time, healthy sperm typically become more flexible to help them penetrate the egg. But in sperm with low motility, this expected change in membrane fluidity doesn’t seem to happen — the membrane stays stiff even when it should be becoming more pliable. What we’ve found so far suggests a link between membrane stiffness and reduced swimming ability, but we don’t know if the stiffness causes the poor movement, or if both are symptoms of another underlying issue. The evidence doesn’t show whether improving membrane fluidity would help sperm swim better, or if this stiffness is just a side effect of other problems. We only have one assertion to work with, supported by 45.0 studies or observations, and no studies contradict it. While this pattern is consistent, the evidence is limited in scope — we don’t yet know how widespread this is across different populations, or what factors might influence membrane stiffness. In everyday terms: if sperm don’t swim well, their outer layer may be too rigid, even when they’re supposed to be getting ready for fertilization. This could be one reason they struggle to reach the egg, but more research is needed to understand why this happens and what, if anything, can be done about it.