Sperm motility parameters for ICSI, explained: VCL, VSL, LIN
What computer-assisted motility measures mean, how they relate to what you see at the microscope, and how to use them in single-sperm selection.
Selecting a single sperm for ICSI is one of the most visual decisions in the IVF lab. Embryologists judge movement in seconds, at the microscope, often under time pressure. Computer-assisted analysis does not replace that judgement, but it can describe the same movement in numbers that mean the same thing from one operator to the next.
From "it swims well" to measurable movement
When software tracks a sperm head frame by frame, it reconstructs its path and derives a few standard measures:
- VCL (curvilinear velocity): speed along the actual, wiggly path the head travelled.
- VSL (straight-line velocity): speed along a straight line from the first to the last tracked point. It reflects how much real progress the cell made.
- VAP (average path velocity): speed along a smoothed version of the path.
- LIN (linearity): VSL divided by VCL. Close to 1 means a straight trajectory; low values mean a lot of movement without much progress.
- STR (straightness): VSL divided by VAP.
- ALH and BCF: the amplitude of the head's side-to-side movement and how often it crosses the average path.
These are the same families of parameters used in computer-assisted semen analysis (CASA) and described in the andrology literature.
How this relates to the WHO categories
The WHO laboratory manual for the examination of human semen (6th edition, 2021) classifies motility as progressive (rapid or slow), non-progressive and immotile. Those categories are an overall description of a sample. In ICSI the question is different: among the cells in this field, right now, which ones move in the most consistent, progressive way? Per-cell measures such as VSL and LIN answer that question directly.
Why consistency matters at the microscope
Visual assessment depends on magnification, the optics of each workstation, the depth of the medium and experience. Two embryologists can describe the same cell differently, and the same person can see it differently at the end of a long day. Numbers do not remove the need for an expert eye, but they give the team a shared language and a record of what was seen.
How SiD uses these signals
SiD follows the sperm in the field in real time during ICSI. After about four seconds of tracking it gives each cell a score on a single 0–100 scale based on objective motility signals, and marks the three highest in the field as A, B and C. The embryologist chooses which sperm to immobilise and inject; SiD records the video, the tracks and the data of each procedure, and asks the embryologist to confirm each pairing before the case is uploaded.
The motility signals SiD uses are described in peer-reviewed work published in RBMO — see our research page.
Practical tips
- Use measures to compare cells within the same field and preparation, not across different setups.
- Give tracking a few seconds: very short tracks give unstable values.
- Keep the video. Being able to review what was selected, and why, is valuable for training, audits and quality programs.
SiD supports, and does not replace, the embryologist's selection. It does not predict fertilisation or pregnancy.