Knowledge Base

Problem

Users are unsure of the conceptual difference between Phase Dispersion and Couplings parameters in CatWalk XT, and how to interpret or report them correctly — including why coupling trial averages do not match the simple mean of run values.

Details

Both parameters describe inter-limb timing coordination but use different statistical frameworks. Confusion commonly arises when users notice that the trial-level coupling value does not equal the arithmetic average of individual run coupling values, leading them to suspect a software error. Affects CatWalk XT 10.6 and 10.7.

Solution

**Phase Dispersion:**
- Measures the timing offset between placements of two specific paws (e.g., LF–RH) expressed as a percentage of the step cycle.
- Values range from −50% to +50% on a circular scale (0% and ±50% are the extremes).
- Reflects coordination consistency: a small spread indicates regular, consistent inter-limb timing.

**Couplings:**
- Describe the timing relationship between two paws within a step cycle, expressed on a 0–100% scale.
- Because gait is **cyclical**, CatWalk treats coupling values as **circular statistics**, not linear statistics.
- On a circular scale, values near 0% and values near 100% are mathematically close to each other (they wrap around). A simple arithmetic mean would therefore be incorrect and misleading.
- This is why the trial-level coupling value displayed by CatWalk XT will **not** equal the arithmetic average of individual run coupling means — CatWalk uses circular mean calculation, which is the statistically correct approach for angular/cyclical data.

**Practical guidance:**
1. Use **Phase Dispersion** when you want to quantify the variability/consistency of inter-limb timing.
2. Use **Couplings** when you want to characterize the typical phase relationship between two paws.
3. When reporting couplings, use circular statistics (circular mean, circular SD) rather than linear equivalents.
4. For detailed formulas and examples, consult the **CatWalk XT Reference Manual (version 10.7)**, Chapter on Gait Parameters.