Start with the activity:

• Strength work needs proper compression, so choose Pro or Universa
• Yoga or pilates? Our favourite is Zenvy, every time. 
• Mixed/hybrid sessions where you might do anything? The One.

From there it's mostly down to length (7/8 vs full) and whether you need pockets for your phone or can manage with a smaller back pocket for your essentials only.

They do three different jobs:

• The Pro is the squat-proof training legging, properly compressive, made for heavy gym work
• The Zenvy is the soft, low-compression yoga, pilates or everyday pick.
• The One sits in the middle: mid-compression, opaque, the most versatile choice of the three. 

If you're only buying one pair, the One makes a great all-rounder, suitable for all workouts and comfortable enough to wear all day. It also comes in a variety of waist heights and leg-lengths so you can find your perfect fit.

Yes, the Pro is the dedicated squat-proof line. Universa also works for heavier training, both are high-compression and hold up well. We wouldn’t recommend lower-compression styles like Zenvy for this kind of workout.

Depends on the kind of running:

• Swift is the everyday miles pair, with cooling fabric, and a reliable choice.
• Epic Fast is lighter and more pared back, built for race day and faster sessions. 
• Tempo Flash brings in weather resistance for colder, wetter runs. Skip the Pro for runs, it's a gym legging.

Mostly yes, but each line fits differently. Nike Pro leggings have a compression-fit, so size up if you don't love that locked-in feeling. One and Universa run true to standard UK sizing. Zenvy has more stretch thanks to the softer fabric, so half-sizes either way usually work.