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The Fit Clinic app!
From version The Fit Clinic 7.33.0:

We’ve made some exciting upgrades to the Apple Watch App powered by Trainerize that will make it easier for clients to train on the go! Now, clients can leave their phones behind and be able to start and track any workout (regular, circuit, and interval), including three new cardio activities, swimming, dancing, and HIIT, directly from their watch!
As always, we've also made some back-end upgrades and bug fixes to make your experience even better.