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The best one is supposed to be the single leg on a table one but I never had coffee tables in the shower (I know, how did I cope) and I'm a sucker for efficiency so I'd just touch my toes with a slight bend at the knees (dropped the soap, again?). At home I'd do a single leg sitting one because you can work each leg separately so you can adjust as needed. It helped my lower back pain a lot in the early days. I'm sure there's some physios doing examples on youtube.
Every ride!