You ask very good questions. Is your mentor TI Coach Stuart McDougal in LA?
I'll address the question of "Never practice struggle' first. While it's true that it's not a good idea to do lap after lap imprinting an inefficiency in muscle memory, at the same time, some of your practice should be designed to expose weak points in your technique or body control.
For example, let's say you're doing a Tempo Trainer set where you gradually increase Tempo, while counting strokes. You start at (to pick a number) 1.30 and increase at .05 second increments and suppose this happens
1.30 15 SPL
1.25 16 SPL
1.20 16 SPL
1.15 17 SPL
1.10 19 SPL
And suppose your Green Zone range of efficient stroke counts is from 15 to 18.
That set provided valuable information: Somewhere between 1.15 and 1.10 Tempo, you abruptly add two strokes (where prior changes were 0 to 1 stroke) AND SPL rose above your Green Zone.
That tells you, for the moment, you should practice at tempos a bit slower than 1.10. That's not practice struggle. It's discovering what conditions CAUSE struggle. After a week or two at Tempos around, say, 1.12 you'll probably find that 1.10 is no longer such a struggle.
The second question to address is what happened during your triathlon swim. The difficult condition you encounter here isn't necessarily the distance--i.e. swimming 1500m continuously, where you normally swim 100m repeats in practice.
It's probably more likely that the combination of being in open water AND feeling race pressure caused you to experience struggle. And there are other ways to address that weakness. For instance practicing in open water with a friend or two and intentionally swimming quite close while intent on maintaining a calm, observant INNER focus (on a familiar focal point). At first swim with small space separating you. Then swim so closely that you have occasional light contact--leg to leg, hip to hip, or arm to arm during recovery.
It's a separate skill to maintain form--and relaxation--in those conditions, after already mastering the ability to maintain it in the pool.
And finally there's the question whether it's necessary to swim, say, 1500m continuous in the pool in order to maintain form for 1500 continuous during an event -- whether that event is 1500m race in the pool, or tri swim leg in open water.
I don't believe so. I strongly recommend that 90% or more of pool practice repeats be 200m or shorter. You simply do more of those repeats to prepare for longer events. I've swum marathons ranging from 10km to 40km or more quite successfully on pool training that rarely exceeded 200m continuously.
I believe shorter repeats are best because you
1) Maintain better form and focus; and
2) Swim faster paces (which means faster tempos with same stroke count) with less fatigue.
than is possible on longer repeats.
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