If you're just coming off an injury then you shouldn't do any max or LT HR tests in order to find your current ~70% value. Use old data and perceived exertion along with HR feedback to keep your workouts easy/aerobic for the time being. For example, if you know your max bike HR was 185 last season, keep training below 130 or so for a few weeks.

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Marty Gaal, CSCS
One Step Beyond Coaching
Triangle Open Water Swim Series | Old School Aquathon Series
Powerstroke® Freestyle Technique DVD