These field-based tests are designed to establish a baseline for heart rate based training. You should be able to perfom up to 40 minutes of maximal intensity exercise safely before completing these tests. Perform the field tests as per the plan. The average heart rate from the 40 min test is approximately 97% of your aerobic threshold. The 10 min tests are to determine maximum heart rate. The accuracy to the test is contigent upon your ability to push yourself very hard and you may, in the early stages of training, lack the requesite strength required to reach a maximal effort. If you feel you did not get accurate results after 4-6 weeks, simply redo the tests during a hard workout cycle. We use Joe Friel's heart rate zones for both the bike and run for our clients on Training Peaks. All of our swimming workouts are done of of perceived exertion levels and as such do not require heart rate zones.