That USTA manual also stressed the importance of anaerobic training:
"Anaerobic Training Guidelines
Anaerobic exercise is characterized by brief, high-intensity periods of activity, where energy
metabolism during the activity periods is predominantly provided by the systems that do not
involve oxygen. As such, anaerobic training should include a series of brief, high-intensity
drills or activities followed by a specified period of rest to allow for replenishment of the
anaerobic energy stores within the working muscles. In general, it is recommended that you
provide a 1:3 work to rest ratio; this means for every second the player works, he or she gets
3 second of rest before the next interval. In other words, the longer the activity, the longer
the recovery interval should be.
With this type of training it is possible to
impact the body’s ability to deal with lactic
acid. With a properly designed anaerobic
training program a player’s body will
adapt to be able to perform at higher
intensities without an appreciable gain in
the lactic acid that is produced. The
intensity level where the body switches
from aerobic to anaerobic energy systems
is commonly called the lactate threshold.
In Figure 2, the lactate threshold is
identified as the intensity where the
lactate levels in the blood start to increase
suddenly. With training, it is possible to
shift the lactate threshold to higher and
higher intensities.
Please note, as important and beneficial
as anaerobic interval training can be for
helping to optimally develop a tennis player, such training is also very demanding. Without
sufficient preparation, progression (within the session and over longer periods of time), warmup,
and recovery, too much of this type of training can readily lead to injury."
http://dps.usta.com/usta_master/usta/doc/content/doc_437_23.pdf
While there is no direct USTA recommendation for using heart rate as a monitor for anaerobic training, there seems a reasonable explanation and estimation of heart rate using the Canconi method on the Sports Fitness website:
http://www.sport-fitness-advisor.com/heart-rate-training.html
"The Conconi Test for Measuring Lactate Threshold
As mentioned earlier, the simplest method for determining the lactate threshold is to assume it occurs at 85-90% of the maximum heart rate. An alternative is to use the Conconi test...
In 1982 Conconi et al, stated that the lactate threshold was linked to a deflection point in heart rate data. Heart rate plateaus briefly before rising sharply again and this is said to correspond with a sudden rise in blood lactate concentrations. There are various protocols used to elicit the plateau Conconi and co-workers refer to. Here is an example:
Equipment
•Treadmill (with metric setting - km/hr and meters)
•Heart rate monitor
•Assistant to take recordings
Procedure
•Begin by warming up at a light pace for 5 to 10 minutes. Set the treadmill to a 1% incline.
•The run should last between 2.5km and 4km to allow sufficient data to be collected.
•Gauge your starting speed. Speed is gradually increased every 200m so start too quickly and you won't last long enough. Start too slowly and you'll be there all day.
•As a guideline 8 - 10 km/hr is a good starting point.
•Increase the speed every 200m by 0.5 km/hr.
•Record the heart rate and speed at each 200m interval.
•Continue until exhaustion and complete a 10 minute cool down.
You can now plot a simple heart rate graph like the one below and read off lactate threshold:
You can see from the graph above the obvious plateau and deflection in heart rate. It seems to correspond with a heart rate of 172bpm. In theory, then an athlete could train at or just above this heart rate training zone and improve their lactate threshold. However, caution is required when using this test as subsequent research has questioned its validity (7,8). It has been argued that the deflection point occurrs only in a certain number of those tested and that it underestimates the lactate threshold exercise intensity.
Heart Rate Training to Increase Lactate Threshold
Here's a simple heart rate training program to increase lactate threshold...
•Assuming your heart rate at lactate threshold is 170bpm
•Start by completing two 6-10 minute runs approximately 5% below the lactate threshold heart rate. In this case it would be 162bpm.
•Rest for 2-3 minutes between runs and complete this twice a week.
•Gradually build up the length of each run or the number of repetitions (up to 6). Also increase your target heart rate up to your threshold (170bpm).
•The target eventually is to reach a sustained 20minute run at or just above your threshold heart rate.
•Complete a thorough cool down at the end of each session. Also re-test your lactate threshold every 6-8 weeks."
Of course, you could be much less scientific, and probably a lot more practical, to run your anaerobic training sessions to get up to a sustained level 4-5 on your heart rate monitor, then rest until you drop to level 3, and then exercise back up to level 5 again.
"Heart Rate Percentage
A very simply method for estimating the anaerobic threshold is to assume anaerobic threshold occurs at 85-90% maximum heart rate (220-age). As mentioned earlier, heart rate varies greatly between individuals and even within the same individual so this is not a reliable test."
http://www.sport-fitness-advisor.com/anaerobicthreshold.html
http://www.howtobefit.com/five-heart-rate-zones.htm