Threshold endurance training

A few weeks ago I blogged on basic endurance training. Today I'm going to continue that post with guidelines for threshold endurance training. Training in this category should be done at a speed that approximates the swimmer's individual anaerobic threshold. The training effects of this type of training are:

• Increased percentage utilization of VO2 max
• Increased lactate removal from muscles and blood
• Increase in number of capillaries around slow twitch and fast twitch muscle fibers
• Increased myoglobin and mitochondria in slow twitch and fast twitch muscle fibers
• Increased stroke volume and cardiac output
• Increased blood volume
• Increased pulmonary capillaries
• Improved blood shunting
• Increased VO2 max, particularly in fast twitch muscle fibers

  The following are suggested set and repeat distances, rest intervals, and training speeds for threshold endurance training sets:

• Set distance. 500 yd or m or 6 min and longer. Set distances of 2000 to 4000 yd or m or set lengths of 20 to 45 min are recommended.
• Repeat distance. Any distance can be used, but repeats of 200 yd or m and work times of 2 min and longer are recommended.
• Rest interval. 5 to 10 sec for short repeats, 10 to 20 sec for middle distance swims, and 20 to 60 sec for long repeats.
• Training speed. Sufficient to produce blood lactates in the range of 3-5 mmol/L, heart rates between 10 and 20 beats below maximum, or perceived efforts in the range of 15 to 16 on a scale of 1 to 20.

Swimming is the best method for threshold training because it produces training effects primarily in and around the muscle fibers used in that training. Some threshold endurance training should occur during all phases of the swimming season so that the aerobic capacity of fast twitch muscle fibers can improve concurrently with that of the slow twitch fibers. The quantity of threshold endurance training should decrease during the final 3 to 4 weeks before the taper to provide time for the slow twitch and fast twitch a (fast oxidative glycolytic) fibers to regain some of the anaerobic capacity they may have lost through endurance training. 

Keep in mind that to much threshold endurance training greatly depletes the muscle glycogen stores, and may eventually lead to overtraining. Swimmers will know that their muscle glycogen supplies are low when they have difficulty swimming at or near previous threshold speed. When that happens, the amounts of threshold and overload endurance training should be reduced for a day or two to provide time for replacementof muscle glycogen.

The goal for threshold training is to increase the swimming speed gradually so that swimmers can maintain balance between lactic acid production and removal. The usual methods for overloading - increasing volume, increasing speed, and reducing rest - do not work well with threshold swimming. Athletes can swim faster than threshold speed but only with increasingly greater amounts of energy provided from anaerobic metabolism which will swift the training effect from one that emphasizes greater oxygen consumption and lactate removal to one that emphasizes improved buffering capacity. Coaches should wait until their athletes show signs of being able to swim these sets faster before asking them to do so. In the absense of blood testing, the three best methods to evaluate when it is time to increase the training speed, reduce the rest, or increase the set length are to monitor threshold speed with test sets (do this set every 2 to 4 weeks, threshold speed will have improved when the average speed is faster that the previous one for the entire set), heart rates (when swimmers can complete the same set at the same average speed with consistenly lower heart rates), or measures of perceived exertion (when swimmers can complete the same threshold set at the same average speed with a sense of reduced effort).