Once you start to trigger the intermediate and fast twitch fibers, your muscles tire more quickly and they run out of glycogen more quickly, but they can contract with about five times the force of the slow twitch. Training the intermediate/fast muscle fibers helps triathletes at every distance and it's important that all triathletes optimize this type of training.
In doing so, you lay the foundation for the rest of the year and your body becomes more economical: Your aerobic enzymes go up; your capillary density (which enhances circulation) goes up; your mitochondria (around the muscle) increases in density and function; and you ultimately end up conserving muscle glycogen.
Come race day, your body will be more equipped to handle and respond to variations in pace and effort.
The best way to work the faster twitch muscles during the base period is to integrate sets of increased pace on slow, longer-distance, aerobic workout days (i.e. Thursdays and Sundays) or do a specific "speed endurance" set once per week.
Use these sets to get started.
Sample Sets for Run, Bike and SwimRun Strides
Develop your "speed endurance" by incorporating strides into your longer training days.
Do a short segment of six to 10 pick-ups or strides that are between 25 and 90 seconds (example: 6x75 seconds). The more seasoned athletes can go up to three minutes. Throughout the segment you should pick up your tempo so that at the end you are running at about your 5K pace.
Allow recovery between each segment so that you can maintain the same pace in the next segment. The ideal rest would be 75 seconds (75 sec. on, 75 sec. rest) however if you are getting fatigued faster, you can increase that recovery time.
Another way to do it is to split the set into two:
- Warm up 15-20 minutes
- Do first set of 6x60 seconds
- Cruise aerobically for 15-20 minutes
- Do second set of 8x45 seconds
As you get more fit, you can increase the number of sets and the length of each segment.