Sprint training is an aspect that can and should be part of your training year-round, and is an aspect that many riders neglect, or often do incorrectly if they do include them. Making a well-designed sprint workout part of your weekly routine is crucial for any cyclist who not only wants to improve not just their final sprint, but also their ability to make speed changes in almost any kind of mass-start bike race.
A sprint, like most efforts, consists of two aspects: cardiovascular and muscular. It's important to consider each aspect separately, and then see how to combine them for maximum effectiveness. From the cardiovascular standpoint, any interval that begins with a maximal effort will require energy quickly. Your body gets that by using adenosine triphosphate (ATP) as a fuel source. To accomplish this, a phosphate bond is broken, releasing immediate energy. This ATP source is replenished by creatine phosphate (CP) stored in the muscle. With this process, no lactic acid is produced, and is said to be anaerobic and alactic. While this system provides energy quickly, the source is limited. Your body can only do this for 8-15 seconds before the creatine phosphate is depleted, and the effort becomes anaerobic and lactic. Once the CP stores are gone for that effort, your body turns to glucose stored in the muscle for energy (a process called glycolysis), and that's when the lactate and other metabolites begin to accumulate. When the effort is over, your body can replenish its CP stores very quickly on its own, within minutes. As a result, you can see how important it is to make sure that a proper sprint effort stays in the 8-15 second range. Any longer than that and you're training a different energy system.
From a purely muscular standpoint, sprint workouts are the place to build speed of movement, strength, and the power that results when you combine these two aspects. Many cyclists have made weight training part of their preparation regimen; sprints are where you can do your weight lifting on the bike, and in a sport specific way. Just as you would follow a lifting program that consisted of adaptive, strength, and power periods, so too can you take that approach with on the bike sprint training. A sprint workout not only helps you with your field sprinting skills, but can also aid your overall race fitness and ability to punch it out of corners, up hills, and when making attacks. Over the course of a 50-lap, 4-corner criterium, you might make 200 small sprints. It's not just about the sprint to the finish. Everyone needs to be a sprinter, just to get to the end of the race.
To begin with, your sprint workout should come as the first day in a string of two or three consecutive training days, and always after a recovery day. Because it's your maximal intensity day, it should happen when you're most rested, and before you attempt any workouts of a lower intensity. You need to be fresh enough to put 100% into each sprint, just as if you were doing a day of squats in the gym. Practically, that means your sprint days will fall on Tuesday and either Friday or Saturday, if you choose to do them twice per week. Tuesday will be your most important day, with the Friday and Saturday sprint day being equally important if there's no race, or secondarily, as a way to open up the day before an event.
There are a number of different ways you can implement a sprint workout. What you do will depend on what phase you're in, what aspect of your form you're trying to focus on, and what discipine you're training for. I'll detail each type of sprint I employ, and when you would take that approach:
Early in the season, when you're in your first 4-12 weeks of training, you want to emphasize technique. You're training for neuromuscular adaptation, strengthening of the tendons, and an increase in your body's ability to store, use and replenish CP. To do this, begin your effort from a walking pace, in the saddle, and in a gear that will take you 8-15 seconds to do 20-30 pedal strokes. Typically this will be the small chain ring, perhaps a 39 x 21-15, depending on your ability level. Burst into the effort (staying seated the entire time), focusing on pulling up and exploding down onto the pedals, and staying very square and rigid in the saddle with no rocking. These should be done on a flat road. It should take you about 5 seconds to get the gear up to speed, and the remainder of the time to spin it out, staying on top of the gear. If you're weight training as part of your program, these sprints should coincide with your adaptive or hypertrophy phase. Your ultimate goal with each of these efforts is to acheive peak cadence, as high as 160 rpms, but not peak power.
Once you've got enough training to move into the more extensive part of your base period, you can begin to emphasize strength in your sprints. The technique is the same as the previous example: in the saddle, starting from a walking pace, for an 8-15 second sprint. The major difference is that now you only want to complete 8-10 pedal strokes over the course of that same time frame. You'll likely find yourself in the big ring; anywhere from the 17-11, again depending on your level. This is very stressful to your knees and back, and should be undertaken with extreme caution. What's crucial here is that your form is impeccably strict to get maximum benefit with minimal injury. The force you sprint with here must generate from your core since that's what will hold you still in the saddle and provide the resistance. If you find that you can't push a big enough gear to get 8 pedal strokes in without failure, the weak link might be your abs and back as much as your legs.
Another difference with these sprints is that you can do them on a slight incline to keep the cadence and resistance consistent throughout the sprint. It's not crucial, and you'll still get plenty of benefit from them if, like me, you can't bear the boredom of doing sprint repeats in the same spot. Again, if you're weight training, these sprints should coincide with your strength phase. Your ultimate goal with these sprints is for peak torque or pedal force. Because cadence is restricted, you will not acheive peak power.
Once you finish your base period and are moving into your real racing season, you want to be able to combine what you've built in speed and strength for a truly powerful sprint. In this period, you want to go back and emphasize the acceleration aspect of the speed sprints, while combining them with the high resistance of the strength sprints. These sprints can be done out of the saddle now as part of the process of putting things together for race day. Again, the time frame is 8-15 seconds, starting from a walking pace, with a goal of 12-15 pedal strokes. Your gearing will be similar to what you used in the strength phase, but now you should accelerate all the way through the effort. These sprints could coincide with the power phase of a lifting program, if that's part of your training, though generally we would expect all weight training to have stopped if racing has begun.
There are many ways to vary these types of sprints. If it's in-season, and you feel like you need to go back and reemphasize a bit of strength in your sprint, then you might find that shifting down as you do an out-of-the-saddle sprint is helpful, and simulates a race situation well. You might want to work on your ability to attack on a climb, so you could introduce some occasional sprints into a longer tempo effort done while climbing. Perhaps it's your acceleration or speed that turns out to be a weakness; in that case you might do a variable power threshold interval, where you sprint and coast for 15 seconds at a time for a period of 8-20 minutes. And if you're doing your sprint workout before a race day, simply to open up, you should almost always emphasize the speed, and peak a gear you can hit peak torque in the first 4-6 seconds, but then requires you to sit down to finish, maxing out cadence as well.
In all cases, the sprint itself lasts 8-15 seconds. Over the 45 seconds that follow the sprint, you'll see your heart rate rise and fall as your body tries to pay back its energy debt and recover. You should consider that whole minute part of the interval, and be sure that there's 1-5 minutes of rest between each interval. With that approach, the tightest your sprint workout would ever be is one sprint every second minute, with one every five minutes my preferred structure. You can also group your sprints into sets of three or five with a longer recovery period between sets.
How many sprints you do in a workout should be dependent on the quality of the sprints. When you sense that you're no longer able to put out the same wattage as the workout goes on, then again, that might signal the end of the workout. I would expect most riders to finish at least 3 sprints at the beginning of their program. Building up to 15 or more in a workout is not as difficult as it sounds, and 60 is possible in a 2 x 20 minute 15 on/off variable power workout. Remember the 200 sprints in a 50-lap race is something you do almost without thinking, so reproducing that in training is also possible. In any case, if you can't change your speed, you can't be competitive in mass-start racing of any kind. It's easy to spend all your energy improving functional threshold power, but being able sprint out of turns repeatedly matters, too. Don't neglect it.