Bicycling Australia
Intervals – ah, intervals. So often spoken about, so little understood (as a cycling coach I speak to many riders who know they should be doing intervals, but do not quite know which ones or why). The interval is really the centrepiece of a modern training program, but what is an interval exactly? There are three definitions for the word used as a noun: 1 – an intervening time; 2 – a pause in activity, and; 3 – a space between two things. All three are relevant to cycling.
Some books stress the duration of the active (or ‘on’ phase) portion of the interval (definition 1). Many coaches stress the importance of the recovery (or ‘off’ phase) portion of the interval (definition 2). Programmable bike computers (most companies’ top models) and velodromes permit easy distance-based intervals (definition 3).
Strictly speaking, any drill you undertake that involves alternating between going faster and slower repeatedly is an interval. I would narrow it down slightly by excluding exercises that do not have a set duration, such as the Fartlek drill. Yet this still-broad definition misses all the subtleties that make intervals effective for so many purposes.
Before I get into the details of intervals I need to cover the terminology. The on portion is the hard part of the session. During the on, the rider can either ride to a prescribed effort or as hard as possible depending on the specific exercise. The off is not usually completely off the bike, but is the easy part of the session. Some exercises prescribe the off-effort as well, but most just leave the cyclist to pedal easily while trying to recover from the on-portion. Aside from stepped intervals, where the on and/or the off vary in length as you go, most intervals can be written as X:Y, where X is the duration of the on and Y is the duration of the off. Examples include 1:3M (one minute on, three minutes off), 20:10S (20 seconds on and 10 seconds off) and 5:15M (five minutes on, 15 minutes off). On top of the terms I will also introduce you to two riders, Al and Bart. They will assist me by providing examples of interval sets (both good and bad) throughout the article.
Humans have three distinct metabolic systems that power our muscular motions (see Metabolic Primer box). There is a short anaerobic process that can drive big efforts for up to 10 seconds. Then there is a longer anaerobic process that sustains efforts for less than 45 seconds. Anything over 45 seconds is aerobically powered. While the chemistries behind these three systems are distinct, they are not mutually exclusive. When Mark Cavendish sprints to a win, he is employing both anaerobic systems on top of a very sustained aerobic effort.
The physiological distinction between the metabolic systems is the basis for how different intervals can affect each system. It also provides an explanation as to why, as a coach, I see so many people make errors in their intervals. If, for example, you are doing one minute on this should be a very aerobic drill. Yes it is brief and yes, there will be an anaerobic component, but most of the people I have watched do one minute intervals start out completely in the anaerobic realm and are forced to back off in order to complete the minute.
With a power meter the difference is easy to spot. Look at figure 1 below. Al completes his minute at an average of 400 Watts with very little variation – just what I want as his coach. Bart goes out as hard as possible, peaks up about 600 Watts and then hits the wall. Power drops off sharply before recovering again and dropping yet again. Without a power meter it feels something like this for Al:
Start off pedalling at a solid pace,
by 15 seconds he thinks he’s gone out too easy,
at 30 seconds he knows he’s gone out too easy,
at 40 seconds it seems much harder than it did just 10 seconds ago,
by 50 seconds he is convinced he can’t make it,
and by gritting his teeth he pulls through the final few seconds to see the clock tick over one minute – done!
For Bart it feels more like this:
Start off fast and hard,
‘This is only one minute after all,’ Bart says confidently to himself,
perhaps as early as 10 seconds and definitely by 15 seconds the pedals are slowing,
the legs feel heavy,
‘Like riding through glue,’ thinks Bart,
despite fighting like mad the speed continues to slow,
it feels better so he cranks it up once more only to hit the wall earlier and harder than before – thank goodness that’s over!
Figure 1. Power output for Al and Bart, Al doing a one-minute interval correctly and Bart doing it wrongly.
Al is doing his interval exactly right. Bart is really doing a 10-second interval badly and ensuring he cannot do many more by riding as hard as he can for another 45 seconds. Al will progress swiftly, developing more aerobic power for events such as 40km time trials. Bart will get frustrated with sore legs, no progress and wondering why all his effort seems wasted.
Metabolic Primer
Within each muscle cell is a vast array of chemicals, including glycogen (a sugar providing readily available fuel) and adenosine triphosphate (ATP). ATP, as the name suggests, has three phosphate molecules attached to it. The third phosphate is attached via a very high-energy bond. Breaking that bond provides a good burst of chemical energy.
The energy held in ATP directly powers every contraction in the muscles. The amount of ready-to-go ATP in the muscles amounts to around 10 seconds of activity. Once the ATP has released a phosphate it becomes ADP (diphosphate) and is available to have another phosphate attached so that it can provide a burst of energy for yet another contraction.
This process of joining a phosphate onto ADP takes some time, so the rate of muscular contraction is necessarily lower after the initial 10-second burst is over. Glycogen is the fuel driving the conversion of ADP back to ATP. Over short periods up to about 45 seconds, energy derived from the glycogen stored in the cell can drive that conversion to ATP (called anaerobic glycolysis because it is the breakdown of glycogen in the absence of oxygen).
Continuation of muscular effort beyond 45 seconds requires both sugar and oxygen delivery to the cell via the blood (called aerobic glycolysis because this sustainable process uses oxygen to break the glycogen down). The muscle’s activity is then dependent on the flow of fuel and oxygen into the cells from the bloodstream – which is the reason that aerobic actions are lower in intensity than anaerobic actions.
Altogether there is only around five grams of ATP in a human body. Over the course of 24 hours the body will use its own weight in ATP – meaning that each ATP is recycled thousands of times per day. The body holds roughly two hours of glycogen supply. This is why you need to consume some calories during longer rides to keep going – without eating anything your body is forced to rely on fat stores (and converting fat to glycogen is slower again than straight aerobic glycolysis, meaning you will slow down). Oxygen is usually the limiting factor in aerobic performance, which is why so many of the nefarious practices of the pro peloton aim to enhance the oxygen carrying capacity of the blood.
Rule #1: The effort needs to be even (within one interval). It doesn’t matter if the interval on-phase is as short as 30 seconds or as long as 30 minutes – if your effort level isn’t even (which is best measured by power output in Watts) across the whole interval then you aren’t doing it properly.
Always remember that you get out of training what you put in. Poorly performed intervals prepare your legs for poor performance. Completing a dozen one-minute intervals would see the average power output per interval dropping substantially from start to finish. Look at figure 2 where Al completes five intervals properly and quits while Bart carries on to 10 intervals. Unfortunately, the last six of Bart’s intervals are more than 10 per cent down on his first one – meaning he should have quit at four! (But probably he would have done five and noticed that the fifth one was weak.)
Rule #2: The effort needs to be even (across intervals). You cannot complete a set number of intervals. Rather you should repeat them until your power drops by more than 10 per cent. The 10 per cent drop is not a hard rule – it works well for most people and most intervals but somewhere that number might be smaller (eg, if you want really high quality training) or larger (eg, when the blood lactate levels are more important than the power output).
Figure 2. Power outputs for Al and Bart, Al doing one-minute intervals correctly and quitting when power output drops by 10 per cent while Bart does five too many (sometimes you have to do that last bad one to know for certain you are done).
The off-phase has a huge influence on how an interval set trains the body. Short recovery periods cause an accumulation of fatigue, reducing the number of repeats that can be completed, but teaching the body to recover more effectively. In this case the session is much more about the recovery than it is about the effort itself. Criterium racing and the point score on the track are two races that require a lot of ability to recover.
Long recovery periods avoid the build-up of fatigue and allow each repeat to be a maximal effort. Going for a prime in a criterium or a mid-race sprint in the point score put much more emphasis on outright power, which is developed through long recovery sessions (but then you need good recovery to keep going in the race!). Figure 3 presents these two scenarios – Al is doing 30:15S at around 600 W to develop a fast recovery ability, Bart is doing 30:300S at around 1200 W (and in fact gets off his bike for several minutes between efforts). Al quits after 11 because that last one is more than 10 per cent down on power from the first one. Bart only shows two efforts but if he had enough time would complete four or five before his power dropped off.
Figure 3. Power outputs for Al and Bart for their 30-second intervals, Al having 15 seconds recovery and Bart having 10 minutes recovery.
Rule #3: The on and the off are equally important for achieving what you want out of an interval session. Ignore this advice at your own (fitness’) peril.
Rule #4: You get out what you put in. If you want to work on the ATP-fuelled anaerobic mechanism, your on-phase is going to be 10 seconds or less. For the glycogen-fuelled anaerobic mechanism, your on-phase is going to be between 10 and 45 seconds. Aerobic workouts will involve intervals over 45 seconds long. There are very few reasons to have an interval longer than 15 minutes.
Rule #5: There is a critical mental factor in interval training. Learning how hard you can go, for how long; adopting an effective pacing strategy (for the TT, bridging across and breaking away); learning to live with the burn in your legs and the fire in your lungs – there are many positive lessons to be gleaned beyond the physiological effects.
Example intervals
Peak power – 8:900S. Eight seconds flat-out should just about exhaust your ATP reserves and train your legs to really produce some power. Fifteen minutes (900 seconds) of rest should fully replenish those ATP stores for the next go.
VO2 max – 20:10S. Twenty seconds would normally be an anaerobic interval, but because the recovery phase is so short you never actually recover from the prior effort. Over the course of five to 10 of these your average workload is in excess of your VO2 max (even though you could not ride at that average intensity for the same duration), so your max goes up.
Lactate tolerance – 30:30S. Thirty seconds is long enough to hurt your legs a bit in the on-phase, but not quite enough to completely recover in the off-phase. The result is your lactate levels go sky-high and you adapt to working with these high levels.
TT prep – 10:5M. A road time trial is all about the sustained effort across the distance. Even if you are not an experienced TT rider and take a minute or two to settle into a rhythm, a 10 minute interval still gives you plenty of time to sit slightly above race-pace (teaching your body to go faster) while a five minute recovery is almost enough to do it all over again (‘almost enough’ is key here).
Summing Up
It is easy for a self-coaching rider to toss some intervals into their program and be happy. If you are just riding for fun then it probably does not matter much how you go about training – as long as you remain having fun. But if you are serious about getting faster you should either get yourself a coach (who will understand the finer points of intervals) or start reading books on coaching and interval training (so that you understand the finer points). Interval sets are effective at making you ride faster but as I have demonstrated above there is much more to it than just going hard for a period of time repeatedly.
Next issue Michael will have an interval training plan for you to implement in your own regime.
