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Tuesday, 01 April 2014 00:00

Endurance

Any sport with events lasting more than a minute or two can be defined as an endurance sport but today I'm going to write about those sports at the more extreme end of the spectrum. Milan Sanremo has been and gone for another year and once again we were treated to that wonderful spectacle of the top pros trying to sprint at the end of 300km in the saddle. MSR is the longest one day race that the professional peloton now races but it hints at a theme that is pervasive throughout both the professional and amateur cycling world - the subtle difference between fitness and endurance. Alexander Kristoff summed it up nicely with his post-race press conference when he said (and I paraphrase), "I would not normally expect to beat Cavendish and Greipel in a sprint but it's different after 300km and I tend not to lose too much power".

So if we think if fitness as the ability to produce a certain amount of power, ie your power at lactate threshold or your power at VO2max for example, then endurance is the ability to maintain that power over a long period of time, or to produce that power at the end of a long ride. It's one of the key factors in deciding professional road races where the result is often decided by a short maximal effort late in a race, but it's also important in amateur races, in sportives (for example the Etape this year finishes with the climb of Hautacam after 150km in the saddle) and also in multistage events where fatigue accumulates through the week. The concept of endurance has been vital in the last few years for some of the PBscience athletes: it's been a key area of focus for Nic who races professionally for Team Vorarlberg, also for the guys and gals we've worked with in preparation for the Haute Route sportive and it goes without saying that endurance played a key role in Pete winning the National vets 24 hour title. Here are some of the strategies that we've used to enable our athletes to produce their best efforts when they count, be that at the end of a tough road race, or several days into a mountainous endurance event.

Consistent mileage - it's hard to look past an accumulation of time in the saddle when it comes to building endurance. An increase in muscle glycogen stores is one of the first adaptations that takes place when you begin a program of endurance training but that tends to max out after 10 weeks or so (Griewe, 1999). We're also looking for a homogeneity of muscle fibres - as you fatigue a set of motor units, you're forced to recruit new fibres that initially may not be as well conditioned as the slow twitch fibres that are the first port of call. It takes a little while to develop the condition in the whole muscle bed so that you don't get wild ups and downs during a long ride. And having spoken about maximising carbohydrate stores, another fuelling issue is the need to become fat adapted, that is to develop the ability to efficiently use fat as a fuel in order to preserve your precious carbohydrate stores until later in the race.

Nutrition - beyond simply riding your bike, a little care can be taken with fuelling to improve or speed the adaptations we're looking for. For example fasted riding or train low race high is a strategy that can be used to aid process of fat adaptation. Of course beyond that, 60-80g of CHO per hour during the race itself is vital to keep stores topped up.

Specific training - I guess the most specific ride you can do in this situation is the long base ride. By long I mean efforts in excess of 3 hours as going beyond this duration places a demand on fuelling beyond the stored glycogen in your muscles/liver/etc. There's something about a 5 or 6 hour training ride that seems to cause an adaptation that is hard to replicate! In terms of a progression, or if you're short on time, you could try a 'tempo finale' or a 'tempo sandwich' (makes me hungry just thinking about it), or a very specific workout for road racers is to complete high intensity intervals at the end of a long ride. Alternatively, if you're fitting training around work, a steady endurance ride in the morning followed by intervals in the evening is a popular way of learning to make a hard effort when fatigued (or atleast not fully recovered).

Weight training - the jury remains out on whether strength training improves cycling performance and the sports science research isn't too enlightening in that respect in my opinion. There are an abundance of studies that are open to criticism; for example studies involving:

  • strength programs involving nothing but leg extensions (who would do that in reality?!)
  • untrained participants
  • unmatched total workloads between strength and non strength training groups
  • etc

That said, there have been a few good studies such as those by Bent Ronnestad that I think do carry weight in this area. One such study for example tested the effect of adding strength training to endurance training on a five minute all out effort. Interestingly for the purpose of this blog, the five minute effort was completed after 185 minutes of endurance riding to simulate the latter stages of a road race. Sure enough strength training improved average power in the strength plus endurance training group by almost 30W. Performance improvements aside, anecdotedly I've found that some element of strength training does allow for improved comfort on the bike on very long rides, whether that's improved core strength, addressing muscle balances or just an increased level of general conditioning I'm not sure but it's a mental boost to know you've ticked another box. Wishing you'd devoted some time to a core program is quite distracting in the 9th hour of a 12 hour ride for example!

Pacing - the above strategies are all ways to improve your endurance, but there is also much you can do in practise to make best use of that endurance. Avoiding hard efforts early in a ride will protect muscle glycogen stores that can be called on later in the ride. The perfect example for this is in road racing where top riders will give themselves sliding room on climbs. If you start the climb at the front of the bunch, you may be able to ride a hill 30s slower than the rest of the riders and still retain contact with the back of the bunch as you summit. It's risky if you think the race might split but otherwise you can move back up when the race eases off having saved yourself a few watts on the climb. Alternatively if you can't climb for toffee then this might help you stay in contact on a hilly course but that's a topic for another day.

Tracking improvements - the scientific method requires that we quantify endurance in some way to ensure that we're actually making improvements. Aerobic decoupling or cardiac drift is one way to assess your endurance (see Helen's presentation below or our factsheet on decoupling). Alternatively, something I've used a little more is a specific performance test where the athlete is required to produce a max effort over a set duration at the end of a long ride. For example, try completing your 20 minute FTP test after a three hour zone 2 ride, or ride a 5 minute VO2max type effort at the end of your long ride. You can come up with something that matches the demands of your event, but make it repeatable.

 

Published in Blog
Saturday, 13 September 2014 20:15

Aerobic decoupling or cardiac drift

PBscience_factsheet_logo

'Aerobic decoupling' or 'Cardiac drift'

The relationship between heart rate and power

With the increasing use of power meters by cyclists looking to monitor their training and racing, the heart rate monitor has taken a back seat in the minds of many athletes (and coaches). By choosing not to monitor heart rate alongside power, a large amount of information is being neglected that can have great value in guiding an athlete’s training. Some of the important concepts are outlined below.

One way to consider these ideas is to think of power being the input to the system and of heart rate being the output. In other words heart rate can be viewed as a measure of how much stress the body is under in order to meet the required work rate. Your heart rate response is very individual so the relationship between heart rate and power (power:HR)  is of little value on its own. However, when we begin to look at how this relationship changes over time it offers a fantastic insight to how you are responding to the training plan.

For more information check out the following links

Fitness versus endurance part 1

The importance of base training

Training in zone 2

Training in zone 3

Riding at a given intensity requires the body to use energy. For durations longer than around 60 seconds, the energy requirements are largely derived from aerobic sources, i.e. using oxygen. This oxygen is transported from the lungs to the working muscles in the blood, so the rate of oxygen supply is directly dependent on the rate of blood supply. This in turn is determined by the product of stroke volume and heart rate (stroke volume is the volume of blood pumped by each contraction of the heart). What we are doing by looking at heart rate at sub-maximal intensities is using it at as a ‘proxy’ for oxygen supply. This enables us to take some of what we learn in the laboratory and apply it to training in the ‘real world’. Of course this is subject to some significant assumptions (i.e. stroke volume being constant) – how these affect the power:HR relationship are examined below.much stress the body is under in order to meet the required work rate. Your heart rate response is very individual so the relationship between heart rate and power (power:HR)  is of little value on its own. However, when we begin to look at how this relationship changes over time it offers a fantastic insight to how you are responding to the training plan.

Changes during a single session

bike-with-srm-power-meter-by-KevinSaundersA common phenomenon for athletes training with a heart rate monitor is the gradual increase in heart rate as a session progresses, despite no increase in intensity. This is sometimes termed ‘Cardiovascular drift’. An often quoted mechanism for cardiovascular drift is the issue of heat and/or dehydration. In hot conditions (for instance training indoors) blood flow is increased to the skin to aid cooling. This extra blood flow is delivered by an increase in HR. Dehydration, which so often goes hand in hand with exercise in the heat, can add to this effect. Dehydration leads to drop in blood volume which ultimately leads to a reduction in stroke volume. As the heart is pumping less blood per contraction it must pump faster to deliver the same amount of blood, and therefore oxygen, to the muscles.

More useful, in terms of an athlete’s fitness, is the observation that changes in the power:HR relationship occur even without an increase in body temperature and in the absence of dehydration. This change in the power:HR relationship over a single session has been termed ‘decoupling’ and has been popularised by coaches such as Joe Friel as a measure of aerobic fitness. Your fitness does not change during an individual workout so it is perhaps not immediately obvious why the relationship should change over a relatively short space of time. For ease of explanation we will assume that you are producing a constant power and examine why the heart rate at that power might change. There is surprisingly little on this phenomenon in the sports science literature so there is ultimately a degree of speculation!

The mechanism behind this is related to oxygen demand. At the start of an endurance training session, the body selectively recruits the (predominantly slow twitch) muscle fibres that are most adapted to aerobic exercise. As these fibres fatigue, you are forced to recruit other fibres, including fast twitch fibres which are not as well adapted to aerobic exercise. In fact studies have shown that fast twitch fibres require approximately twice as much oxygen as slow twitch fibres for the same power output (Coyle, 1992). This extra oxygen requirement is delivered by an increase in heart rate.

Another cause of decoupling may be changes in the blood chemistry itself affecting oxygen delivery. Oxygen is carried in the blood by joining with a protein in red blood cells called haemoglobin. Under ‘normal’ conditions, haemoglobin is very efficient but during exercise, increased temperature, CO2 production, and acidity all have a negative effect on its efficiency. If the blood is less effective at carrying oxygen then more of it must be pumped to the muscles to meet the oxygen demand. Again this leads to an increase in heart rate.

Armed with this information it becomes clear that causing decoupling is an important training goal when looking to build endurance at certain parts of the season. The guiding principle of training is that we apply a stress to the body and then allow it to recover, grow stronger, and be better able to deal with the stress in future. Moderate decoupling is a sign that your slow twitch fibres have been sufficiently stressed and that the larger, less aerobically fit fibres are receiving some training stimulus. At the other end of the scale, excessive decoupling may point in the direction of too high of a training load. Analysing the amount of decoupling in your endurance training sessions can be an effective means of judging the training load and provide valuable insight into how well your endurance ‘base’ is progressing. Once you can complete a given workout with minimal signs of decoupling, it is time to increase the training load: either by adding to the length of the session or increasing the intensity.

Well trained endurance athletes will have minimal decoupling even in long duration workouts. In other words their heart rate for a given power will be the same (or very similar) at the end of a ride as it was at the beginning. This would be shown by a decoupling of less than around 5% - or in other words power and heart rate remain coupled. In fact when in top condition, it is possible to complete back to back rides of several hours in zone 3 with minimal decoupling!

How do we measure Decoupling?

Decoupling is normally measured as the percentage change between power:HR in the first half of the workout and power:HR in the second half of the workout. For example if you cycle for one hour at 200W with a HR of 150bpm and then cycle back for one hour at 200W with a HR of 160bpm then you would calculate your decoupling as:

Decoupling_example_calculation

Thankfully, this calculation is performed automatically in the Training Peaks WKO+ software! (see figure below)

WKO_decoupling

 

Changes over a number of sessions

The power:HR relationship can also be useful in tracking changes in your condition over an extended period of time. Often changes in this relationship can be the first sign that it is time to update training zones. If over a number of sessions your power is higher than normal for a given heart rate, or your heart rate is lower for a given power, then it may be a sign that your fitness has improved. A difference in one session may not mean very much but if you see a consistent alteration in power:HR over a sequence of training rides, possibly accompanied by a decrease in effort levels at a given intensity, it might be time to ‘tweak’ your training zones. Alternatively for greater precision, this might be the perfect time to schedule a week of power profiling or a lab test.

Monitoring fatigue

PowerTap_SLC_sized

In a similar way to showing changes in fitness, power:HR can also be used to monitor how tired you are. The use of HR monitors and power meters has enabled accurate tracking of training load through metrics such as Training Stress Score and TRIMPS but equally important for the working athlete is the effect of the ‘non-cycling’ stressors in your life. The quality and duration of sleep, stress at work and in relationships, and nutritional choices amongst others all have an effect on how well you adapt to training and how fatigued you are at any point in time. Analysing your training load alone may suggest that you should be fine to complete the days prescribed training session. However other factors in your life may mean otherwise. An athlete needs to consider ‘total stress’.

Changes in your power:HR relationship can help to measure this fatigue. To confuse issues, for a given power, heart rate may be higher, or vice versa, lower than normal when tired. When the body is under stress, a number of hormones (such as cortisol) are elevated and can affect your heart rate response. Take a look at your power:HR relationship in the warm up to your training session. Is it within a normal range for what you would expect? If not consider other factors that might influence your heart rate – is the temperature particularly hot/cold? How do you feel? If you are sore, lethargic or the power target requires a greater effort than expected AND your power:HR is different then take it as a sign that you are inadequately recovered and consider whether it might be wise to abandon the training session or just have an easy recovery spin. Ultimately the aim of training is to stress your body to adapt and become stronger but there comes a point where extra training will just make your more tired rather than more fit. Use power:HR as a tool to help identify this point and prevent the need for an extended period of recovery.

Summary

For more information check out the following links

Fitness versus endurance

The importance of base training

Training in zone 2

Training in zone 3

While it is true that heart rate is affected by many external factors that limit its effectiveness in monitoring training intensity, when combined with the use of a power meter it becomes a very powerful tool. Used with other information such as your rating of perceived exertion (RPE) for each session it can provide valuable information on how well your endurance is developed, the first signs of improvement in your training zones and an indicator of your fatigue levels on a day to day basis.

None of this is possible unless you remember to wear your heart rate strap for EVERY session and make detailed comments in your training diary on how the session felt!

 

Published in Free Factsheets