Shoe weight and running speed - the scientific literature

Shoe weight and running speed - the scientific literature

Whilst I have done a few tests of my own on the effect of different running shoes on physiological effort associated with running, I thought it might be interesting to draw together the literature on the subject.

There have been quite a few studies, but many of them are inaccessible to the general public because they were published in Scientific Journals that are not open access. Where possible I have tried to get hold of these articles and I have explained what the authors did and what they concluded. This is very much my opinion of the research - you may well have a different view point (please feel free to comment). For my starting point I used an article by Greg Crowther, published in Northwest Runner back in 2001.

The early years

The first relevant article I can find seems to be Hettinger & Muller (1953). They published an article on the effect of shoe weight on the energy consumption during walking and carrying (sorry for my poor translation). I have managed to access it, but it does stretch my German to its limit. They investigated a range of conditions (in 265 trials) from barefoot, plimsolls (UK terminology), half shoes, normal laced shoes, ski-shoes, riding boots, military boots and a variety of other test shoes. I only mention them since it is the most complete test list of shoes I have come across - very Germanic! Whilst there is much good data to consider - most of it is hard to connect with more modern studies. However, the conclusion that; when walking and carrying a weight of 30 kg, energy consumption increases more rapidly in shoes than barefoot seems interesting. The next relevant study was by Catlin & Dressendorfer (1979) where the authors, apparently, looked at the effect of shoe weight on the energy cost of running. Since I am unable to get hold of the article and PubMed does not have an abstract for it I am limited to recounting what Jones et al. (1986) state about it. The study looked at marathon runners, on a treadmill wearing lightweight racing flats (520g total weight). They recorded a 0.9% increase in energy cost for each 100g increase in shoe weight (per pair).

Myers & Steudel (1985) reported the results of four people running at a stately 10.3 km per hour (5:50 min per km) on a treadmill (paced using a metronome) with weights (3.6 kg in total in canvas belts filled with lead shot) attached to them in four locations - waist, upper thigh, upper shank and ankle. They made measurements of oxygen consumption at steady-state (after 8-15 mins of running). The authors also modelled the physical work required to accelerate the additional weights using a few simple equations. They found that 3.6 kg around the waist had the smallest effect causing a 3.7% increase in energy (oxygen) consumption. Moving the weights (split equally across the limbs) caused a greater increase in energy consumption with the size of the increase related to how close the weights were to the foot. Thus, the ankle weights produced a 24.3% increase in energy consumption compared to running without the added weights and 20.7% compared to running with the weights around the waist. Thus, they demonstrated that weight around the feet has a much bigger effect (on the flat and on a treadmill) than weight on the waist. Interestingly they state the kinetic energy changes of the legs account for about a third of the energy used when running. They also noted that this kinetic energy component increases with the square of the running speed whilst the mechanical energy in moving the trunk rises linearly resulting in high energy costs for limb movements at higher speeds.

In the same year (Martin, 1985) the results of weight loading (thighs or feet) of 15 men running at 12 km per hour (5 min per km) was published. I only have access to the abstract, so I can make little comment other than to report what was summarized within it. The increase in oxygen consumption was 7.2% per kg of load (on the foot) which, when extrapolated to 3.6 kg, produces a value of ~26% and is in agreement with Myers & Steudel (1985). They also go on to state that the energy increase can be attributed to the additional weight rather than a change in gait.
Jones et al. (1986) looked at the energy cost of women walking and running in athletics shoes (514g) and leather military boots (1,370g)! They report an 8.3% increase in energy cost associated with the weight increase of 857g giving rise to the memorable number of 1.0% increase in energy cost per 100g increase in footwear weight.

Miller & Stamford (1987) went a little bit further, investigating men and women, ankle and arm weights during both walking and running! Again the experiments were on a treadmill but, at a range of speeds. The maximum running speed was halfway between Martin (1985) and Myers & Steudel (1985) at 7 mph (11.2 km per hour). The bottom line is that men and women had the same energy consumption and it rose by 8% per kg on the ankle (i.e. similar to Martin's study). Weights on the hand produced a greater increase (13%) in oxygen consumption - but, subjects ran with a 90 degree bend in the elbow with arms swinging with each step. Interestingly they also report that the increase in oxygen consumption rose linearly with weight added to the ankle all the way up to 4.5 kg.

Barefoot - the ultimate weight reduction

Warburton  (2001) published a brief review of barefoot running. In it he states that Laboratory studies show a 4% reduction in energy cost when running barefoot. But he also states; "Competitive running performance should therefore improve by a similar amount, but there has been no published research comparing the effect of barefoot and shod running on simulated or real competitive running performance." This short coming may have been solved by Buchholz (2007). He wrote a Masters Thesis on performance differences between bare foot and shod running, however, an electronic copy is not available. Divert et al. (2008) attempted to compare barefoot with shod running in order to test whether shoe weight or a change in foot strike was responsible for the differences in oxygen consumption reported. They used 12 subjects running at 13 km per hour on a treadmill with a range of shoe and additional weights. They found that additional mass increased oxygen consumption and that efficiency was reduced when wearing shoes.  They hypothesized that the damping effect of shoes reduced the storage of elastic energy lowering the efficiency of running in shoes.

Hanson et al. (2011) compared the cost of running barefoot and shod both on the treadmill and overground using both men and women at 70% of VO₂max (10.7 km per hour). They found that shoes increased oxygen consumption and heart rate on both the treadmill and overground. Interestingly, the effects were more pronounced overground where using shoes increased oxygen consumption by 5.7% compared to on a treadmill where shoes increased oxygen consumption by 2%. An electronic copy is available at the time of writing, and care needs to be taken with reading it. Fig. 1, for instance, appears to show no significant oxygen uptake differences between the groups - which is compatible with their data since the SEM will be dominated by the variation between subjects rather than the different test conditions. It is hard to see why this figure is included and the statistically significant data (i.e. mean relative change in oxygen uptake for each condition) is not plotted - hey ho! But, there are studies which report the opposite. Jack Daniels, who worked for Nike in the early 1980s, states that as shoe weight is reduced there is a point at which further weight reductions began to increase the cost of running. This may well be the same observation reported by Franz et al. (2012). Again this is a treadmill comparison of barefoot versus shod. However, in this case running (12 km per hour) in shoes produced a lower oxygen uptake and the authors concluded that running barefoot offers no metabolic advantage over lightweight cushioned shoes. The paper got a lot of publicity across the web, but without being able to see it I cannot comment further.

Is strike type important?

To finish off I want to just bring a recent article to your attention, whilst it is not directly related to shoe weight it does impact on shoe type. Perl et al. (2012) looked at running economy in minimal shoes versus standard running shoes using a range of foot strike patterns. Again I am hampered by not being able to access the paper, but here is the gist of what they report in the abstract. First, they controlled for shoe mass (I don't know how) and they found that regardless of whether you forefoot or rearfoot strike minimal shoes require less energy (2.4% less for forefoot, 3.3% less for rearfoot). That is an effect of the shoe type not weight. Again, like Divert et al. (2008) they suggest that shoe cushioning may get in the way of the more efficient elastic recoil in your musculature.

Conclusion

It is clear that adding weight to your feet will cause a rise in oxygen consumption. Treadmill data suggests about 1% for each 100g of total shoe weight. There is only one study of oxygen consumption that used a hard surface (an indoor track) and that was by Hanson et al (2011). However, they only tested one shoe type against barefoot running, and the shoe weight does not seem to be given. Others (e.g.  Buchholz, 2007) must have run tests on the effect of shoe weight overground, however, they are hard to get hold of - perhaps because they are done as student projects and without laboratory control are considered unsuitable for publication in the scientific literature.

It is also clear that the physics dictates that increasing the mass at the end of a lever system must result in an increased energy input. The counter argument is that some of the weight, in a shoe, improves efficiency by altering the gait or foot-strike or improves race performance by allowing the runner to get to the finish line without injury. Minimal running shoes may also present a benefit beyond weight reduction in allowing the musculature to return more elastic energy. However, barefoot running tests have produced mixed results. There are suggestions that this may depend upon gait or foot-strike, experience, adaptations etc. However, I think it is probably fair to say that there is not yet enough evidence to be at all sure.

Finally, there is the BIG question as to whether a reduction in energy consumption leads to better race performance. Certainly extrapolating from mathematical models like Rapoport's (which is a good read) then one would conclude that it can. But, races are about more than simple energetics......

References

Buchholz MP Performance differences between the conditions of running with the foot bare and running with the foot shod (2007) Thesis (MS) Springfield College Listing

Catlin MJ, Dressendorfer RH (1979) Effect of shoe weight on the energy-cost of running Medicine and Science in Sports and Exercise 11, 80.

Divert C, Mornieux G, Freychat P, Baly L, Mayer F, Belli A (2008) Barefoot-shod running differences:shoe or mass effect? Int J Sports Med, 29, 512-518. PubMed

Franz JR, Wierzbinski CM, Kram R (2012) Metabolic cost of running barefoot versus shod: is lighter better? Med Sci Sports Exerc Mar 2 [Epub ahead of print] PubMed

Hanson NJ, Berg K, Deka P, Meendering JR, Ryan C (2011) Oxygen cost of running barefoot vs. running shod. Int J Sports Med PubMed Full Text

Hettinger T, Muller EA (1953) Der Einfluss der Schuhgewichtes auf den Energieumsatz beim Gehen und Lastentragen. Arbeitsphysiologie 15, 33-40. Full Text
Jones BH, Knapik JJ, Daniels WL, Toner MM (1986) The energy cost of women walking and running in shoes and boots. Ergonomics 29, 439-443. Full Text
Martin PE (1985) Mechanical and physiological responses to lower extremity loading during running. Med Sci Sports Exerc 17, 427-433. PubMed 

Miller JF, Stamford BA (1987) Intensity and energy cost of weighted walking vs. running for men and women. J Appl Physiol 62, 1497-1501. PubMed Full Text

Myers MJ, Steudel K (1985) Effect of limb mass and its distribution on the energetic cost of running. J Exp Biol. 116, 363-373. PubMed Full Text

Perl DP, Daoud AI, Lieberman DE (2012) Effects of footware and strike type on running economy. Med Sci Sports Exerc [Epub ahead of print] PubMed
Warburton M (2001) Barefoot running. Sportscience 5. sportsci.org/jour/0103/mw.htm

A quantification of the reduction in effort when running in lighter shoes - another test

Difference in running speed, at equal physiological effort, in lighter and heavier shoes

In a previous blog I reported a comparison of 'physiological effort' of running at the same speed in two different running shoes. I compared the ProGrid Ride (740g total weight for the pair) with the Hyperspeed (425g total weight for the pair) both UK size 9. I conducted the test on a flat tarmac track (2km at 4:50 mins per km) alternating between shoes. The result was that when using the Hyperspeeds I had roughly 1.8% fewer heart beats per km than when using the ProGrid Rides. I then went on to suggest that this might extrapolate to a 1.8% faster speed for the same heart rate.

I posted a link to the results on RunningAHEAD and JRMichler suggested a better test. He said; " The next logical experiment would be to run 1.8% faster with the lighter shoes and compare average heart rates. "

It is indeed a good idea. The last test was fixed speed, on this one I set out to run at a speed that I predicted would produce equal heart rates with both shoes. I started with the ProGrids and ran a warm-up 2km along the route (here is the run). I then set-off at 4:30 min per km pace for the first test run which I abandoned just over halfway when I turned at the wrong pole......I walked back.

The next two runs in the ProGrids were done (1 & 2) in 9:03min and 9:02min for the 2km - roughly 2s slower than I had planned. I then attempted to run ~2% faster in the Hyperspeeds. The first run was in 08:51min and the second in 08:49min (2.14% faster than in the ProGrids).

In Figure 1 I have plotted the speed (in km per hour) and the heart rates for each run.

Figure 1. Speed and heart rate data for six runs (in sequence) on a 2km flat tarmac path in two different running shoes. The first run was done at an average of 137 beats per minute (as reported by Garmin Training Center) whilst the next four runs were all done at 138 beats per minute. On the final run my heart rate was 1 beat per minute higher. I have set the y-axis ranges to allow the difference in heights of the individual bars to be judged. I am not attempting to imply either a large or a small effect - it is ~2%.

I shall restrict my analysis to the middle four runs for a couple of reasons. First, the middle Hyperspeed runs are bracketed ProGrid runs making for a symmetrical arrangement. Second, all of those runs resulted in the same heart rate (138 beats per min). The average speed for the middle two ProGrids was 13.30 km per hour and for the Hyperspeeds it was 13.58 km per hour. Thus, I ran in the Hyperspeeds 2.14% faster than in the ProGrids but (to integer accuracy) the heart rate was identical. Thus, it would appear the lower heart beats per km using the Hyperspeeds in my previous test does translate to a faster speed (~2%) at the same heart rate.

For interest I have included in Figure 2 an analysis of heart beats per km for the same data set.

Figure 2. Heart beats per km for the same runs as shown in Figure 1. This form of analysis normalizes for speed and heart rate producing a value of heart beats per km that is fairly insensitive (although not completely) to speed. It shows that the Hyperspeeds require fewer heart beats to be taken per km run. The y-axis minimum is the value that I predict I would need to run a world record marathon.

This is consistent with the idea that losing 150g from each foot results in either a lower physiological effort for the same speed, or higher speeds for the same heart rate (the choice is yours!).

A quantification of the reduction in effort when running in lighter shoes

Are lightweight running shoes worth the money?

Yesterday, I attempted a little experiment to see if lighter trainers really could really make running easier. Was the £51.99 I spent at SportsShoe on my lightweight Asics Gel Hyperspeed 5 really worth it?

For those of you who don't want to read the detail, my preliminary answer is: Yes! The Hyperspeeds gave me a about a 1.8% reduction in effort and therefore probably also a similar improvement in running time compared to my previous running shoes.

I have previously run all of my marathons in a pair of Saucony ProGrid Ride 3 (£39.99 from SportsShoe) and I have not had any problems with them. But, they are reasonably heavy (UK 9 ~350g) and after missing my 'good for age' at the London Marathon (3 mins on the loo just over halfway resulting in a disappointing 3:15:42 finish) I decided I wanted to maximize my chance of a fast time 3 weeks later in Prague. So, at the last minute and with no time for any testing I bought a pair of Hyperspeeds. They are much lighter than the ProGrids and my first impressions were that they felt good (3 km test jog a few days before the Prague Marathon). At that point I weighed-in at about 62 kg having put on about 1.5 kg since the London Marathon and I figured that being towards the lower end of the weight spectrum I was unlikely to suffer too much from the Hyperspeeds reduced cushioning. At Prague the race went well and I finished feeling great in 3:07:52 - a fair improvement on London. The Hyperspeeds were definitely light and I had the impression that they might have been solely responsible for the speed improvement. Now, 2 weeks on and well into my recovery (two days ago I just got a PB on my 5 km of 19:07 in my trusty ProGrids) I thought I should compare the two shoes directly with one another.

Method

The test I designed was fairly simple and based on the principle that within a session heart rate reports physiological effort. As you increase your muscular effort, your heart has to pump more blood. If all you are doing is the same type of exercise (i.e. running) with no massive change in posture, muscle groups, hydration etc then increased effort equals increased heart rate. Taking speed into account is very important since increasing speed increases the effort and therefore heart rate. I used two techniques for making sure that speed could not confound my results. First I ran at a flat pace, the same for each run (within a couple of seconds) and then I also normalized the data for speed by calculating the number of heart beats per km (more about that form of analysis later). To make sure that wind speed and hills and road surface did not also confound the results I ran on a flat tarmac path running along the course of an old railway track (now the Cambridge Guided Busway) using an out-and-back course. I made sure that intervals between the tests were similar and I started and stopped my watch at fixed points (a white line on the path) whilst running at a constant pace. The route was 1 km out and 1 km back with the turn around at a 'Cycle Dismount' post where I swung 180 degrees around it using my right arm (you have to make these things fun!). I stuck to the same running-line and I did not drink or take any loo stops between runs. I did five runs in total. The first was a warm-up run in the ProGrids and it also served as a test of the pace that I felt comfortable with (I had done a hard day gardening, I had raced the previous day and it was warm...). I then did each subsequent run alternating between the shoes. I had intended to do six runs, but a loo stop called a halt to the experiment.

I wore my Garmin 305 Forerunner watch and heart rate monitor strap with the autopause and autolap function switched off. I left the recording mode set on the automatic setting. I used a display showing average heart rate, average pace, instantaneous heart rate and instantaneous pace in order to get each run at roughly the same pace.

Results

Data from each of the five test runs is shown below (Table 1). The Time was that recorded by the Garmin, the distance was always 2 km (not the noisy satellite data from the Garmin). Since the Garmin Forerunner only reports average heart rate to integer accuracy, which is not good enough for this study, I have had to use a slightly more complicated technique to extract the heart rate data. I have used a form of signal processing that relies on averaging multiple values in which noise dithers values around a mean (I am guessing most people don't realize that noise is often very useful when manipulating data which is reported with low resolution). I summed the product of the integer heart rates, for each of the Garmin's ~1s sample periods, and the length of the sample period. This produces a mean value for heart rate that has more than integer accuracy.

Num	Shoe	Time	Speed	Heart rate	'Physiological effort'
		mm:ss	km per hour	beats per min	beats per km
1	ProGrid	09:42	12.37
2	Hyperspeed	09:38	12.46	125.4	604
3	ProGrid	09:40	12.41	127.7	617
4	Hyperspeed	09:39	12.44	125.8	607
5	ProGrid	09:39	12.44	127.9	617

Table 1. Data for the 5 sequential 2 km runs with alternating shoes. The first run was a warm-up from resting and, since the first run always shows a lower heart rate, was excluded from subsequent analysis. 'Physiological effort' is the term used to describe the number of heart beats taken per km covered. For this data sequence no other correction was made to this metric - although my previous work has shown that it typically rises during longer runs. Click on the shoe name to take you to the Garmin Connect data.

From Table 1 shows that all of the test runs were completed to a 1 s accuracy. The average heart rate for the ProGrid was 127.8 bpm compared with 125.6 for the Hyperspeed. Correcting for the small differences in running time I have calculated that the average heart beats per km for the ProGrid was 617 beats per km compared with 606 beats per km for the Hyperspeed. This equates to  ~1.9% reduction in physiological effort which, if sustained over a marathon distance, might be expected to result in a similar percentage time improvement (~3min 30s for a 3hour 10min runner). Equally, such a reduction in effort may be expected to drop my PB at 5 km from its current 19:07 to 18:46. Obviously only two tests with each shoe is insufficient for testing of statistical significance, so a note of caution is required (and I would not dream of publishing this in a scientific journal as it stands!).

Figure 1. Bar chart of the heart beats per km from the four test runs.  The y-axis is scaled so that the lowest value (510 beats per minute) sits approximately where I would need to be if I wanted to break the current marathon world record. Thus, you can judge to what extent the shoes might contribute towards an attempt by an only just 'good for age' runner on the world record!

For those would prefer figures (Figure 1) shows the heart beats per km for the four test runs. The effect certainly looks consistent. However, more data is still required. I will do another repeat of this test, but I want to try a faster speed. A simplistic physical analysis suggests that the effect of weight should become progressively greater as speed increases since the kinetic energy is proportional to velocity squared. Since the shoe has to accelerate from rest to roughly twice your bodies travelling speed with each stride, the energy input must rise dramatically with speed. To see the results of my next test, done at a higher speed and using a constant effort click here.

Health Warning

The most obvious question is: "Would this work for me?". Well, if you are running a marathon there are lots of things that can stop you getting a PB and lightweight shoes might not help. The most obvious mistakes are poor pacing (flat is best), unrealistic expectations from current training, not taking weather into account, over-hydration and injuries. But, if you are light enough to tolerate minimal shoes and fast enough to benefit from them with a decent running form then the answer is probably yes! There is a well established literature on the topic of shoe weight and energy consumption using treadmills - I have reviewed some of it here. But, there are probably cheaper ways of getting faster - but, they usually involve more training, less eating and generally reduced fun!

Two hearts beating as one

Two hearts beating as one

Before indulging in some data analysis I just wanted to make sure that we have a good grasp of the heart's function within the circulatory system and its control. To start with, I think it is useful to divide the circulatory system into six parts each with separate functions but physically and functionally coupled together.

Arteries and arterioles

Moving away from the heart, through the circulatory system, we have the arteries and their branching smaller arterioles. The arteries have an important function in storing blood for short periods of time just after each heart beat. The rise in pressure at the end of each heart beat (systole) pushes blood into the arteries. The rise in pressure expands the arteries and gives the blood somewhere to go since it cannot escape through the rest of the circulatory system fast enough. This expansion of the arteries reduces the peak systolic pulse pressure and their elastic recoil as the heart is relaxing maintains your blood pressure at high enough levels to perfuse your brain. Elastic arteries allow for lower heart rates and lower peak pressures - it is definitely worth trying to keep your arteries fat free and in good condition! The smaller arterioles branch off the arteries and they are innervated by nerves which generally cause them to contract. We call this sympathetic tone. This contraction causes the vessels to be small in diameter and stop the blood escaping too fast from the heart. By doing so they reduce the amount of work the heart has to do. The arterioles are the 'taps' of the circulation, they are the points at which flow is controlled.

Capillaries

Rather short vessels that connect the arterioles with the venules. These vessels are slightly smaller than the diameter of red blood cells (bad news for sickle cell suffers with rather stiff red blood cells...). Capillaries have walls that are one cell thick and no cell in your body is very far from a capillary. Interestingly most capillaries don't have much blood flowing through them at rest - they tend to be shut. Because they are very small they are intrinsically very strong (Laplace's Law) but they are quite permeable to water. Diffusion across capillary walls is fast and this is where most O2 is delivered and CO2 picked-up. So, the function of the capillaries is to allow for exchange of metabolic substrates and waste products.

Venules and veins

If you read the previous blog, you won't be terribly surprised to find out that these vessels are really important. At rest most of your blood is sitting in your veins doing remarkably little except staying away from your heart and stopping it from having to work too hard. Venules collect blood from the capillaries and carry it to the veins. They are also quite permeable to water and a rise in venous pressure can easily give rise to a loss of fluid into the intercellular space (the space between the cells that make up your body). This swelling occurs when venous pressure rises due to a failure of valves, fluid overload, heart failure or a loss of muscle pumping through sitting still for too long! Veins can contract and push a lot of blood into the circulatory system. When blood is pump through the heart into the arteries and the capillaries it can - if the control system isn't working - simply pool in the veins. That lack of blood flow back to the heart stops it pumping enough blood to maintain blood pressure and a faint usually results....a common circulatory failure rapidly and automatically cured by the adoption of a prone position!

Right side of the Heart

The right side of the heart accepts blood returning from the body and pushes it on to the lungs. If does this at rather a low pressure to stop water being squeezed out of the lungs. Starling's Law of the heart ensures that all of the blood returning to the heart gets pumped onwards, not by altering heart rate, but by raising stroke volume when more blood fills the ventricle. The intrinsic control prevents venous pressure rising too high. This is particularily important for blood returning back to the left hand side of the heart from the lungs.

Left side of the Heart

If you take a heart in your hands almost all of it is made up of the left hand ventricle. The right side is pretty small - not that it doesn't pump the same amount of blood - it is just the left hand side has to develop a much higher pressure than the right hand side. Blood flowing into the left side, by virtue of Starling's Law, gets pumped away through the aorta at just the right rate. This mechanism results in a matching of the amount of blood pumped by each side. The thick ventricular wall is made of muscle that can generate the high pressure necessary to push the blood onwards around your body. Rather oddly the heart muscle does not get nurished by the blood in the ventricles. Instead blood in the main artery (aorta) flows off down the coronary arteries to supply the heart muscle. Most importantly this only happens when the heart is relaxed. Here in lies the rub. The faster you ask your heart to beat, the more you make it work, the less time it is relaxing and the less blood flows through the muscle. This is a really silly bit of design and is one of the features that stops the heart beating at very high rates. (It is a bit more complicated than that since mice can have heart rates of about 1,000 beats per minute....and I measured my daughter's guinea pigs' heart rates at about 300 beats - but that is another story....). When those coronary arteries get blocked pain and then damage can result....

The control system

The default setting of the control system is to shut pretty much everything down to a minimal energy saving level. At rest only a small proportion of your capillaries are fully open and the heart is pumping only 5 or 6 L of blood per minute (or something close to that). The control system has set your arterioles to a narrow diameter to stop blood flowing away from your heart too rapidly, the veins relaxed so blood can pool in them. Because of the constricted arterioles systolic pressure is high and the arterial pressure sensors (baroreceptors) keep the heart rate low. The relaxed veins keep venous return to the heart low and thus the heart contracts relatively weakly. This is a nice energy saving setting.
When you exercise everything changes. Oddly the arterioles contract more, further reducing general capillary blood flow - but that is to keep blood pressure high. The veins contract injecting blood back into the circulatory system towards the heart. This raises the force of contraction of the heart. Heart rate gets controlled by the baroreceptor reflex to keep blood pressure in the right range. Then, the exercising muscle through a process of autoregulation begin to dilate arterioles and take the blood that they need by virtue of their metabolic activity.
Thus, you and your circulatory system are a bloody marvel. Heart, arteries, capillaries, veins, sensory receptors, brain and nerves functioning together to allow a massive increase in work rate. Fantastic.

But, as athletes all we record is heart rate....disappointing....

Art of the Heart: Part I

Art of the Heart: Part I

GPS watches with inbuilt heart rate monitors are excellent tools for tracking fitness. But, I am always a bit concerned about whether athletes and coaches know how to interpret the data. Once you start to get interested in heart rates and begin studying them in detail, things can look quite confusing. For instance two runners - one older and one younger might have very similar performance times, but run at very different heart rates: and not as you might expect. The older runner might be working at heart rates above an 'age-predicted' maximum which the other younger runner might have a lower heart rate at the same speed 20 beats per minute slower. This seems to make no sense. Then you might find that one day your average heart rate rises or falls by 10% at the same speed and distance. Surely all of this means that heart rates are variable and useless? Indeed, that is the stated opinion of some of our high level coaches.

However, there are good reasons for these variations - and once you understand why heart rates can vary they can become a useful tool. Since the heart is a pump, the rate at which it beats tells you something about the metabolic work being done. But, the complication lies with the fact that each beat can pump a different amount of blood. The amount of blood ejected from the heart with each beat is known as the stroke volume. That volume is a proportion of the blood that fills the heart whilst it is relaxing (diastole). If you multiple the stroke volume by the heart rate you can calculate the amount of blood pumped: the cardiac output. The first thing to realize is that the heart has an internal, intrinsic control (known as Starling's Law of the Heart). That control process results in the heart pumping the blood that returns to it through the venous supply, i.e. if you give the heart blood to pump, it will work as hard as necessary to get rid of the blood.

Starling's Law of the Heart

Venous blood, returning to the heart through the vena cava (great vein), flows through the right atrium and into the right ventricle. Almost all of the blood flowing into the right ventricle does so as a result of the pressure gradient caused by the elastic recoil of the ventricle as it relaxes and the venous pressure. The contraction of the atrium adds a bit more blood - and it isn't terribly much at rest, although during exercise it is more important. The force of contraction of the ventricle is determined by the amount of blood that has entered (i.e. the amount of stretch of the ventricle). So, the more blood returning the greater the contraction and the more blood is pumped. The heart acts to get rid of the blood that flows back to it. This is a very important property of the heart since it makes sure that our pulmonary pressure (blood in the lungs) does not rise too high and cause us to lose fluid into the lungs (and therefore suffocate). People suffering from left ventricular failure - where the left side of the heart cannot generate sufficient pressure to get rid of the blood have exactly this problem - they effectively drown.

So, the stroke volume of the heart is determined by venous return (blood flowing back to the heart). It is this variability in venous return that underlies much of the variability of heart rates. When venous pressure is high the heart fills with a lot of blood and each beat pushes out a lot of blood. The result is a low heart rate. When venous pressure falls the stretch is less and heart rate rises. The same amount of blood is pumped, by your heart rate differs. This change in stroke volume is not reported by GPS/heart rate monitor watches and if you  don't realize what is happening you may well end-up making the wrong conclusions....or worse still training at  a different intensity than you intended.

Maximum heart rates

The next problem is maximum heart rates. The term, as used by most people, is nonsense.  Sure, there is a maximum rate at which a heart can beat before the output decreases - but even that probably depends upon the state of the rest of the circulatory system. When you exercise, you are driving your muscles using motor nerves: your heart gets driven by changes that result from this. The maximum heart rate you can achieve will depend upon the amount of muscle you use, the duration of the exercise, your motivation and the state of your circulatory system (in particular the venous portion of it). If you are fatigued, fluid loaded as a result of previous exercise, aerobically fit and exercising for a prolonged period you will not get anywhere near an age-predicted value for heart rate. If your arteries are a bit stiffer, you have good muscle mass, middling fitness and your venous pressure is lower and you are doing a brief period of exercise you should be able to achieve a much higher heart rate than the age-predicted value.

Resting heart rates

Resting heart rate is the value you might get towards morning before you wake-up. Its value is dependent on both long and short term exercise as well as emotional state. It may be of some value - I know some Elite level coaches use it to determine levels of fatigue. I am not a great fan of it and rarely either measure of use it.

Changes in heart rate during an exercise session

Now this is the most useful measure. When you exercise you will notice that your heart rate first rises rapidly and then after a minute or two it either continues to rise or it wobbles around some value. These numbers are the ones that mean something! When you exercise there is an initial drive to the heart to increase the force and rate of contraction. There is also a rise in venous return and a change in arterial resistance. All of these will set stroke volume and heart rate at some value. Then as you exercise muscles will begin to metabolize, use stored energy and release waste products. It is the change in these metabolites and waste products that open up blood vessels and increase the blood flow to the exercising muscles. The result is a general (but small) decline in arterial blood pressure as the peripheral resistance decreases. The baroreceptor reflex (the process that maintains arterial blood pressure) maintains the arterial blood pressure by altering both heart rate, venous return and contraction force. The change in heart rate is what the GPS watch notices.

Once the rapid rise in heart rate is over, your body is in steady-state (almost) with sufficient blood flowing through your lungs and muscles to sustain the exercise intensity. Or, at least if the exercise intensity is low enough that is what happens. If you exercise more intensely then the metabolites/waste products continue to feedback causing ever greater amounts of blood flow and your heart rate keeps rising. The point at which it stops rising depends upon whether your muscles can continue to produce the force and whether your brain can keep driving them.

The beauty of measuring heart rate during a session is that your heart rate, within that aerobic zone, is nearly proportional to the amount of work you are doing. Of course, there are factors that will cause heart rate to drift upwards at constant effort. Thermoregulation places an extra-load on the circulatory system requiring the heart to pump more blood to the skin, and a progressive dehydration will also reduce stroke volume causing heart rate to rise to maintain cardiac output. Also, fatiguing muscles will also require more blood. But, the great thing relationship here is that the number of heart beats required to cover a km remains roughly constant and independent of speed. It is this relationship that I want to consider in my next post.