Note this is the first edition of the book, published 2009. The second edition, published in December 2012, appears to be not yet available on Kindle.
First, an obvious. He uses as an example of the advantage of weight loss on performance the now infamous case of Lance Armstrong. Lance lost weight from cancer and went from getting passed in time trials to dominating them. We now know there's a lot more to that than losing his "linebacker's build" from cancer. Of course the full story came out after the book was published in 2009 but this was after David Walsh's book, so there was certainly plenty of evidence out there that Armstrong had "confounding factors", and I'd expect he could have used a more scientifically sound example.
The section of main interest here is "The Right Body For the Job". He goes through various endurance sports and describes the optimal body type for each.
Regarding cross-country skiers he writes:
The average height of an olympic cross-country skier is 5 foot 10 inches, and that of their female counterpart is 5 foot 7 inches. Height provides a mechanical advantage for poling, which is important for the generation of forward thrust for a cross-country skier. However, with height comes mass, and mass is the enemy of performance in cross-country skiing because it increases gravitational and frictional resistance. That's why you don't see as many 6-foot-5 athletes on the competitive ski trails as you do, say, on the volleyball court.
This is promising but not very insightful. The male height is close to population averages for cultures with ample access to protein-rich foods, the female height slightly taller. Why is it that the mechanical advantage dominant up to the average US male height, then the weight gain becomes dominant later? It's quite vague.
On cyclists, he argues:
Whereas power-to-weight ratio is the critical variable in climbing, in time trialing it is raw, sustainable power output which matters most.
This would be true if weight wasn't correlated with cross-sectional area, except it is. Consider a simple model of constant BMI. Weight is proportional to width multiplied by height multiplied by depth. If I assume height is proportional to the square root of weight, and depth is proportional to width, then width and depth are each proportional to the 4th root of weight. If I assume area is proportional to width times height, neglecting depth, then area is proportional to the 3/4 power of weight across a population.
But for a given rider mass is fixed, so in this case cross-section will increase proportional to the square root of weight increase. If I further assume that the bike is responsible for approximately 30% of total wind resistance, that leaves 70% to the rider, and then a 1% increase in weight will result in a 0.35% increase in wind resistance, assuming the body has fixed coefficient of drag. So if you are training for a big flat time trial and you gain 3% of mass but increase power 1%, feeling good about it because you read power was all that mattered in this book, the analysis suggests you're no better off from the perspective of wind resistance, and adding in rolling resistance likely results in you going into the red.
So size, while not as detrimental as it is to climbing, is nevertheless still very important for time trialing, even on flat roads.
He goes on to say, comparing cyclists to runners who have smaller legs on average:
Cyclists have greater leg muscularity because legs do essentially all the work in cycling whereas running is a full-body activity.
I propose an alternate explanation: when cyclists ride their legs spin around, coupled together by the pedals, When one leg drops, the other rises, the falling leg pushing up the rising leg. There is no fundamental energy dissipation with this motion, although in practice there will be energy losses associated with spinning the legs in small circles.
Runners also experience energy dissipation from spinning limbs around, which they do at roughtly the same rate as cyclists, but the difference is the legs aren't coupled together and so muscular work must be done to life each legs separately. There is thus an additional energy cost with raising each leg which isn't present for cyclists. And there's not much pay-back for lifting the legs: when the foot crashes back to the ground, much of the energy is lost (shoes can store and return only around 10 joules, a small fraction of the peak potential energy). Whereas with a cyclist, as the leg drops it does continuous work on the chain, propelling the bike.
The result is a cyclist gets power from leg muscles at less cost than the runner. The runner pays a greater penalty for leg mass. Indeed, leg and foot mass, which must be raised and lowered each step as well as accelerated forward, is the most costly mass for a runner. If you had to carry a pack when running, you'd put it on a relatively inert position like the back or waist rather than on your foot. I'd never, for example, use one of those shoe lace-attached key packs when running a race if I had an option.
Therefore people with powerful, muscled legs are disadvantaged when running more than when cycling. The best runners are those with slimmer legs. Mechanically the slimmer legs are more efficient: power may be less, but required power is less still, disproportionate to the difference in total body mass.
Of course, more muscle means more power in every encurance sport, but unlike in other endurance sports, that mass comes at no cost in rowing, because there is no gravitational resistance to overcome and the extra weight has very little effect on frictional resistance between the boat and the water.
When I showed up at college as a Freshman, I was immediately recruited to be a coxswain on the rowing team because I was small and light. I instead joined the sailing team, to which I was also well suited. And there was good reason for this: because boat drag is strongly dependent on weight. In equilibrium a boat displaces its weight (with the weight of its contents) in water. More weight = more water displaced. And displacing water takes energy. So you want a boat to be as light as possible (which is why they're made of carbon fiber) and you want the crew as light as possible. Of course, increased power may offset increased weight, which is why good rowers are muscular, but they're also exceptionally lean, because only weight directly contributing to propulsive force is justified.
But I further disagree that "more muscle means more power". I suppose it depends on how you consider power, but I consider power to be useful power available for propulsion. If I have beefy arms as a cyclist, it seems plausible the metabolic load of supporting this additional mass, since muscle requires blood flow, will rob my legs of blood flow which they could better use to help me move the pedals. Rowers happen to use a wider range of muscles than cyclists, but they still want muscle development optimized for rowing, no more.
The interesting discussion here is that swimmers have higher body fat than other endurance athletes. The question becomes whether this is simply because body fat doesn't matter as much for swimmers or because it actively helps?
The proposed hypothesis is that the buoyancy of fat is an advantage. For open water swimming, insulation would be an additional advantage, but this probably isn't the case for thermoregulated pools. I don't think you can assume any characteristic feature of world-record-class swimmers is accidental or due to neglect.
I've one issue with his dicussion of runners. He notes that elite endurance runners are relatively small due to the advantge of being light. I completely agree. But I don't think this is obvious.
It might seem intuitive that taller runners would be faster. There's a guy at work I've run with who's much taller than I am: he's maybe 185 cm, while I'm 169 cm. When we run together, I feel like I'm struggling to keep my legs turning over fast enough while he takes advantage of a longer stride to just glide along.
Having made an amateurish attempt at working out running power-speed equations, I'll take a stab at explaining why our difference in height alone isn't the reason he's faster. Here's the formula I derived:
P = M g² (1 − 2 C Lstep / v)² / 16 C +
[ 2 ( Mfoot g hfoot − Eshoe ) + Mk,foot v²/ (1 − C Lstep / v)² ] C.
I define terms in the original post, but in summary is M (total mass), Mfoot (leg mass lumped into an effective foot mass), C (running cadence: rate at which each foot lands per unit time), v (run speed), Lstep (how far the body moves while each foot is planted), g (gravitational acceleration), and Eshoe (energy stored and returned in the sole of each shoe, around 10 joules). Note both C and Lstep appear in multiple places, either adding to or subtracting from total power, suggesting optimum values for each.
One way to increase stride is to keep your planted foot on the ground for the same amount of center of mass motion and just launch your body into a higher trajectory, landing further. This comes with a high energy cost, because you've got to jump higher off the ground with each foot-stroke, and much of the energy is lost when you land, so it needs to be re-supplied next step. The other way is to keep your foot planted for more center-of-mass motion progress. This reduces the ballistic time, which reduces the amount of potential energy you need to supply, but it also increases the amount you've got to swing that foot and leg forward as well as reducing the time available to swing it forward before it lands ahead of your body. You thus need to supply additional kinetic energy for this to happen. And of course there's flexibility constraints on how long you can leave your grounded foot planted.
Legs get thicker as they get longer, so the mass increases disproportionate to the length, and energy increases proportional to mass, so longer, thicker legs with a longer stride aren't necessarily more energy efficient. In sprints, energy efficiency isn't as important as muscular strength, but in endurance events, the heart & lungs are limiting, so energy efficiency is more critical. The result is shorter, skinnier legs can beat out longer, thicker legs over long distances as long as flexibility is sufficient to cover the required angle of motion. At some point, though, the running cadence needed to keep up becomes too great given the limits height imposes on the center-of-mass motion over which the ground foot can stay planted, and so midgets don't win marathons.
This running discussion is probably too much detail for what is essentially an introduction, so I don't blame Fitzgerald for failing a detailed discussion of running kinetics. However, even his short descriptions of the issues in the other sports borders on misleading. It's a disappointing start to a popular book. But his strength is in nutrition, so perhaps that aspect is better.