How nature programmes get things out of proportion


Michaela – stronger than a stoat


I have a lot of time for Chris Packham and BBC’s Spring Watch. But the other day his co-presenter Michaela Strachan repeated the age old nature programme gimmick of scaling up small animals “feats of strength” to human size and then being amazed.

“Wow,” said Michaela, “That stoat carried the rabbit all the way back to its kits, and the rabbit weighs more than it does.  We’ve worked it out astoat with rabbitnd it’s as if I carried a red deer stag by my teeth.”

Weight lifters sometimes go in for that kind of mistaken mathematics as well, with smaller bodybuilders or weight lifters often claiming that “pound for pound” they are stronger than their bigger rivals and that therefore they have better muscles.

When gymnasts have to lift their own bodies on the parallel bars, it is always harder for a big person to do it than a small one. This is not because the small person has done more exercise.

The mistake here is to assume body weight is proportionate to strength. Body weight is not proportional to strength. What determines the strength of a muscle is its cross sectional area, not its weight or volume. A thicker muscle can exert a bigger force, a longer, thinner one with the same volume would be weaker.  Never mind the size, feel the thickness!  Your strength is proportionate to the cross sectional area of your muscles.

Thankfully, as you grow, the cross sectional area of your muscle increases, as does its length and overall volume. But here’s the key to it all: your muscle volume and weight increases at a faster rate than your muscle cross section does.

So if you are a tall person trying to lift yourself up on the parallel bars, you will find it harder than you did when you were smaller, because your weight has increased by more than your muscle cross section.

Heavyweight boxer Mohammed Ali was 1.91m high and in his prime weighed in at 107kg while Amir Khan, a light welterweight, weighs just 63kg and his height is 1.65m

Amir: Feel my cross section

So Ali is roughly 1.16 times taller than Khan but 1.7 times heavier.

mohammed ali

Ali – it’s all about the cross section

Every inch increase in the length of our muscles is cubed when it comes to the volume of the muscle but only squared when it comes to cross sectional area.

So Ali’s muscle cross section will be larger than Khan’s but nowhere near 1.7 times larger.

Obviously Ali is stronger than Khan and Michaela is stronger than the stoat. “Pound for pound” Khan is stronger than Ali and the stoat is stronger than Michaela Strachan. But that’s not a fair way to look at it. Cross section for cross section they are all roughly the same strength because all four of them are very fit.

So how could Michaela really take on the stoat in a fair challenge? The stoat has a cross sectional area of about 12 sq cm.  You can probably fit around 40 stoat waists into Michaela’s waist. The stoat carried a rabbit weighing a kilogramme. So, to match the stoat, Michaela should carry something weighing 40kg. This is about the weight of a large family dog, like a German shepherd. It’s roughly what British soldiers carried on their backs when they yomped across the Falkland Islands.  Not easy, but certainly more achievable than a male stag.

I’m sure she’d do it to feed her kits.