Running with more muscle: body mass and running economy

Extra mass costs you on a run: carry more kilos and every kilometre costs more energy, roughly in proportion to the weight you added, at least in the men tested.¹ But muscle isn't the same as dead weight in a vest, and the one HYROX lab study found body mass and muscle mass weren't linked to run times while body fat was.² So the short answer is that muscle costs a little on the run, and how much depends on whether it brings more strength and engine with it.

What carrying more weight costs#

Running on flat ground costs about 3.40 joules per kilo of body mass per metre, and in Minetti's treadmill data that cost didn't change with speed.³ Every extra kilo is carried for every metre.

The cleanest test of added mass is to strap weight on runners and measure them. Gaddie's group did that with 36 runners at marathon pace, adding 1.6, 2.4 or 3.2 kg to the torso.¹ Energy cost rose from 13.2 kcal per minute unloaded to 13.5, 13.6 and 13.7. In the men, the rise followed body mass almost one for one: their model is a 0.98% increase in energy cost for every 1% of added mass, minus a small constant. The women's response was messier, about 3.5% up with the lightest load and no clear step after that.

Hills make it worse. In Minetti's slope data, running up a 45% slope cost 18.93 joules per kilo per metre, more than five times the flat cost.³ Nobody races up 45%, but the direction holds on any climb: extra kilos cost more per metre the steeper it gets, because you're lifting them as well as moving them. On a flat HYROX course that matters less than on a hilly trail race.

That's the case against mass. It isn't the whole story, for two reasons. A vest adds weight and nothing else. Muscle adds weight and can add force, and force is what running economy partly depends on.

Why per-kilo numbers punish heavier athletes#

There's a catch in how the numbers are reported. Almost every engine number you see is divided by body mass: VO2max in ml per kilo per minute, running economy in ml per kilo per kilometre. That assumes oxygen use scales one to one with mass. It doesn't.

Bergh's group measured 134 endurance athletes and fitted oxygen uptake against body mass.⁴ Submaximal oxygen uptake rose with mass to the power 0.76, and maximal oxygen uptake to the power 0.71. Both are well under 1. The implication is that a heavier athlete tends to use less oxygen per kilo at a given speed, and to show a lower VO2max per kilo, even at a similar level of fitness. That's why running researchers have recommended reporting economy per kilo to the 0.75 power instead of per kilo.⁵

To put a number on it: by Bergh's exponent, a 74 kg athlete would be expected to show about 2% lower VO2max per kilo than an equally fit 69 kg one, and a 79 kg athlete about 4% lower, just from how oxygen scales with size. That's a cross-sectional pattern across different people, though, not what happens when one person puts on muscle.

What 5 kilos does to a real score#

The hybridAF score divides both halves by bodyweight: lifts as a multiple of your weight, VO2max per kilo. That's a design choice of ours, and the standards page says outright that dividing lifts straight by bodyweight slightly favours lighter lifters. Here's what that means on real numbers.

My last weigh-in on file is 69 kg, with a bench at 1.82 times bodyweight, a deadlift at 2.21 and a squat at 1.31, and a VO2max estimate of 47. Now add 5 kg, then 10, and change nothing else: same kilos on the bar, same litres of oxygen per minute.

Same lifts, same engine, more bodyweight

bodyweight (kg)	Strength Index (points)	Engine Index (points)	Hybrid Score (points)
69	73	57	64
74	69	47	56
79	65	38	48

Ken's lifts and VO2max estimate held constant in absolute terms and re-graded at each bodyweight with hybridAF's scoring code (src/lib/score-grading.ts, src/lib/engine-grading.ts), age 20. Read 2026-10-22.

At 74 kg the VO2max reading drops from 47 to 43.8 and the Engine Index from 57 to 47. The Strength Index slips from 73 to 69 (73 is what these rounded ratios give; my live figure is 74). The Hybrid Score falls from 64 to 56. At 79 kg it's 48. The engine side drops harder because, at my age, each ml/kg/min near the middle of the VO2max ladder is worth about 3 Engine Index points.

That's not a verdict on gaining muscle. It's the break-even. Five kilos is about 7% of my weight, so to hold every number I'd need about 7% more on each lift and about 7% more absolute oxygen uptake. If the new muscle brings that, the score holds. If it brings more, the score goes up. The scaling above suggests a heavier athlete with the same fitness will read slightly low on any per-kilo engine number, ours included, so the per-kilo VO2max part of this is a little harsh on mass.

If you want to run your own version, the Hybrid Score calculator takes a bodyweight, your lifts and a run, and grades them with the same code.

What muscle can buy back#

Running economy, the oxygen cost of a given pace, is a big part of the answer. Among elite runners with similar VO2max, it predicts performance better than VO2max does.⁵ Strength is one of the things that improves it.

A well-known trial here is Støren's.⁶ Seventeen well-trained runners, eight of whom added heavy half squats (four sets of four-rep maxes, three times a week, eight weeks) to their normal running. Their squat 1RM went up 33.2%, running economy improved 5.0%, and time to exhaustion at their maximal aerobic speed went up 21.3%. VO2max didn't change, and neither did body weight. The control group didn't change at all.

That last detail is the key one for this question: Støren's runners got stronger and more economical without getting heavier. Reviews of running economy list resistance and plyometric training among the strategies that have improved it, most likely through neuromuscular changes.⁷ The proposed mechanism is better use of elastic energy and less energy wasted in braking.⁵

So it helps to separate two kinds of "more muscle". Strength gained without much extra weight, like in Støren's trial, has a track record of helping economy. Extra size adds mass that has to pay for itself in force and engine.

What the HYROX data shows#

The one lab study of HYROX, Brandt 2025, measured body composition in 11 recreational athletes before a simulated race and correlated it with their times.²

What tracked HYROX run times (Brandt 2025, n = 11)

	Value (rho)
Body fat %	0.68
Muscle mass %	-0.07
Body mass	-0.11
VO2max	-0.73

Spearman correlation with accumulated run time in a simulated Open race; positive means a higher value went with slower runs. The text reports VO2max as -0.73; the table prints 0.73. Source: Brandt et al., Front Physiol 2025, Table 3.

Body fat percentage went with slower runs (0.68). Body mass and muscle mass percentage didn't go with anything (minus 0.11 and minus 0.07, nowhere near significant). VO2max went with faster runs. Their body masses ranged from 62.4 to 108.5 kg, so there was real spread to find an effect in.

Eleven people can't settle anything, and correlation isn't cause. But it points the same way as the rest: carrying extra weight that does nothing is expensive, and being heavy for a hybrid athlete isn't automatically the problem.

What this means if you lift and run#

Don't compare your per-kilo VO2max with a much lighter training partner's at face value. On Bergh's exponent, an 85 kg athlete would be expected to read about 7.5% lower per kilo than a 65 kg athlete of the same fitness, so a gap of nearly 4 ml/kg/min at around 50 can be size, not engine.⁴ A race time isn't divided by anything. If you want to know whose engine is better, race each other.

If you're in a phase where you're deliberately putting on size, track bodyweight next to your lifts and your runs, not on its own. A heavier scale with a heavier bar and a steady 5K pace is a good block. A heavier scale with the same bar and a slower 5K is the expensive kind of mass. The ratio tells you which one you're in long before a race does.

What none of these studies did is add muscle to the same people and then measure their running. That trial hasn't been run, as far as these sources show, so the real answer for a specific gain in size is still a judgment call. The scoring logic gives you a way to keep it honest: if your bodyweight goes up, your lifts and your engine have to go up at least as fast.

For the strength side of this, see strength training for runners and running for lifters. For where your per-kilo VO2max sits, the VO2max by age chart. The rest is on the engine hub.