Body size and the pump: weight, height and forearm muscle

#Weight: the same hold, a harder grip

#The arithmetic

Your hands carry whatever part of your weight your feet don't. On a vertical wall that might be around 40 % of your body weight; on a roof, most of it. That load is shared between the hands that are holding on. Then:

grip per hand as % of max = (body mass × share on the hands ÷ hands holding) ÷ your maximum grip on that hold

A 70 kg climber with 40 % of their weight on two hands holds 14 kg per hand. If their one-hand maximum on that edge is 45 kg, that's 31 %, below critical force. Mid-move, with one hand off, the other hand holds 28 kg, or 62 %: inside the band where blood flow stops. That's why the moves pump you and the stances don't.

#See it

Choose a wall angle, then change body mass and strength. The "what if" lines below compare 5 kg heavier, 5 kg lighter, 10 % stronger, and better footwork.

Notice that better footwork often changes the number as much as a big change in weight or strength. Moving 10 % of your body weight from the hands to the feet (from 40 % to 30 % on a vertical wall) lowers the grip you need as much as getting about a third stronger.

#Strength-to-weight is what matters

#Muscle size: bigger isn't automatically better

#What a bigger forearm gives you

A thicker forearm muscle can usually produce more force, so the same hold becomes a smaller share of your maximum. In recreational climbers, the deep finger flexor was thicker in men than in women even after adjusting for body size. Each extra millimetre of its thickness went with about 57 % higher odds of being a lead climber rather than a boulderer.

#What it costs

Force comes with pressure. When a muscle contracts, the pressure inside it rises and squeezes its own blood vessels. Researchers have proposed that a larger, stronger muscle working at the same share of its maximum builds more absolute force and pressure, which may partly explain why men often tire faster than women in sustained holds. The picture depends on the task, though. In repeated grips (closer to climbing than one long hold), women and men tired at the same rate, and in elite climbers the forearm re-oxygenated equally fast in both sexes.

#Where the threshold sits varies

The grip at which blood flow stops isn't one number. It has been reported anywhere from about 20 % to 70 % of maximum depending on the muscle and fibre type, and 45–75 % for the forearm. People differ, and the same person's threshold may shift with muscle size and training.

#Height and arm span

Taller climbers and climbers with a long arm span (a high "ape index") reach further, which can mean fewer moves. But they usually weigh more, and their limbs act as longer levers. When studies put body measurements into the same analysis as trainable factors, body shape explained very little:

No study we found measured the pump directly against height or arm span.

Warning

Don't chase a lower body weight to climb harder. Eating too little for the training you do (low energy availability) harms health and performance: bones, hormones, immunity, mood and strength. That's what the IOC consensus on Relative Energy Deficiency in Sport (REDs) warns about. If weight is a question for you, talk to a sports doctor or dietitian. Young climbers should never diet for climbing.

#The evidence

Numbers from adults only.

• Measured Critical force relative to body mass explained 61 % of sport and 26 % of bouldering ability. Giles et al. 2021: 129 adult climbers. Int J Sports Physiol Perform 16(7):942–949.
• Measured Mean critical force was 41.0 % of maximum (SD 6.2). Giles et al. 2019: 11 adult male climbers. Int J Sports Physiol Perform 14(7):972–979.
• Measured High-level climbers are small in stature with low body mass and body fat; absolute strength isn't unusual, but strength-to-mass is high. Watts 2004, a review. Eur J Appl Physiol 91(4):361–372.
• Measured Strength and strength-to-weight were higher in climbers than non-climbers and in men than women. Forearm re-oxygenation during repeated grips was faster in climbers, with no sex difference. Strength-to-weight correlated with on-sight grade in the female climbers (r² = 0.946). Philippe et al. 2012: 12 elite climbers (women and men) and 12 non-climbers. Eur J Appl Physiol 112(8):2839–2847.
• Measured Deep finger flexor thickness was greater in men even after adjusting for body size and training; each 1 mm went with 57 % higher odds of being a lead climber (OR 1.573). Caro-Betancur et al. 2026: 50 recreational climbers (33 men). Sci Rep 2026.
• Measured In repeated handgrips at 50 % of maximum (5 s on, 5 s off), women (341 N max) and men (480 N max) lasted equally long (793 vs 685 s, not significantly different) and fatigued at the same rate. Gonzales & Scheuermann 2007: 11 women, 11 men. J Sports Sci Med 6(1):98–105.
• Measured Women are usually less fatigable than men in similar-intensity isometric contractions, but the difference depends on the task. Hunter 2014, a review. Acta Physiol 210(4):768–789.
• Measured The occlusion range is wide and muscle-dependent (around 20–70 % of maximum). Higher absolute force and muscle mass may shorten hold times through higher pressure inside the muscle, and a weaker muscle needs a higher share of its maximum for the same task. Heinzl et al. 2025, summarising earlier work. Sci Rep 15:1250.
• Measured Body measurements explained 0.3 % of sport-climbing performance, and trainable factors 58.9 %. Mermier et al. 2000: 44 climbers (24 men, 20 women). Br J Sports Med 34(5):359–365.
• Measured Body shape and muscle measures explained under 4 % of climbing ability, and trainable variables 46 %. Laffaye et al. 2016: 41 climbers (15 novice, 16 skilled, 10 elite). Scand J Med Sci Sports 26(10):1151–1159.
• Measured Low energy availability harms health and performance. Mountjoy et al. 2023, IOC consensus statement on REDs. Br J Sports Med 57(17):1073–1098.
• Model The calculator's share of weight on the hands for each wall angle is an illustration, not a measurement.

#Model assumptions

• Grip per hand = body mass × share on the hands ÷ hands holding. The share for slab (20 %), vertical (40 %), overhang (60 %) and roof (85 %) is illustrative; real values depend on the footholds and your position.
• "Max one-hand grip on this hold" is the most weight one hand could hold on that particular hold. It's lower on small or sloping holds.
• Critical force is set at 41 % (the Giles et al. 2019 mean) and the occlusion band at 45–75 % (Barnes 1980). Your own values may differ.
• The time estimates use the critical force model with a 3,000 %·s reserve.
• It leaves out muscle size and pressure effects, and fatigue that lowers your maximum as you climb.

#Try it

#Common mistakes