Foundations · L3 · The mechanics
Levers, Tension & Your Wrist
Why a 10 kg dumbbell can make your muscle pull like it's holding 100 kg — and why grip angle decides whether that force bends your wrist or runs safely through it.
Your muscles never feel "the weight" directly — they feel torque, set by leverage. Understand that one idea and you'll know why a lift has a sticking point, why machines feel different from dumbbells, how far to lower the weight, and — the personal payoff — exactly why your program keeps your wrist neutral to protect the healed scaphoid.
1Every rep is a tug-of-war of torques
A joint doesn't respond to force, it responds to torque — a turning effort, equal to force × lever arm (the perpendicular distance from the force to the joint).1 Two torques fight at every joint:
- The weight's torque = load × the horizontal distance from the weight to the joint.
- The muscle's torque = muscle force × its tiny lever arm (the biceps pulls only ~3.5 cm from the elbow).
To lift, muscle torque must beat the weight's torque. Here's the kicker: the muscle's lever (~3.5 cm) is about ten times shorter than the weight's (a ~35 cm forearm). So to balance a 10 kg dumbbell, the biceps must generate the pull of roughly 100 kg. Your muscles always work at a mechanical disadvantage — that's normal, and it's why tension, not the number on the dumbbell, is the real stimulus from Lesson 2.
2Drag the curl — watch the force explode
Slide the arm through a curl. The weight never changes, but the muscle force and the difficulty swing wildly with the angle — because the lever arm does.
Difficulty peaks when the forearm is horizontal — that's where the weight's lever arm is longest, so the torque (and the muscle force) is greatest. That point is your sticking point.1 At the very bottom and top, the lever arm shrinks toward zero and the lift feels easy. Same dumbbell, completely different demand.
3Strength curves — and why machines feel different
That rise-and-fall of difficulty across a movement is its strength curve. Different lifts have different shapes:
A free weight's resistance follows gravity's lever arm — heaviest where your limb is horizontal, whether or not that's where you're strong. A cam machine reshapes the resistance through the range to better match your strength curve (easing the sticking point); bands get harder as they stretch, which suits ascending lifts.2 None is "best" — they just load the curve differently. For you, machines are also a gift for the wrist: a fixed neutral handle removes the balancing demand.
4How far to move: train the stretch
Range of motion is a real variable, not a vanity one. Across the research, full range of motion produces equal or greater growth than partial range, and the most valuable part appears to be the lengthened (stretched) portion of the lift — loading the muscle while it's long.34
Lower under control through a full range, feeling a stretch in the working muscle, then drive back. If you ever shorten a lift (fatigue, a cranky joint), keep the stretched end and cut the top. (That "lengthened partials beat full range" is a promising but still emerging claim4 — don't over-rotate on it; just don't cut the stretch.)
And tempo? You don't need to grind in slow motion. Rep durations anywhere from about half a second to eight seconds grow muscle about the same; only deliberately very slow reps are worse.5 Control the lowering, lift with intent, don't bounce.
5Your wrist: where the force goes
Now the personal payoff. The scaphoid is a small boat-shaped bone on the thumb side of your wrist that bridges the two rows of wrist bones — so it carries a lot of load and shear. Its worst direction is loaded extension: the wrist bent backwards under weight (with the hand thumb-side-out), which bends and shears the scaphoid.6 A flat-palm push-up forces the wrist to ~80–90° of extension under your bodyweight; a straight barbell does the same to keep your palm under the bar.
A neutral grip — palms facing each other, on dumbbells, a neutral handle, or your fists — keeps the wrist straight, so the load runs axially down the forearm bones instead of bending the joint. Toggle it:
Wrist straight: the load (green) runs straight down the forearm bones. The scaphoid is barely bent — this is your default for presses, rows and squats.
Choose tools that let the wrist stay straight: neutral-grip dumbbells, machine handles, goblet holds, a barbell RDL (the bar hangs from straight wrists), and fists/handles for push-ups. It's not weakness to avoid the barbell bench — it's routing ~100 kg of internal force away from a bone that can't take it bent. A mixed grip keeps heavy pulls from slipping without bending the wrist. See the Program Card for the full swap list.
6Check yourself
What you can now say
- Joints feel torque = force × lever arm, so the muscle pulls ~10× the weight on the bar.
- A lift is hardest where the weight's lever arm is longest — the sticking point.
- Lifts have strength curves; free weights, cams and bands load them differently.
- Train a full range, especially the stretched part; tempo 0.5–8 s is fine.
- A neutral wrist routes force down the forearm, away from the scaphoid in extension.
§References
- Kompf & Arandjelović (2016). Understanding and overcoming the sticking point, Sports Med. PMC4887540.
- NSCA. Forms of variable resistance / strength-curve matching (cams, bands), Strength & Cond J. journals.lww.com.
- Schoenfeld & Grgic (2020). Effects of range of motion on muscle development, SAGE Open Med. PubMed 32030125.
- Kassiano et al. (2023) & Wolf et al. (meta) — long-muscle-length training. PubMed 37015016 · SportRxiv.
- Schoenfeld, Ogborn & Krieger (2015). Repetition duration & hypertrophy meta-analysis, Sports Med. PubMed 25601394.
- Erhart et al. (2020). Wrist movements induce torque/shear in the scaphoid, J Orthop Surg Res. PMC7457810.
Lesson L3 of the dad-mod course · all lessons · glossary