Training science
Lying Leg Curl returns 4.50 per unit of neural cost. Seated Leg Curl returns 4.50. Nothing in the 272-movement library returns more. The Romanian deadlift — the exercise that has replaced the leg curl in most serious programmes — returns 0.94.
Every movement in the database carries two numbers: neural cost, 1 to 10, and stimulus, also 1 to 10. Divide the second by the first and you get the return — what the exercise pays you for each unit of systemic fatigue it charges.
The highest return anywhere in the library is 4.50. Sixteen movements reach it, all of them at cost 2 and stimulus 9, and three of those sixteen are hamstring movements: Lying Leg Curl, Seated Leg Curl and the Glute-Ham Raise. Glutes hold another three. No other muscle group holds more than two.
So the hamstrings are unusually well served at the top of the library. The odd part is which hamstring exercises are up there, and which are not.
Twelve ways to train the posterior chain, sorted by what they pay back.
| Movement | Neural cost | Stimulus | Return |
|---|---|---|---|
| Lying Leg Curl | 2 | 9.0 | 4.50 |
| Seated Leg Curl | 2 | 9.0 | 4.50 |
| Glute-Ham Raise | 2 | 9.0 | 4.50 |
| Nordic Hamstring Curl | 2 | 8.5 | 4.25 |
| Swiss-Ball Leg Curl | 3 | 8.5 | 2.83 |
| Single-Leg Romanian Deadlift | 6 | 7.5 | 1.25 |
| Kettlebell Swing | 6 | 7.0 | 1.17 |
| Romanian Deadlift | 8 | 7.5 | 0.94 |
| Good Morning | 8 | 6.5 | 0.81 |
| Hip Thrust | 8 | 6.0 | 0.75 |
| Stiff-Leg Deadlift | 10 | 5.5 | 0.55 |
| Barbell Deadlift | 10 | 4.5 | 0.45 |
The top of that table is a machine, a second machine and a bodyweight exercise. The bottom is the barbell. A leg curl returns 4.8 times what a Romanian deadlift returns, and exactly ten times what a barbell deadlift returns, for the same target muscle.
This is the paradox in the title. The leg curl is the exercise nobody films, nobody writes a programme around, and plenty of coaches have quietly dropped. On this arithmetic it is the best-value hamstring exercise there is.
Not because a leg curl is a better hamstring stimulus than a Romanian deadlift. It is not, particularly. The stimulus scores are 9.0 and 7.5 — close, and the hinge is arguably underrated at 7.5.
The gap is entirely on the cost side: 2 against 8. A Romanian deadlift asks your spinal erectors to hold a loaded trunk in place, your grip to survive the set, your hips and knees to coordinate under axial load, and your whole system to brace. All of that is real training, and all of it is charged to the same account.
A leg curl asks for knee flexion against a pad. The fatigue it generates is almost entirely local — in the hamstring, where you wanted it — rather than systemic. You walk out of the gym with trained hamstrings and an intact nervous system.
A Romanian deadlift trains your hamstrings and taxes everything else. A leg curl trains your hamstrings. That is the whole difference, and it is worth 4.8 times.
You might expect the injury-prevention literature to point back towards the barbell. It does not.
The best-evidenced hamstring intervention in sport is the Nordic hamstring curl — cost 2, stimulus 8.5, return 4.25, fourth in the table above. Van Dyk, Behan and Whiteley pooled 15 studies covering 8,459 athletes in the British Journal of Sports Medicine in 2019 and found that programmes including the Nordic exercise roughly halved the rate of hamstring injuries — a 51 per cent reduction.
No barbell. A bodyweight exercise at the bottom of the cost scale.
The same pattern holds one joint over. Harøy and colleagues, also in BJSM in 2019, gave 339 Norwegian footballers a single adductor exercise — the Copenhagen adduction — against 313 controls, and the intervention group's risk of reporting groin problems was 41 per cent lower. One cheap isolation movement, three times a week in pre-season and once a week in-season.
Two of the strongest prevention findings in team sport come from exercises that would cost you 2 on this scale. That is not a coincidence, and it is not an argument against barbells. It is an argument that cheap does not mean weak.
Three honest limits, because the table above invites one bad conclusion.
First, a leg curl trains knee flexion. The hamstrings also extend the hip, and the hinge trains that, under load, in a position sport actually uses. Drop the hinge entirely and you have trained half a muscle group.
Second, part of what makes a Romanian deadlift expensive is what makes it valuable: bracing a loaded trunk is a trainable quality, and it does not appear on a leg curl machine at any volume.
Third, the return number says nothing about transfer to a sprint. A high return means the stimulus is cheap, not that it is specific.
Keep one hinge in the week, on the day you can afford an 8. Then stop treating the rest of your hamstring work as a hinge problem.
If the plan currently reads Romanian deadlift on Monday and Romanian deadlift again on Thursday, the second one is costing 8 and returning 0.94 on a day when the account is already low. A leg curl in that slot gives you 9.0 of stimulus for 2 — and leaves Thursday's actual session intact.
The machine in the corner that nobody uses is not there because it is a soft option. It is there because it is the cheapest way to buy a hamstring.
A ten-second check-in — sleep, mental energy, soreness — and a score from 0 to 100. The hinge appears on the days you can pay for it. On the other days, the session still trains the same muscle.
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