Leg curl and leg extension

#Leg curl — what the pattern is responsible for

Knee flexion, isolating the hamstrings from their hip-extension job. See the hamstrings anatomy lesson — hip angle at the time of the curl changes hamstring length throughout the movement, and that is not a minor detail.

#Tier 1 — Equivalent

Seated leg curl ↔ Nordic curl (assisted or partial-range for most lifters). Both keep the hip flexed, training the hamstring at a long length throughout the knee-flexion range. The Nordic curl adds a substantial eccentric-strength demand that most lifters cannot yet control through a full range — start partial and build the range over weeks.

#Tier 2 — Close, with a stated cost

Lying leg curl, if a seated machine is unavailable. Trains knee flexion adequately but with the hip extended, which slackens the hamstring throughout — a real, not cosmetic, difference from the seated version. Use it if it's what you have; know that switching to seated when available is a genuine upgrade, not a lateral move.

#Tier 3 — Last resort

Stability-ball hamstring curl, dumbbell-between-feet curl. Functional at very light loads or with no machine access. Load ceiling is too low for meaningful long-term progression.

#Leg extension — what the pattern is responsible for

Knee extension in isolation, closing the rectus femoris gap that squatting and pressing patterns leave open — see the quadriceps anatomy lesson.

#Tier 1 — Equivalent

Leg extension, seat reclined ↔ Sissy squat. Both extend the hip while extending the knee, keeping rectus femoris genuinely long throughout. The sissy squat additionally loads the vasti at length; a real, slightly more complete stimulus if you can execute it with control.

#Tier 2 — Close, with a stated cost

Leg extension, seat upright (the more common machine default). Still trains the vasti well; trains rectus femoris less completely than the reclined version, because the hip stays more flexed throughout. Recline the seat if the machine allows it — this is a free upgrade, not a different exercise.

#What Jeff Nippard's guide gets wrong here, specifically

His substitution table's advice for leg extension is: "If you're unable to perform leg extensions, simply remove them from the program since they are not a mandatory exercise for quad growth."

This sidesteps the actual question. Leg extensions are not "mandatory" in the sense that no single exercise ever is — but the specific gap they close (rectus femoris, undertrained by every squat and press pattern) does not close itself just because the exercise is removed. Dropping it without a replacement means accepting that gap, which is a defensible choice if no substitute is available, but it should be stated as a trade-off rather than dismissed as a non-issue. Tier 1 above (sissy squat) is the actual substitute if the machine is unavailable — not removal.