Leg extension weight needs a machine-specific baseline
In short
Leg extension torque curves from a variable cam resembled a dynamometer at 100°–35°, peaking near 70°–65°. This does not provide a machine-to-machine weight conversion. Keep equipment and settings attached to each training record.
Similar torque did not guarantee similar muscle activity. A September 29, 2026 study compared a variable-cam leg extension system with an isokinetic dynamometer. Torque describes the turning effect around a joint. A number on a weight stack is therefore a poor description of everything happening across a repetition, even before a lifter tries a different machine.
Where did the two devices agree?
Twenty-two men, averaging 20.55 years old, completed four assessments in randomized order, two per device. Researchers assessed concentric knee extension from 100° to 0° at 5° intervals. The closest agreement covered the initial and middle range. Muscle activity still differed between conditions. Neither a similar sensation nor an electrical activity reading alone establishes which device delivers better training outcomes.
| Comparison | Observation | Interpretation limit |
|---|---|---|
| Variable cam versus dynamometer | Similar torque curves at 100°–35° | No universal stack-weight conversion |
| Curve peak | Around 70°–65° | Not a prescription to train only there |
| Muscle activity | Differed between conditions | No long-term growth ranking |
What weight should you use at another gym?
This experiment did not compare identical stack labels across commercial gym machines. It cannot tell you which number on a new machine equals 40 kg on your usual one. A practical editorial recommendation is to start a fresh baseline when the equipment changes. That keeps a new setup from looking like an unexplained strength gain or loss. It is a way to interpret a log, not a progression method tested in this study.
- Record the machine and the seat, backrest and pad settings.
- Use a manageable load to establish a controlled movement range.
- Compare load and repetitions within the same equipment and setup.
A useful record identifies the task behind the number. Keep the load actually lifted rather than inventing a conversion to your old machine. A leg extension result also does not become a squat 1RM for the Muscle Index calculator. For a separate question about training transfer, read how squat and leg extension gains transfer. Improvements within one exercise and improvements in a different lift require different comparisons.
This was an acute measurement study, not a long-term hypertrophy or injury-prevention trial. It cannot establish that a particular cam is best for every lifter, or that training only around the peak angle improves results.
Frequently asked questions
Should leg extension weight stay the same across machines?
The study supplied no machine-to-machine weight conversion. Establishing a manageable baseline with the new setup is a logging recommendation, not a tested prescription from the paper.
Should repetitions be limited to 70°–65°?
That interval describes the approximate torque peak. The experiment did not compare training only there with training through other ranges.
Does a variable cam build more muscle?
This study did not measure long-term muscle growth. Torque curves and muscle activity differences cannot establish a hypertrophy advantage.
Source: PubMed · Frontiers in Sports and Active Living (published )