Three signals to modify

Position degrading — the pelvis rocking, ribs flaring, shoulders shrugging. Feeling it in the wrong place — neck strain in an abdominal exercise, lower back in a hip exercise. And pain during the movement, which is categorically different from effort.

All three mean the exercise as performed is not doing what it is meant to. Continuing simply practises the compensation.

The order of modification

Range first. Most exercises become appropriate at a smaller range long before they become appropriate at a lighter load, and reducing range keeps the movement pattern intact.

Then simplify — bend a knee, take one leg out, reduce the lever. Then change the spring, and finally substitute a different exercise entirely.

Modification order
StepChangeWhy this order
1Reduce rangeKeeps the pattern intact
2Shorten the leverReduces load without changing spring
3Remove a limbHalves the demand, keeps the shape
4Change the springCoarse but effective
5Substitute the exerciseLast resort

Modification is not failure

A modified exercise performed well produces more benefit than a full version performed poorly. This is genuinely true rather than consolation, because the compensated version trains the compensation.

Experienced practitioners modify constantly and think nothing of it.

When to skip entirely

Pain during the movement, a contraindication for your condition, or an exercise you cannot perform safely even fully modified. Skipping one exercise costs almost nothing in a session of twenty.

Tell your teacher why rather than quietly opting out — it usually produces a better alternative.