Lying Dumbbell Leg Curl

You choose a dumbbell too heavy to grip with the feet, so it slips and breaks the set.

You swing the weight up with momentum instead of curling it with the hamstrings.

You let the hips rise off the bench as the knees bend.

Squeeze the dumbbell hard between the arches of your feet before the first rep.

Move slowly in both directions; control matters more than load on this variation.

Keep the hips pinned to the bench so the knees are the only joint moving.