Unilateral Romanian Deadlift

The deadlift is also known as the king of all exercises. Looking at the list of muscles involved above, you can infer why.

The calf muscles bring the lower legs, which extend forward, back into alignment above the ankles when you stand up and also provide stabilization throughout the entire movement.
The quadriceps work hard to extend the knees.
The hamstrings and glutes work to bring the upper body upright by aligning the thighs and back.
The erector spinae, which runs along the spine, helps straighten the back.
The abdominal muscles mainly provide stabilization (the rectus abdominis as an antagonist to the erector spinae and the oblique abdominals as a brake against lateral tilting)
The muscles in the upper back and neck engage to pull the shoulders upward and backward.

The Romanian deadlift falls between a normal deadlift and the straight leg deadlift. In the straight leg deadlift the legs remain nearly straight, so the quadriceps are barely engaged (only to stabilize). The hamstrings are stretched more and therefore loaded more. You also bend farther forward from the back, placing more load on the lower back and glutes.

In the Romanian deadlift, the knees are bent slightly more, but not as far as in a normal deadlift. In the unilateral Romanian deadlift, you perform the exercise on one leg. The other leg moves backward for balance. The hamstrings and gluteal muscles are also engaged to keep this leg extended backward.

Unilateral Romanian deadlift execution

1
Move your glutes backward and your chest forward while bringing your leg backward (the leg on the opposite side from the hand holding the dumbbell). Make your back slightly more arched than its natural curvature. Allow the knees to bend slightly, but not as far as in a normal deadlift (approximately halfway).
2
Continue until you feel the greatest possible stretch in the hamstrings.
3
Raise yourself again by pushing your pelvis forward and lifting your back (see notes). In the same movement, bring the extended-back leg back and let it hover just above the floor*. Continue until your back is upright with its natural curve. Just as when you normally stand upright. Do not extend it too far and therefore do not create an exaggerated curve in your back at the end of the movement (no hyperextension).
4
Repeat.

Unilateral Romanian deadlift

Muscles involved:

Legs:

Hamstrings
Quadriceps (vastus lateralis, vastus medialis, vastus intermedius and rectus femoris)
Sartorius

Glutes:

Gluteus maximus, medius and minimus

Pelvis:

Iliacus (hip flexor)
Psoas major (hip flexor)

Calves:

Gastrocnemius
Soleus

Abdominals:

Rectus abdominis (rectus abdominis muscle)
Obliquus externus abdominis (external oblique muscle)
Obliquus internus abdominis (internal oblique muscle)
Transversus abdominis (transverse abdominal muscle, 'corset muscle')

(Lower) back and spine:

Erector spinae
Lumbar multifidus
Quadratus lumborum

Back and neck:

Trapezius
Rhomboid
Levator scapulae

Unilateral Romanian deadlift setup

Stand with the dumbbells in your hands, with a dumbbell directly in front of the legs and your palm facing backward.
Tighten your abdominals and glutes like a natural weightlifting belt to protect your back.

Unilateral Romanian deadlift technique

The Romanian deadlift places more emphasis on the hamstrings, lower back and glutes than the normal deadlift.

The farther you tilt forward, the greater the stretch on the hamstrings. This also applies to the extent to which you keep the legs straight. To avoid bending the legs more than desired and to avoid falling forward, move your glutes backward from the ankles (and therefore not from the knees).

*You can perform the unilateral Romanian deadlift on one leg per set as described here, but also per repetition, alternating legs.