The lying leg raise is done by lying on the floor on the back. It is done without apparatus except possibly cushions or weights for added resistance.
Practitioners generally caution to keep the lower back in contact with the floor and place hands to sides or under lower back for support.
Due to leverage, the hardest portion of a supine (laying) leg raise is generally the first part when the legs are on the floor, as this is when the femur is parallel with the earth and perpendicular to the pull of gravity.
A seated leg raise is halfway between a laying raise and a hanging/suspended/tractioned one. They are done seated on an elevated surface. Usually the hands are placed on the surface (or arm rests) and bear some of the body's weight to lessen the weight borne on the buttocks and increase abdominal recruitment.
Leg raises can also be performed hanging onto an overhead bar. These are known as hanging leg raises and are more challenging than lying leg raises.
This variation of leg raise allows the pelvis to freely rotate. It is generally the more difficult variation for the abdominal muscles due to having to support the pelvic weight as opposed to simply stabilizing its alignment.
Alongside this however, it is also potentially the easiest to use bad form on, because users may use a swinging motion to 'cheat' by building up momentum. If the abs are not properly engaged, the spine can easily hyperextend and go into anterior pelvic tilt. It should be going into posterior pelvic tilt if the movement is being done to target the rectus abdominis.
This movement can also be done with "ab slings" which hold the humeri in ~90 degrees of shoulder flexion. This allows to do a more traditional crunch (exercise) by bringing the knees up to touch the elbows. It is however possible to assist in this movement by using the lats and other muscles to perform shoulder extension.
The demands of force on the muscles can be increased through altered posture or apparatus.
Extending the knee joint (often called a straight leg raise]) increases the demands of leverage on both hip and spine flexors. It also reduces the contribution of the rectus femoris muscle via over-active insufficiency, creating more work for the non-quadriceps hip flexors.
The generic term "leg raise" usually indicates a bended knee, though the term "knee raise" is used to distinguish it from the category which includes both variations.
The exercise can also be done weighted, such as wearing ankle weights or weighted boots, holding a dumbbell between the feet, or slipping one's feet through kettlebells. These weights are also affected by the increased leverage of a straightened knee.
Some prefer to lift as high as possibly through a full range of motion for each repetition. Others do partial range repetitions to focus on more difficult portions of the exercise (generally when the leg is parallel with the ground).
"Leg raise" usually indicates that the mobilization only occurs in the joints of the leg. Both ab muscles (rectus and transversus abdominis) are used to isometrically stabilize the spine to resist extension and posterior pelvic tilt that would otherwise occur with the legs lifted in front.
When the lumbar spine flexes, the rectus abdominis is worked dynamically instead of isometrically, increasing its activity, but lessening the stabilizing effect of the TA. This variation is called a leg-hip raise.
Leg-hip raises are usually done suspended as the pelvis is more free to rotate. With laying/seated leg raises, the friction between the posterior of the pelvis with the surface it is in contact with impedes rotation.
Some advise to lower down to starting position slowly and with control. Others do the exercise more dynamically and with speed.