Lie on the mat on your back and put your feet up on the bench with your knees at about a 90-degree angle.Keep your feet shoulder-width apart. Put one hand down and let it rest on the mat next to your body. Hold your head behind your ear with the other hand and point the elbow outwards.
From the starting position lead the motion by contracting your abdominals while moving the arm with the hand holding your head in a diagonal direction to the opposite knee. Lift your shoulder blade from the mat when you are in the upper position. Exhale as you are going up and inhale as you return to the starting position. Do the required number of repetitions and alternate your hands, namely the sides of the oblique cruches.
Make sure your legs and feet are in the correct position. Avoid bending at the waist when you are in the lower starting position. Watch for correct breathing.
It is important to learn the correct breathing technique to do this exercise well.
Oblique crunches are an interesting alternative to crunches. They strengthen the abdominal muscles from a different angle and involve a turning motion.
Their main disadvantage is that you cannot really increase the resistance by holding a weight when performing them (although it is possible to use resistance bands).
There are many different ways to do oblique crunches. One possibility is to hold the hands touching your head (do not lock your fingers behind the neck, this will push your head forwards and can lead to problems with upper back) touching the opposite knees with your elbows. You can either lay on the floor/mat with the legs under 45-degree angle or with the legs in 90-degree angle (more common form).
You can also do several crunches with one hand pushing against the floor, then change the hands and continue on the other side.
Another variation is to cross your legs, do several crunches; then change the legs.
Notice: there is a difference between crunches and sit-ups. In sit-ups you lift your upper body completely into or very close to sitting position. This movement is mainly strengthening internal muscle called psoas, not the external visible abdominal muscles.