Reverse Abdominal Crunches – Biomechanics
Reverse abdominal crunches are widely practiced in fitness training. Too many people yearn to do this exercise to reduce their waist size, even without knowing if they are competent to do this exercise (because incompetent people lead to back injuries).
Reverse abdominal crunches are performed by raising both legs at the same time, keeping them straight. The lower extremities contribute about 32% of the total body weight (This approximate figure is based on Williams and Lissner, 1962. Vide: Textbook: Basic Biomechanics Explained by John Low & Ann Reed). We will take an example of an individual who weighs 100 kg. When this individual attempts to perform reverse sit-ups (straight leg raises), he must lift legs whose mass is around 32 kg. The straight leg raise is the bilateral flexion of the hips and the agonist of the hip flexion is the iliopsoas muscle. But to avoid bending the knee under the influence of gravity, the quadriceps must also work. The role of the abdominal muscle is to stabilize the proximal attachment sites of the iliopsoas (and also the quadriceps), that is, the lumbar vertebrae and the pelvis. If the abdominal muscle cannot stabilize the lumbar vertebrae and pelvis, lumbar lordosis inevitably occurs. In fact, the challenge of the reverse abdominal crunch exercise is not only to raise the legs, but also to strictly ensure the “stabilizing” function of the abdominal muscles.
To understand the amount of gravitational torque acting on the hip joints during abdominal reverse crunches, one must know how to predict the ‘segmental center of gravity’ of the lower extremities. The segmental center of gravity of the lower extremities may be located just above the knee joint, as the thigh contributes 10% of the total body weight, the leg contributes 4.5% of the total body weight, and the foot contributes 1.5% of total body weight (based on Williams and Lissner). Gravity tends to act at the segmental center of gravity. To calculate the amount of gravitational torque (GT), several factors must be considered such as (a) effort arm of the muscle (b) moment arm of the muscle (c) resistance arm (d) mass of the lower extremities. Except the moment arm of the muscle, the value of all other factors can be predicted.
Suppose the length of the lower limbs of this 100 kg individual is 90 cm (from hip to heel). We can also assume that the segmental center of gravity of the lower extremities is located 40 cm from the hip joint and the iliopsoas muscle inserts 10 cm from the hip joint on the lesser trochanter.
Now;
1. Arm of effort = 10 cm (the distance between the hip joint and the insertion point of the iliopsoas)
2. Resistance arm = 40 cm (the distance between the hip joint and the center of gravity of the segment)
3. Lower limb mass = 32 kg (16 kg per limb)
Gravitational torque (GT) = [mass x acceleration due to gravity] x resistance arm in meters
= [32 kg x 9.8 ms-2] x 0.4m
= 125Nm
Anti Gravity Torque (AGT) = GT / Force Arm in meters
= 125Nm / 0.1m
= 1250 newtons
(Note: AGT must be produced by the iliopsoas muscle. Each iliopsoas muscle must generate more than 625 Newtons to cause straight leg raising.)
To prevent lumbar lordosis, the abdominal muscles must also generate more than 1250 Newtons to stabilize the lumbopelvic unit. This large force requirement to produce AGT is also needed in the initial few degrees of leg elevation because as the angle of elevation increases, the resistance of the gravity arm decreases. Therefore, the AGT requirement is directly proportional to the ‘variable resistance gravity arm’. We need to input another factor (cosine θ) into the formula to calculate AGT as follows;
AGT = GT x cosine θ / Arm of effort in meters
Where, Θ – indicates the angle between the raised legs and the floor
Just having two examples, let us understand that (a) lower θ, higher antigravity torque (b) higher θ, lower antigravity torque.
Example: 1 (angle between the raised legs and the ground = 30 degrees)
Antigravity torque = 125 x cosine 30 degrees / Arm of effort in meters
= (125 x 0.866) / 0.1m
= 1082.5 Newtons
Example: 2 (angle between the raised legs and the ground = 60 degrees)
Anti-gravity torque = 125 x cos 60 degrees / Arm of effort in meters
= (125 x 0.5) / 0.1m
= 625 newtons
These analyzes clearly indicate that the closer the legs are to the ground (a) the greater the GT (b) the greater the AGT (c) the greater the stabilizing role of the abdominals (d) the greater the magnitude of lumbar lordosis if the abdominal muscles are not strong enough. Only people who have the ability to control lumbar lordosis in the initial few degrees of leg elevation can express themselves as eligible to perform reverse crunches. Ineligible individuals, who cannot control lumbar lordosis in the early degrees of leg elevation, could lead to unnecessary unrecoverable injury.