Common Errors in Abdominal Training
It is common for inappropriate or ineffective exercises to be implemented when there is an infatuation with challenging a specific muscle group (through every angle and method possible). The rectus abdominis is one such group. Often exercisers attempt every type of crunch and ab-related exercise to encourage a “six-pack.” In many cases the exercises are hip flexor dominated and actually provide little improvements in the trunk flexors, as seen in the anchored incline sit-ups, leg lifts, and 90 degree hanging knee raises. It is important for personal trainers to recognize these exercises place certain individuals at risk for low back pain and in some cases disc injury.
Examining the regional anatomy should clarify any confusion; the rectus abdominis attaches to the pubic symphysis of the pelvis, whereas the psoas major (hip flexor) attaches to the femur and the lumbar spine. Since the abdominals do not attach to the femur, the abdominals do not move the legs, but rather the pelvis and the upper spine via the anterior pull on the ribs. Performing straight leg lifts illustrates this as the movement is controlled by the hip flexors, only using the abdominals to maintain stability until a change in pelvic tilt occurs. Under the long resistance arm (straight legs) the pelvis commonly migrates forward causing disc compression making it contraindicated for most individuals. Therefore when examined in greater detail, the movement does not challenge the rectus abdominis in a manner that promotes hypertrophy or significant strength adaptations and the joint mechanics involved place the lower back at increased risk for injury. Personal trainers should evaluate movement techniques and mechanics to ensure undue stress is removed from the spine rather than increased due to poor exercise selection.
A certified personal trainer’s prudent examination of the biomechanics involved in abdominal movements would find the fibers of the rectus abdominis serve as the prime mover for isolated resistance exercises like the reverse crunch and curl-up where a posterior pelvic tilt is used. In other cases, the rectus abdominis serves as a phasic accelerator, like in chopping exercises often accompanying some level of hip activity in a closed chain position. Standing abdominal work generally encourages a more efficient connection between force couples of the anterior chain and places less stress on the lumbar spine due to reduced torques in the area. But what about hanging, closed chain exercises?
Similar to supine leg lifts, hanging leg raises present an inherent conflict between force couples, but with less consequence to the intervertebral discs as the resistance arm is shortened and the torque on the lumbar spine reduced. Again, the fact that the rectus abdominis does not attach to the femur suggests the leg lifting aspect is primarily controlled by the hip flexors until a posterior tilt is attained. This requires a movement range much beyond 90 degrees of hip flexion for abdominal focus. From 0-90-0 degrees the hip flexors undergo concentric and eccentric contractions during the movement, while the rectus abdominis serves as one of the stabilizers to deal with the resultant torque produced. When swinging (momentum) the mass of the legs against gravity, the torques increase and may demand significant deceleration, placing stress on the lumbar spine, depending on the length of the lever arm. Personal trainers should also recognize that when performed in a controlled limited range, movements are less risky but are still primary managed by the hip flexors as a posterior pelvic tilt is not attained.
In summary, personal trainers should be conscious of the fact that the insertion of the hip flexors causes a consequent stress on the lumbar spine and pulls the superior aspect of the pelvis anteriorly during the concentric phase of the movement. During all exercises that focus on the rectus abdominis, a posterior pelvic tilt should be attained unless standing closed chain or specific hip activity (overhead chops) or rotation are used (MB side throws). When the lower body becomes the resistance arm and an anterior pelvic tilt increases so does the lumbar convexity and disc compression forces. This consequently, places the lumbar area in a higher risk for injury. This potential hazard is increased further if a personal trainer or partner forcefully pushes (throws) the legs to the floor in an attempt to provide additional challenge to deceleration. The anchors of the hip flexor exaggerate the lumbar convexity and increase compressive forces. Personal trainers can modify the supine leg lift exercise by beginning the movement with the hips and knees flexed to 90° and contracting the abdominals to pull the pelvis posteriorly reduces spinal stress and enhances rectus abdominis activation. This movement can be implemented with or without external load from a supine or inclined position. If hanging leg raises are employed the emphasis should be on the degree of flexion. The knees should reach the chest to optimally encourage a posterior pelvic tilt and then decelerated in a controlled manner. Personal trainers can spot the technique to ensure a full range is attained and the client decelerates without a swinging action.