Figure 1: The muscles commonly referred to as ‘hip flexors’- iliacus and psoas (minor and major).
It is commonly thought that exercises such as leg raises or other hip flexor exercises work the lower abdominals. Many people are mislead to believe this due to the localised muscular fatigue or burning sensation felt in the pelvis area. The primary muscle used during any hip flexion exercise is known as the iliopsoas. The iliopsoas is made up of the iliacus and psoas (minor and major) muscles (see Figure 1) which work to flex the femur (thigh bone) at the hip joint. During all exercises that flex the hip, the rectus abdominis only contracts isometrically (not producing any movement or change in its length while contracting) to:
1. Posture the spine and pelvis
a. Supports weight of lower body so the lumbar spine does not hyperextend excessively
b. Maintains optimal biomechanics of iliopsoas
2. Counteract iliopsoas’ pull on the spine
While the rectus abdominis is still working (to a lesser extent than other abdominal-specific exercises), there is an increased element of risk from performing such hip flexor exercises. People with weak abdominals can strain the musculature of the lower back in turn leading to what is commonly known as lower back pain (LBP).
By looking at rectus abdominis’ structure and points of attachment (see Figure 2) a better understanding of how it behaves biomechanically can be obtained, therefore allowing more targeted exercise to be prescribed. As the fibres of the muscle contract longitudinally, the spine can flex (bend forward) and the pelvis rotate posteriorly (tuck under). With a fixed pelvis, rectus abdominis can only flex the trunk to around 30 degrees with any further flexing being taken over by the hip flexors. An example of a commonly prescribed exercise where this occurs is the ‘sit-up’. Rectus abdominis is only responsible for the first 30 degrees of the movement, leaving the remaining 50-60 degrees to be completed by iliopsoas. Therefore, more targeted exercise aimed at isolating rectus abdominis should only incorporate movements where the spine is flexed to around 30 degrees. Exercises that also posteriorly tilt (tuck under) the pelvis are excellent in isolating rectus abdominis; examples of exercises that achieve both these movements are crunches, vertical leg crunches, and reverse crunches. Incorporate these into your workout as they can help you to achieve that ‘6 pack’ or set of ‘washboard’ abs!
Firgure 2: Rectus abdominis – originates on the pubic crest and inserts on ribs 5, 6, and 7 and the xiphoid process of the sternum.
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