This particular study will help personal trainers, and group exercise instructors select the best exercises for their clients across all populations.  The study targeted abdominal exercise done without equipment to investigate the effect of changing the position of the upper extremities relative to 3 different feet and hip positions. With each change of the hands from arms crossed on the chest to the hands supported by the ears there is an increase in the external torque which must be countered by the torques of the abdominal muscles.

We all know that strength exercises for the abdominals play an important role in low back pain and prevention of low back pain.  The exercises selected were curl-up exercises where the spine is flexed by the rectus abdominis and the hip flexors are basically not involved.  Furthermore, curl-ups involve no lumbar spine flexion, which implies the absence of compressive loads.  This means that such exercise can be done by patients with low back pain.

The subjects were 3 women of similar age, all physically active, healthy and had never experienced disabling low back pain.  The activity of the rectus abdominis and erector spinae were studied. The curl up was analyzed when the women were precisely in the defined position isometrically.  The reason for not analyzing the muscle activity during isotonic contractions (joint movement) was due to subjects in other studies often performing sit ups or curl ups improperly by initiating the movement with the aid of the arm extremities instead of the  flexing of the upper body.  The shoulders were always raised above the ground and the lumbar spine touching the ground.  The angle between the trunk and the ground was 30 degrees.  Electromyographic activity was investigated (EMG) in selected skeletal muscles. Rectus Abdominis RA and Erector Spinae ES.  A set of 6 pairs of active electrodes were placed on the muscles bellies alongside the muscle fibers.  The duration of each isometric exercise was 5 seconds.  The successive exercises were separated by 1 minute breaks.  The sequence of exercises in each test was random which meant different combination of exercises for each subject. 

Each subject performed a total of 9 different exercises.  The variables for the upper body extremities were 1)the arms extended, raised off the floor and parallel to  the torso, 2)the hands behind the neck, and 3)the arms raised and extended overhead in a straight line with the ears. There were also 3 positions of the lower extremities.  The variables were 1)the hips flexed at 90 degrees and with the knees flexed at 90 degrees (feet off the floor), 2) the feet planted on the floor and the hip and knees flexed at 90 degrees, and 3)the legs extended straight and supported by the floor.

The maximum activity of the rectus abdominis was also the position with the lowest erector spinae activity and that was with the arms raised and extended overhead by the ears with the hips bent at 90 degrees and the knees bent at 90 degrees (feet off floor).  The highest EMG activity for the erector spinae was found with the upper extremities position with the hands on the neck and the hips flexed at 90 degrees and knees flexed at 90 degrees (feet off floor).

The statistical analysis conducted for the spinal muscles indicated that changing exercise initial positions (different positions of the upper and lower extremities) has a statistically significant effect on the electrical activity of the erector spinae. Changing the position of upper extremities with a fixed position of lower extremities causes statistically significant changes in the EMG activity of spinal muscles only for extreme positions of upper extremities (arms extended downward or arms extended upward),  In the same positions of upper extremities, changing the position of lower extremities does not alter the activity of spinal muscles.  Again, the least activity in the ES was with the hands at the neck and the hips and feet flexed at 90 degrees and feet off the floor.

By modifying curl-up exercises by diversifying the positions of the upper extremities has more important consequences than simply making the exercises more interesting (less boring). Knowledge of this will help the exercise leader select appropriate loads for clients of different strengths and ages.

Reference:

Rutkowska-Kucharska, A and Szpala, A. Electromyographic Muscle Activity in CurlpUP Exercises With Different Positions of Upper and Lower Extremitites. J Strength Cond Res Volume 24: 11: 3133-3139, 2010