An electromyographical evaluation of the validity of the 2-minute sit-up section of the Army physical fitness test in measuring abdominal strength and endurance

Military Medicine, Nov 2002 by Szasz, Anna

Clinical Report

Objective: The aim was to evaluate electromyographically the validity of the sit-up section of the Army Physical Fitness Test as a way to assess abdominal endurance. Methods: Twentyeight Reserve Officer Training Corps students at the University of Scranton (Scranton, PA) performed the test while eight electromyography surface electrodes picked up the activity of the rectus abdominus and rectus femoris muscles. The researchers analyzed three groups of five contractions from the beginning, middle, and the end of the test. The percentage area of each group of five contractions for each channel and for each of the 28 participants was averaged. Results: Two paired sample t tests were used to analyze the change in area for the left hip flexor and left abdominal muscle (t = 5.951, p

Introduction Many physical fitness tests were designed to evaluate the endurance, flexibility, and strength of members of healthy populations, such as children and those in the armed forces. These physical fitness tests were not always based on scientific research. The purpose of our study was to provide some scientific research to determine the validity of the 2-minute sit-up section of the Army Physical Fitness Test (APFI). Knapik1 stated, "Most factor-analytic studies involving push-ups and sit-ups do not provide details on test administration. This makes it difficult to relate these studies to the standardized APFT push-ups and sit-ups."

The instructions for the 2-minute sit-up section of the APFT stated that the test was simultaneously evaluating the abdominal and hip flexor muscles. In a correctly executed sit-up, hip flexor activity was required. However, in this experiment, the investigators expected that hip flexor activity would dominate during the entire sit-up, especially at the end of the 2-minute period, secondary to the fatigue of the abdominals.2

Kendall et al.3 agreed that physical fitness tests often were inaccurate and did not detect deficiencies in the muscular strength or endurance of the performer. Unfortunately, these tests have become an evaluation of the performance, rather than a measure of physical fitness of the performer. Emphasis is on excesses, for example, the speed of performance or number of repetitions, rather than on quality and specificity of movement."3 Kendall et al.3 repeatedly emphasized the importance of performing the sit-up slowly and maintaining the trunk in a curled position to accurately assess abdominal strength. In contrast, the APFT design requires the performer to do as many sit-ups as possible in 2 minutes. Kendall and colleagues3 stated that the performer began with good technique, but as the abdominal muscles fatigued, the performer began to arch his or her back and relied on hip flexors to execute the sit-up.

The activation of the hip flexor and abdominal muscles has been evaluated in various sit-up positions and other hip and trunk flexion exercises. According to Anderson et al.,4 the abdominal muscles were activated in all tested positions, but hip flexor activity varied. Sit-ups performed with flexed knees and hips with feet supported increased hip flexor activity.4,5 In theory, one would assume that flexing at the hip would cause the hip flexors to shorten and decrease their activity. This was true for the iliopsoas, which only crosses at the hip joint. This, however, was not true for the rectus femoris, which crosses two joints. The positions of both joints, the hip and the knee, affected muscle length. Although hip flexion shortened the rectus femoris, knee flexion caused the muscle to lengthen.5

The researchers were interested in identifying the correct electrode position and alignment to maximize the detection of the electrical signal of the rectus abdominus and rectus femoris. Aligning the electrodes parallel to the muscle fiber orientation has been found to be important for optimal pick-up of electromyography (EMG) signals. The study by Ng et al.6 looked at muscle fiber orientation of the abdominal muscles. In this study, the rectus abdominus was divided into upper and lower segments with the umbilicus dividing the two segments. The muscle fiber orientation of the upper rectus abdominus was found to be 2 deg inferolateral from midline, whereas the lower rectus abdominus was 8 deg inferiomediallially from midline.6 In a study by Guimaraes et al.,5 an ideal position for electrodes monitoring electrical activity of the rectus femoris is midway between the patella and the anterior superior iliac spine. Another study by Zuniga et al.7 indicated that placing the electrode in the middle of the muscle belly would obtain maximal EMG potentials.

Operational Definitions

- Abdominal strength is the electrical activity of the rectus abdominus as compared with the rectus femoris.

- Abdominal endurance is the electrical activity of the rectus abdominus throughout the 2-minute period of sit-ups.

- Fatigue is the reduction of electrical activity on the EMG.

Materials and Methods