Date of Award


Document Type

Scholarly Project

Degree Name

Doctor of Physical Therapy (DPT)


Physical Therapy

First Advisor

Schawnn Decker


Abdominal Muscles; Electromyography; Muscle Strength


Background and Purpose: Back pain and instability are common diagnoses treated by physical therapists. Treatment often includes abdominal or core strengthening exercises that act to improve stability about the lumbar spine. Hip flexors prevent the weaker abdominal muscles from working to their full potential and increase the lumbar lordosis placing the spine at risk of injury. Hip extensors work with abdominal muscles to reduce the lumbar lordosis, and restrict the hip flexor activity, allowing a stronger abdominal contraction. The purpose of this study was to determine if a co-contraction of the hamstring muscles during an abdominal crunch increased the electromyographic (EMG) activity of the abdominal musculature and decreased the EMG activity of the hip flexor musculature.

Subjects and Methods: Thirty volunteers between the ages of 22 and 56 participated in the study, including 15 men and 15 women. An EMG analysis was used measuring the activity of the upper rectus abdominus, lower rectus abdominus, external oblique, rectus femoris, semitendinosus, and biceps femoris while performing five abdominal crunches without and five abdominal crunches with a hamstring contraction by pressing their heels into a chair. Participants were positioned with their hips and knees at 90°. Subjects performed three practice trials prior to data collection and one minute rests in between trials. Each participant's EMG data was normalized to the respective maximal voluntary contraction (MVC) for each muscle group analyzed.

Results: A significant (p<0.001) decrease in abdominal activity during the crunch exercise with hamstring contraction was observed compared to the crunch exercise without hamstring contraction. Specifically, decreased activity of the upper (-54.8%) and lower (-7.65%) rectus abdominus muscles and external oblique (-1.42%) muscle was observed. There was an insignificant increase in hip flexor (rectus femoris, +11.77%) activity during the crunch with hamstring contraction.

Discussion and Conclusion: This study found a decrease in abdominal EMG activity and an increase in hip flexor activity during the abdominal crunch with hamstring contraction. Factors to consider are the position of the hips and knees at 90°, feet not being flat on a stable surface, and difficulty coordinating all aspects of the abdominal crunch with hamstring contraction.