Background The length at which a muscle/sarcomere operates in vivo (operational length) and the length at which it generates maximal stress (optimal length) can be quite different. In a previous study, we found that the rabbit external anal sphincter (EAS) operates on the ascending limb of the length–tension curve, in other words at lengths shorter than its optimal length (short sarcomere length). In this study, we tested whether the human EAS muscle also operates at a short sarcomere length.
Methods The length–tension relationship of the EAS muscle was studied in vivo in 10 healthy nullipara women. EAS muscle length was altered by anal distension using custom-designed probes of 5, 10, 15, and 20 mm diameter. Probes were equipped with a sleeve sensor to measure anal canal pressure. The EAS muscle electromyograph (EMG) was recorded using wire electrodes. Ultrasound images of anal canal were obtained to measure EAS muscle thickness and anal canal diameter. EAS muscle stress was calculated from the anal canal pressure, inner radius, and thickness of the EAS muscle.
Key Results Rest and squeeze stress of the anal canal increased with the increase in probe size. Similarly, the change in anal canal stress, i.e. the difference between the rest and the squeeze, which represents the active contribution of EAS to the anal canal stress, increased with the increase in probe size. However, increase in probe size was not associated with an increase in the external anal sphincter EMG activity.
Conclusions & Inferences Increase in EAS muscle stress with the increase in probe size, in the presence of constant EMG (neural input), demonstrates that the human EAS muscle operates on the ascending limb of the length–tension curve or at low sarcomere lengths. We propose that surgically adjusting EAS sarcomere length may represent a novel strategy to treat fecal incontinence in humans.