GENERAL INFORMATION
The sphincter ani externus is a circular skeletal muscle located around the anal canal. It plays a critical role in controlling defecation and maintaining continence by regulating the opening and closing of the anus. This muscle is part of the pelvic floor and works with other pelvic muscles to maintain bowel control and support the organs in the pelvic region. Unlike the internal sphincter, the external sphincter is under voluntary control, allowing conscious regulation of bowel movements.
ORIGIN
The sphincter ani externus originates from the perineal body (central tendon of the perineum) and the anococcygeal ligament.
INSERTION POINT
This muscle inserts into the skin surrounding the anus and the anococcygeal ligament.
MAJOR ARTERIES
- Inferior Rectal Artery: Branch of the internal pudendal artery supplying the sphincter ani externus.
- Perineal Artery: Another branch of the internal pudendal artery that supplies surrounding areas.
NEURAL INNERVATION
- Inferior Rectal Nerve: Branch of the pudendal nerve (S2-S4), providing motor innervation to the sphincter ani externus, allowing voluntary control over the muscle.
TRIGGER POINT
Trigger points in the sphincter ani externus can lead to discomfort or pain during defecation or sitting. Overuse or straining during bowel movements, constipation, or prolonged sitting can activate these trigger points, causing spasms or tightness in the pelvic floor muscles.
CONCENTRIC FUNCTION
The sphincter ani externus contracts concentrically to close the anal canal, maintaining continence and preventing involuntary passage of stool.
ECCENTRIC FUNCTION
This muscle relaxes eccentrically to allow the controlled release of stool during defecation, providing a gradual opening of the anus and preventing uncontrolled evacuation.
ISOMETRIC FUNCTION
The sphincter ani externus provides isometric contraction to maintain anal closure, helping to prevent leakage of faeces or gas during rest or increased abdominal pressure.
RELATED MUSCLE – SCIENTIFIC NAMES
- Levator Ani: Supports pelvic organs and assists in maintaining continence.
- Puborectalis: Forms a muscular sling around the rectum and assists with maintaining continence.
- Coccygeus: Supports the pelvic floor and stabilises the coccyx.
- Sphincter Ani Internus: Works involuntarily to maintain constant tone in the anal canal.
- Obturator Internus: Supports the lateral walls of the pelvis and stabilises the pelvic floor.
ANTAGONIST – BULLET POINTS
- Sphincter Ani Internus: Relaxes involuntarily when the external sphincter contracts during continence.
- Levator Ani: Assists with the voluntary contraction of the sphincter but can relax during defecation.
- Rectal Musculature: Eases the evacuation process as the sphincter ani externus relaxes.
COMMON INJURIES
- Anal Sphincter Dysfunction: A condition where the muscle weakens or is damaged, often leading to faecal incontinence. It may occur due to childbirth, ageing, or trauma.
- Chronic Constipation: Constant straining can lead to overuse of the sphincter ani externus, causing fatigue, spasms, or pelvic floor dysfunction.
- Pelvic Floor Dysfunction: When the muscles of the pelvic floor, including the sphincter ani externus, become too tight or too weak, leading to difficulty controlling bowel movements.
- Haemorrhoids: Swelling of blood vessels around the anus can cause discomfort and affect sphincter function, often exacerbated by excessive straining.
- Fissures: Tears in the anal lining, often caused by hard stools or excessive strain, can result in pain and dysfunction of the sphincter ani externus.
EXERCISES
- Kegel Exercises: Contract the pelvic floor muscles by squeezing the sphincter ani externus for 5-10 seconds and then relaxing. Repeat to strengthen the muscle and improve bowel control.
- Pelvic Tilts: Lie on your back and gently tilt the pelvis upwards, engaging the pelvic floor and abdominal muscles to strengthen the sphincter and surrounding muscles.
- Bridge Pose: Lie on your back with your knees bent, lift your hips, and engage the pelvic floor and sphincter muscles to build strength and improve pelvic control.
- Squats: Perform bodyweight squats while focusing on contracting the pelvic floor muscles at the bottom of the squat. This exercise enhances both pelvic strength and lower body power.
- Deep Breathing with Pelvic Engagement: Breathe deeply while contracting the sphincter ani externus on the exhale, improving control and strengthening the pelvic floor muscles.
STRETCHES
- Happy Baby Pose: Lie on your back, hold your feet with your hands, and gently pull your knees towards your chest, stretching the pelvic muscles and relieving tension in the sphincter ani externus.
- Seated Forward Bend: Sit with your legs extended and reach towards your feet, gently stretching the lower back, hamstrings, and pelvic floor muscles.
- Butterfly Stretch: Sit with the soles of your feet together, allowing your knees to fall to the sides. This stretch opens up the hips and relaxes the pelvic floor.
- Child’s Pose: Kneel and sit back on your heels while extending your arms forward, gently stretching the pelvic floor and relaxing the sphincter muscles.
- Knee-to-Chest Stretch: Lie on your back and pull one knee towards your chest, holding it with both hands. This stretch helps stretch the lower back and relieve tension in the pelvic floor.