The jackknife position, also called the Kraske position, is a variation of lying face-down in which the patient’s hips are elevated while the head and feet angle downward. It’s used primarily in surgery and medical exams to give clear access to the anal and perineal area. The operating table is flexed at the hip break point, sometimes up to 90 degrees, creating a V-shape with the hips at the highest point.
How the Position Looks and Works
Picture someone lying face-down on a table that bends in the middle. A pillow or bolster is placed directly under the hips at the table’s break point, and the table is then flexed so both the head end and the foot end angle downward. The result is that the buttocks become the highest point of the body, spreading the gluteal tissue and exposing the anus and perineum for the surgeon or examiner.
In a full surgical setup, the arms are positioned on padded armboards with the elbows bent at 90-degree angles. The hips and knees are also flexed to roughly 90 degrees. A mirror is often placed beneath the table so the anesthesia team can monitor the breathing tube without moving the patient. The abdomen hangs freely rather than being compressed against the table surface, which reduces pressure on blood vessels in the abdominal space and helps minimize bleeding during surgery.
A simpler version, sometimes called the modified jackknife position, involves just placing one or two pillows under the front of the hips while the patient lies prone on a flat table. This gives a milder hip flex that’s enough for office-based exams and minor procedures.
When It’s Used
The jackknife position is the standard setup for surgeries and procedures involving the anus, rectum, and perineum. Common examples include hemorrhoid removal, pilonidal cyst excision, anal fistula repair, and rectal biopsies. It’s also used for more extensive operations like perineal resection, where surgeons remove diseased tissue from the anus, rectum, and surrounding structures, sometimes including reconstruction with tissue flaps.
Outside the operating room, a modified version is routinely used in clinic settings for anorectal exams. The patient kneels on a shallow ledge of the exam table, lays their chest flat, and the table tilts head-down to bring the anal area into view. A sheet is held between the patient and the examiner to provide privacy during the process. Certain spinal procedures, particularly those involving the lower back, also use prone jackknife positioning to open up the space between vertebrae.
Effects on the Body During Surgery
The jackknife position produces significant cardiovascular changes that the anesthesia team actively monitors. Research using impedance cardiography found that cardiac output, the volume of blood the heart pumps per minute, drops by about 34% after a patient is placed in the jackknife position. In one study, cardiac output fell from an average of 7.4 liters per minute at baseline to 4.9 liters per minute immediately after positioning. The cardiac index, which adjusts that number for body size, dropped by a similar 33%.
This happens for two main reasons. Blood pools in the legs because they’re angled downward, which means less blood returns to the heart. At the same time, the prone posture increases pressure inside the chest, further reducing the heart’s filling volume. The body compensates by tightening blood vessels to maintain blood pressure, but when this is combined with spinal anesthesia, which also lowers blood pressure, the drops can become clinically important. In rare cases, the combined decrease in blood return to the heart can trigger reflexes that slow the heart rate and, if uncorrected, lead to a sudden drop in circulation.
Breathing mechanics also change. The prone position with hip flexion can reduce lung compliance and thoracic volume, making it slightly harder for the lungs to expand. However, when the abdomen hangs freely rather than being compressed, this effect is somewhat offset. Proper table setup and bolster placement are key to keeping the airway and breathing stable throughout the procedure.
Pressure Points and Nerve Protection
Because the patient is face-down and unable to shift their weight, several areas of the body are vulnerable to pressure injury during prolonged procedures. The face, knees, toes, elbows, and the bony ridges of the hips (the anterior iliac crests) all need careful padding. The face is padded by the anesthesia team, and it’s critical that the hip bones rest so the abdominal wall actually hangs free rather than being pinched by the hip support. The knee rest is adjusted to prevent excessive pressure on the kneecaps.
Nerve injuries are a recognized risk with any surgical positioning, and several nerves deserve specific attention in prone and jackknife setups. The brachial plexus, the nerve bundle running from the neck through the shoulder into the arm, can be stretched if the arms are positioned too far overhead. Arms are typically kept with elbows bent at 90 degrees to prevent this. The ulnar nerve at the elbow is protected by avoiding excessive elbow flexion and by padding the groove where the nerve runs along the inner elbow. The radial nerve, which wraps around the upper arm bone, can be compressed if the arm presses against a hard surface for too long.
Periodic checks throughout the surgery confirm that padding hasn’t shifted and that the patient’s position hasn’t drifted. Even small changes during a long procedure can create new pressure points or stretch a nerve that was initially well-protected.
What Patients Experience
If you’re scheduled for a procedure in the jackknife position, you’ll be placed face-down after anesthesia is administered, so you won’t need to hold the position on your own or feel any discomfort from the table angle. The surgical team handles all the positioning while you’re under anesthesia. For office-based exams using the modified version, you’ll be guided into the position on the exam table, typically kneeling and then leaning forward onto your chest. It can feel awkward, but the table does most of the work, and a privacy sheet keeps you covered.
After surgery performed in the jackknife position, some patients notice mild facial puffiness or redness from being face-down, which resolves within hours. Soreness at pressure points like the hips or knees is occasionally reported but is generally minor when padding was properly placed.
The Jackknife in Fitness
The term “jackknife” also refers to a popular core exercise. The jackknife sit-up involves lying flat on your back with arms extended overhead and legs straight, then simultaneously lifting your arms and legs to meet above your midsection, forming a V-shape at the top of the movement. It targets the abdominal muscles and obliques while also engaging the hip flexors. The challenge comes from keeping both arms and legs straight throughout the movement, which demands balance and body control in addition to raw core strength. You lower back down without letting your hands or feet touch the floor between reps.