Saddle Trauma and the Mechanical Bull

TASA ID: 4298


The author is a forensic engineer with a Ph.D in biomechanical engineering and is licensed in several states as a structural and professional engineer. The author is employed by Packer Engineering Group, a firm that investigates accidents and injuries, and provides expert opinions to their clients as well as in a court of law. 

Packer was contacted because of a lawsuit involving an injury on a mechanical bull. The firm was requested to investigate reported injuries, validate the injured individual’s account of the accident and the cause of his injuries, and issue a written report.

This report was, therefore, written as a response to the lawsuit. Details were redacted and replaced with general terms for anonymity.


On September of 2015, while attending a state fair, the subject, a healthy 53-year-old male, elected to ride on a mechanical bull. Immediately upon the commencement of the ride, the subject claims to have received bodily injury that caused him to fall off the ride. This injury so incapacitated the subject that he was taken to a local hospital and was subsequently airlifted to another hospital. What was diagnosed was an internal hemorrhage as well as damage to his pelvis and genitals.


The following written materials were reviewed by the author in the investigation:

  1. Photographs, videos, acceleration graphs, testimony, plaintiff medical records, and other documents submitted by client, including depositions as described below
  2. Deposition of the ride operator
  3. Deposition of the subject
  4. Rosenstein D I, MD, FACS, FRCS (Urol), Alsikafi N F,MD, “Diagnosis and Classification of Urethral Injuries”, Urol Clin N Am 33 (2006) 73-85
  5. Ten Eyck R P, MD, Capt, Longmire A W, MD, “Mechanical Bull Injuries: The Empty Saddle Syndrome”, Ann Emerg Med. 1981; 10:11 (November): 582/51
  6. Green R S, BSC, MD, FRCPC, Maier R, MD, “The Urban Cowboy Syndrome Revisited: Case Report”, South Med J. 2003;  96(12)
  7. McElhaney, J H, Roberts V L, Hilyard J F, “Handbook of Human Tolerance”, Japan Automobile Research Institute (JARI) 1976; 489
  8. Henry Dreyfuss Associates, “The Measure of Man & Woman, Human Factors in Design” John Wiley & Sons, New York (2002)ISBN 0-471-09955-4
  9. https://www.horsejournals.com/riding-training/tack-gear/western/saddle-fit-and-heavier-rider
  10. http://www.healio.com/orthopedics/journals/ortho/2009-6-32-6/%7B38c37dc7-a541-45c7-9a58-adfd65be1d89%7D/fixation-of-pubic-symphyseal-disruptions-one-or-two-plates

An evaluation was made from these written materials and considering the author’s knowledge, experience, training, background and education. In addition, an analysis was performed using general mechanics and biomechanics principles.

Medical Record Review

  • The subject was presented at the ER at a local hospital. The subject was admitted in September of 2015 and was given a blood transfusion due to hemorrhaging in his abdomen, (retroperitoneal hematoma). Separation between front pubic bones were noted, (diastases of the symphysis pubis) along with an outward separation between the sacrum and right iliac bone (slight distraction of right sacroiliac joint)
  • Due to need for specialized care, the local hospital decided on the following morning to transfer the subject to a specialized medical center via helicopter. A pelvic binder (which is a wrap tightly around the hips) was attached at the medical center to keep the pelvic bones together. A catheter was also inserted. Bruising and swelling of genitals was noted. (Type 1 urethral injury without extravasion.)
  • On that morning at the medical center, the subject had a plate attached to his pelvis to close the gap between the two halves at the front, plus, a screw was attached to close the slight gap between the pelvic and spinal bones (open reduction and internal fixation of pubic symphysis and percutaneous screw fixation in the right sacroiliac joint). The subject was released from the medical center a week later.
  • Other medical visits include:
    • Physical therapy starting ten days after hospital release,
    • Urological concerns starting October of 2015 with penile and erectile concerns.
  • The subject’s weight at the time of the hospitalization in the medical center was recorded as 262 pounds and 285 pounds on two separate documents, his height as 6’-3”.

Testimony From the Deposition of the Ride Operator

  • The operator rode live bulls for a living over a five-year period.
  • No one other than the subject had ever been injured on one of the operator’s mechanical bulls.
  • Mechanical bulls are designed to mimic the riding of an actual bull and were originally created for practice.
  • The only malfunction of one of the operator’s mechanical bulls has been a chain break during operation. (A rider was on it, but not the subject.) The malfunction caused the bull to stop bucking and just move in a circle.
  • The operator stated that the subject said he was sore before going on bull.
  • The operator stated that the ride lasted only 20-30 seconds.
  • When the subject’s ride ended, he appeared to the operator to be sore.
  • An exhibit for the legal case showed approximately 1” of depression when an investigator pushed down with two hides on the bull, although at time of the subject’s ride there was only one hide on the bull.
  • The subject did everything correctly when riding the bull.

Testimony From the Deposition of the Subject

  • The subject rode a mechanical bull five times prior to riding the operator’s bull at the state fairground.
  • The subject noticed nothing unusual about the way the operator’s bull operated with other riders.
  • The subject was not prepared when the ride started and was not up tight against the hump.
  • The mechanical bull bucked, almost vertically, and the subject slid down and heard a popping noise.
  • Once the bull returned to level, the subject fell off the back of it.
  • As soon as he fell off the bull, the subject felt a pain in his lower back.
  • The subject quickly became incapacitated, and only moved about five feet away from the bull and could move no further.
  • The subject required a wheelchair and was transported from the fairgrounds to the ER at the local hospital where he received a transfusion.
  • The subject was then transported to a specialized medical center by helicopter for surgery.
  • The subject’s genitals were swollen and solid black. His bladder was pushed over and he had problems urinating.
  • Additional medical concerns of the subject included a recent surgery for complications stemming from the accident.

Literature Review

  • “Diagnosis and Classification of Urethral Injuries”
    • Urethral injuries result from blunt trauma to the pelvis, and usually have an associated pelvic fracture.
    • Riding mechanical bulls will cause an anterior injury to the pelvis known as “straddle trauma”.
    • In a straddle injury, the genitals are crushed against the pubic bone causing the membranous urethra to stretch until it ruptures and diastasis of the pubis symphysis.
  • “Mechanical Bull Injuries: The Empty Saddle Syndrome”
    • Twenty-nine injuries were accumulated over a four-month period from personnel at Keesler Air Force Base Hospital, plus one from Kirtland Air Force Base Hospital.
    • The common type of fall is off the side of the bull as it rotates while bucking.
    • The publication concludes by stating that mechanical bulls have provided a new source of trauma.

Case Study: The Urban Cowboy Syndrome Revisited

  • A healthy, 32-year-old male attempted to ride a mechanical bull.
  • While straddling the seat, the mechanical bull was inadvertently engaged, striking the subject in the perineal area.
  • The patient was thrown to the ground, which was padded with foam.
  • Witnesses stated that there was no head trauma and no loss of consciousness.
  • The subject was able to ambulate after the fall, but immediately felt low back pain.
  • At the hospital, a radiograph of the pelvis showed a symphysis diastasis.
  • The patient had blood in his urine (hematuria.)
  • An abdominal CT scan showed a symphysis diastasis, mild widening of the bilateral sacroiliac joints and a retroperitoneal hematoma.

Injury Biomechanics

The subject had a separation of the two bones at the front of the pelvis. Two bones, one each side of the pelvis, are connected in the front with a cartilaginous joint known as the symphysis, and at the back to either side of the sacrum. The separation is commonly a result of pressure from the inside, which occurs at childbirth. It can also take place when one side of the pelvis is pried away from the other, as can happen when landing forcibly on one foot. See figure 1. (Although this figure does not relate to the injury mechanism occurring in this case, it will be used to illustrate the pelvic components and forces involved in separating the pelvic bones.)

If the right femur was to be forced vertically (at A) while the left femur was not, a shearing force (at B) would attempt to rotate the ilium (at C) and pry the two halves apart at the front (at D) and at the back (at E). Location D represents the pubic symphysis joint, and location E represents the ilium-sacrum (sacroiliac) joint.

Figure 1. Outward rotation of pelvic ilium causing trauma.

Relating to the injuries possible on the mechanical bull (straddle trauma), there are several types of biomechanical forces that can lead to diastasis symphysis. See Figure 2. It is possible to put a prying force by pressing the legs together and causing an outward shear at the acetabular joint, as shown in the left-hand photograph. This is represented by the arrows at A pointing medially (inward). Also, excessive width of the bull could cause a force laterally (outward), causing the legs to become bow-legged. In either case, a shearing force at the acetabular (hip) joint could cause outward shear as shown at B. In the same manner as Figure 1, outward force at the symphysis joint at C could occur. It is unlikely that this would be a common occurrence, nor relate to the injuries of the subject. As shown in the right photograph of Figure 2, a force vertically upward (shown at A), or from the front (shown at B) can similarly cause separation of the symphysis joint. The case represented by A, which can result from a fall from height onto a hard surface, has been known to damage the tailbone (coccyx) and sacral-lumbar spine. However, vertical loads while riding atop a horse or bull are typically distributed to the buttocks and back of the legs, and include a thick layer of tissue protecting the pelvis.


Figure 2. Forces imposing potential injury while riding a mechanical bull.

Loads from the front (as shown by B in the right photo of Figure 2), will come in direct contact with the pubic bone, which is protected by a relatively thin layer of tissue. This will happen when the mechanical bull bucks.

Figure 3. Maximum tilt angle

Figure 3 is still footage from the operator’s bull ridden by a child. The photographs and video submitted to the author shows this as the maximum degree of tilt during a buck. If not tucked close to the hump, the rider would slide into the hump. If, as indicated in the subject’s testimony, the tilt was quick enough to get him airborne, contact would be made at the acceleration of gravity (free-fall).

Figure 4. Ratio of actual force to permissible force on pelvis for different heights of free-fall

The velocity of the pelvis that is created by the mechanical bull is required to stop in a short distance (decelerate) based on the thickness of the padding involved. The thickness is found by adding the physical pad or pads placed on the back of the mechanical bull to our own internal padding between our skin and bones. This is most critical in the vertical direction, where the deceleration of the pelvic bone is due to velocity created by the ride, plus the one g force that exists due to gravity. Figure 4 illustrates a case as reported in this case’s testimony, where the subject was in free-fall against the hump on the mechanical bull. The chart shows that, even with only a few inches of free-fall directly to the pubic bone, enough force can be created to exceed the strength of the pelvis. The horizontal line designating a ratio of 1.00 occurs at a height of only 2 inches! That is because, even with no motion taking place, a heavier individual supporting his or her entire weight on their pelvis is not what nature intended! Thus, heights of free-fall 2 inches or greater represent those capable of causing injury. Data in Figure 4 is based on the following:

  1. Strength tests performed on the human pelvis of cadavers,
  2. A uniform deceleration into 2” of padding between the mechanical bull’s solid structure and the pelvic bone, consisting of 1” of thickness from the pad on the bull, and 1” of thickness between the surface of the subject’s skin and the pelvic bone,
  3. The subject’s recorded weight of 265 pounds. Note that these forces are due to the velocity of the impact in free-fall, plus the subject’s self-weight.
  4. The forces in Figure 4 do NOT include those generated if the free-fall of the pelvis was met with a mechanical bull on its way back up.


  • Mechanical bull riding is inherently dangerous, and causes injuries to healthy individuals on properly functioning and properly operated rides.
  • The type of injury experienced by the subject has been reported in the literature. This includes separation of the pubic bones (diastasis symphysis and sacroiliac joint distraction), trauma and bruising to the genitals (contusions, ecchymosis, hematuria, urethral damage) and internal bleeding (retroperitoneal hematoma).
  • At 6’-3” and between 263 and 285 pounds, as indicated in the medical center records, the subject is placed in the 99th percentile, which means larger than 98% of adult males and over 99% of adult women.
  • Forces to the subject’s pelvic structure during mechanical impacts would be exacerbated by his size as compared to the majority of riders. The subject’s size would cause him to move independently of the mechanical bull more so than the average rider. This is due to his greater rotational inertia, causing his mass to rotate more in opposition to that of the mechanical bull, and his center of mass’s lateral inertia, at a greater distance from the riding surface, causing more lateral motion in opposition to that of the mechanical bull.
  • The injuries reported in the literature, known as saddle trauma, take place while on the mechanical bull, and already occur prior to the rider falling off the bull. There were no reported injuries found in the literature from impact onto a padded surface from an individual who was uninjured while falling off the bull.
  • Engineering calculations support those facts presented in the literature; that forces to the pubic bone due to falls from height, even from short distances, can exceed the strength of the pelvic bone.
  • There is no evidence that the operator’s bull malfunctioned precisely at that moment when the ride with the subject began. There was no malfunction causing a bucking beyond the maximum operating angles that was observed in the pictures and videos presented to the author. There was no reported malfunction before or after the subject’s ride. Thus, a malfunction during the subject’s ride that somehow repaired itself is not consistent with normal mechanical behavior. As previously stated, the injuries to the subject were entirely possible without any mechanical malfunctions whatsoever.
  • The author has no evidence available that would prove or disprove improper operation of the bull by the operator. The operator and his family are involved with bull riding of the non-mechanical kind, which is far more dangerous than the mechanical variety, and they are dedicated to this sport; i.e. the operator is not a “carnival operator.” However, improper operation is beyond the author’s scope to determine.


In summary, based on the review of written materials, analysis, and our knowledge, experience, training, background and education, the author was able to conclude the following to a reasonable degree of engineering and scientific certainty:

  1. The subject’s injuries requiring hospitalization in September of 2015 were caused by the mechanical bull ride that took place the same day. The biomechanical forces involved in the mechanical bull ride were capable of causing the injuries that the subject suffered. The type of injury experienced, known as a “straddle trauma” have occurred while riding a mechanical bull. Furthermore, active hemorrhaging was discovered that began immediately after the subject’s ride on the bull.
  2. The mechanical bull was performing properly when the subject rode it. The mechanical bull operated properly before and after the subject’s ride. It is highly unlikely that the mechanical bull failed only during the subject’s ride, and returned to normal operation permanently afterwards. As indicated in the case study, a matching combination of injuries took place on a properly operating bull. The mechanical bull did not require a malfunction in order to cause the injuries to the subject.
  3. It is not conclusive whether the operator operated the ride improperly during the subject’s ride. There is conflicting testimony regarding the preparation and duration of the subject’s ride. There is no evidence that the operator, who had expert experience at bull riding, and had no reason to be inconsistent, would have done something different for the subject than all other riders. At over 260 pounds and 6’-3”, and potentially sore from work at the time of his ride, the subject may have made a poor choice by riding the mechanical bull. A mechanical bull is inherently dangerous, and literature shows that injuries occur regularly on properly performing, properly operated mechanical bulls. The author has insufficient evidence to know whether the operator operated the ride improperly at the time of the subject’s injuries.

This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

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