Flexion tests are a key component of the diagnostic work-up for lameness in horses. [1] These tests exacerbate pre-existing soreness or irritation in the limbs to make subtle lameness more obvious. [1]

Veterinarians most commonly use flexion tests during lameness examinations and pre-purchase evaluations to provide additional information to owners about potential sources of lameness. [1]

To perform a flexion test, veterinarians hold the limb in a flexed position for around 60 seconds, then evaluate the horse trotting to identify any increased lameness. A positive flexion test showing increased lameness indicates potential injury or soreness in the flexed joint.

Flexion tests have several limitations that make them prone to error, such as identifying lameness when no lameness is present. Any positive flexion test results require further investigation, such as X-rays or nerve blocks, to confirm the diagnosis.

How Do Flexion Tests Work?

Flexion tests increase the amount of pressure on the bones and soft tissues of a joint. [1] This additional pressure can irritate a pre-existing injury or area of soreness, resulting in a positive flexion test (increased lameness after flexion). [1]

Increased pain from a flexion test can result from: [1]

  • Compression of soft tissues on the flexed side of the joint
  • Stretching of soft tissues on the extended (non-flexed) side of the joint
  • Increased pressure within the joint capsule due to compression of joint fluid
  • Increased pressure in the bones on either side of the flexed joint
  • Activation of pain nerves in and around the joint

Since there is a wide range of structures that may be irritated by a flexion test, results of testing do not point to a specific diagnosis. [1] Further investigation into joints that “flex positive” is necessary to determine the cause of lameness. [1]

Interpretation of Flexion Test Results

Flexion tests begin with assessing the horse’s baseline lameness level, or how the horse looks without any other interventions. [1] Veterinarians typically examine lameness by having the horse trot away and towards them, however other options such as lunging or riding are also possible. [1][2]

Once the baseline is established, flexion tests are performed on all four of the horse’s limbs, even if it seems obvious that there is pain in one particular joint. [1] It’s important to check all four limbs as other sources of pain might be masked by lameness in the most severely affected limb. [1]

After performing the flexion holds on each limb, the horse trots out and back again for direct comparison to the baseline movement. [1] A positive flexion shows significantly worsened lameness after flexion compared to the baseline. [1] A negative flexion will show 3-5 steps of irregular movement that quickly resolves to the baseline lameness level. [1]

When a positive flexion result occurs, the veterinarian interprets the result in the context of the horse’s overall lameness. [1] For example, a positive lower limb flexion result could indicate pain within the fetlock, pastern, or coffin joints. [1] It could also indicate pain in the soft tissues surrounding these joints. [1]

Completing the flexion tests helps narrow down the specific area for further diagnostics. [1] Additional diagnostics for lameness include perineural analgesia (nerve blocks), X-rays, and ultrasound. [1]

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Types of Flexion Tests

Different types of flexion tests are designed to target specific joints, which helps narrow down the source of lameness in the affected limb. [3]

Lower Limb Flexions

Lower limb flexions target the fetlocks, pastern, and coffin joints and can be performed on the horse’s front and hind limbs. [3]

To perform lower limb flexion, all three joints are usually flexed at the same time. [3] To achieve this, the veterinarian picks up the leg, then pulls up on the toe of the hoof with moderate pressure and holds for 30 seconds. [3]

It’s also possible to flex the fetlock joint and the pastern and coffin joints separately to some degree. [3] Completely separating the flexion of the three joints is impossible, but careful manipulation can put more pressure on either the fetlock joint or the pastern and coffin joints for more localized analysis. [3]

Flexing the fetlock involves lifting the hoof and supporting the leg by the cannon bone with one hand, then using the opposite hand to pull up on the pastern. [3] To flex the pastern and coffin joints, one hand holds the fetlock in an extended position while the opposite hand lifts up on the toe. [3]

Upper Front Limb Flexions

The upper limb joints in the horse’s front limb include the knee, elbow, and shoulder joints. [3]

Flexing the knee involves picking up the limb and grasping the front of the cannon bone with both hands, then lifting upwards to bring the cannon bone parallel to the ground. [3]

Complete separation of flexion of the elbow and shoulder is difficult, and flexing one of these joints always puts some degree of flexion on the other. [3] To flex the elbow, the veterinarian lifts the leg and stretches it forward so that the upper leg is parallel to the ground. [3] To flex the shoulder, the veterinarian pulls the limb backwards to lift the elbow. [3]

Upper Hind Limb Flexions

These flexions target the hock and stifle. [3] The tendons and muscles in this area prevent complete separation of hock and stifle flexion. [3] Both tests may be necessary to confirm the location of lameness. [3]

Hock flexions involve lifting the hind limb and drawing the hoof forwards. The veterinarian then grasps the toe of the hoof and lifts upwards, bringing the cannon bone parallel to the ground. [3]
Stifle flexions involve lifting the limb and stretching it backwards, then applying upward pressure on the upper limb. [3]

Accuracy of Flexion Tests

Flexion tests are a subjective method of evaluating lameness, meaning they do not provide a measurable outcome for the veterinarian to evaluate. [1] Flexion tests have several limitations, including false-positive results, poor repeatability, and controversial predictive value for future lameness. [1][2]

False-Positives

Flexion tests are prone to false-positive results, which are a positive flexion test occurring in a horse with no lameness issues in the flexed area. [1]

Studies show that clinically sound horses with no observable lameness can flex positive (i.e. have a positive flexion test result). [1][4][5]

One study showed that 20 sound horses out of 50 flexed positive using the typical pressure applied during flexion tests. [6] When the applied pressure increased, 49 out of the 50 horses had a positive result. [6]

In this study, only 10 of the 50 horses had abnormalities on X-rays that may have contributed to a positive flexion test. [6] This result suggests there is up to 50% probability that a positive flexion test in an otherwise sound horse can incorrectly indicate a potential abnormality associated with lameness. [6]

From these studies, many veterinarians do not necessarily take a positive flexion test as an indication of a lameness issue. [4] Careful interpretation and further diagnostic testing, such as nerve blocks or X-rays, are necessary to confirm a positive flexion test result. [1][6]

Repeatability

Repeatability of results is a concern with flexion tests in horses. The outcome of flexion tests may depend on several factors, including: [5]

  • Limb position during flexion
  • Amount of force applied
  • Duration of flexion
  • Interpretation of lameness
  • Number and frequency of flexes during evaluation
  • Other health factors such as age and sex of the horse

Studies show significant variation in how veterinarians perform flexion tests. [5][7] One study examining the force applied to the limb during a flexion