ISOMETRIC TRUNK STRENTH
ICF Category	Measurement of impairment of body function – pain in back; pain in lower limb; control of complex voluntary movements
Description	Clinician assesses the performance of trunk flexors, trunk extensors, lateral abdominals, transversus abdominis, hip abductors, and hip extensors.
Measurement method	Trunk Flexors The patient is positioned in supine, the examiner elevates both of the patient’s fully extended legs to the point at which the sacrum begins to rise off of the table. The patient is instructed to maintain contact of the low back with the table as they slowly lower their fully extended legs to the table. The examiner observes and measures when the lower back loses contact with the tabletop due to anterior pelvic tilt.
	Trunk Extensors The patient is positioned in prone, with hands behind their back or by their sides. The patient is instructed to extend at the lumbar spine and raise the chest off of the table to approximately 30 degrees and hold the position. The test is timed until the patient can no longer hold the position.
	Lateral Abdominals The patient is positioned in sidelying with hips in neutral, knees flexed to 90 degrees, and resting the upper body on the elbow. The patient is asked to lift the pelvis off the table and to straighten the curve of the spine without rolling forward or backward. The position is held and timed until the point where the patient can no longer maintain the position.
	Transversus Abdominis The patient is positioned in prone over a pressure biofeedback unit which is inflated to 70 mmHg. The patient is instructed to draw in their abdominal wall for 10 seconds without pelvic motion while breathing normally. The maximal decrease in pressure is recorded.
	Hip Abductors The patient is positioned in sidelying with both legs fully extended, in neutral rotation and a relaxed arm position, with their top upper extremity resting on the ribcage and hand on abdomen. The patient is instructed to keep the leg extended and raise their top thigh and leg toward the ceiling, keeping the limb in line with the body. Patients are graded on quality of movement by established criteria.
	Hip Extensors The patient is positioned in prone with their knees flexed to 90 degrees and the soles of their feet on the table. The patient is instructed to raise their pelvis off of the table to a point where the shoulders, hips, and knees are in a straight line. The position is held and timed until the position can no longer be maintained.
Nature of variable	Continuous, Ordinal
Units of measurement	Seconds to hold position, muscle performance assessment, change in mmHg using a pressure biofeedback device
Measurement properties	The double leg lowering assessment for trunk flexor strength has demonstrated discriminative properties in identifying patients with chronic low back pain.157, 181, 322 If patients demonstrate anterior pelvic tilt with hip flexion greater than 50 degrees in males and 60 degrees in females they were more likely to have chronic low back pain.322 The assessment of trunk extensor strength has been highly correlated with the development and persistence of low back pain.7, 160, 214 Males who are unable to maintain an isometric hold of 31 seconds, 33 seconds for females, are significantly more likely to experience low back pain (+LR=4.05–6.5, −LR=0.24–0.02) with good reliability (ICC=0.89–0.90).7 Lateral abdominal strength has been measured in healthy controls and found be reliable (ICC=0.97).92, 207 Performance of the transversus abdominis has been evaluated and found to be reliable (ICC= 0.58 95% CI 0.28–0.78).67, 157, 281 A 4mmHg decrease in pressure is established as normal whereas the inability to decrease the pressure biofeedback device measure by 2 mmHg is associated with incidence of low back pain.157, 168, 251 The hip abduction test has demonstrated discriminative ability to predict patients who will develop pain with standing (+LR= 2.68–4.59).221, 222 Endurance assessment of the bridge position to assess gluteus maximus strength has demonstrated good reliability (ICC= 0.84).262 Mean duration of hold for patients with low back pain is 76.7 seconds compared to 172.9 in persons without low back pain.262
Instrument variations	There are numerous alternate test positions for all described muscle groups. For trunk flexion test variations include a bent double leg lowering and sit up tasks. For trunk extension numerous variations have been described including the Sorensen test and prone double straight leg raise.7, 160, 214 Transversus abdominis performance has been described by a palpatory method.67 Hip abduction and hip extension strength can both be assessed with manual muscle testing.173 Clinician’s selection of test may be dependent on patient’s level of conditioning and symptoms behavior.