Isometric strength assessment, Part II: Static testing does not accurately classify validity of effort

Area of Science:

• Physical therapy
• Neurology
• Rehabilitation medicine

Background:

• Isometric tests are frequently used in clinical settings to assess muscle strength and effort.
• The validity of commonly administered isometric tests, such as the Leg Lift and Arm Lift, in accurately reflecting maximal voluntary effort is often assumed.
• However, the reliability of these tests when individuals attempt to feign weakness requires careful examination.

Purpose of the Study:

• To evaluate the accuracy of two common isometric tests in distinguishing maximal voluntary effort from feigned weakness.
• To determine if the Leg Lift and Arm Lift tests serve as reliable indices of effort validity.

Main Methods:

• Thirty-four healthy subjects participated in the study.
• Each subject performed two testing conditions: one with maximal voluntary effort and another attempting to feign 50% of their maximum effort.
• Performance was assessed using isometric measurements during Leg Lift and Arm Lift tasks.

Main Results:

• During the feigned weakness condition, 58.5% of subjects (20 out of 34) produced contraction velocities (CVs) of 15% or less during the Leg Lift test.
• The 95% confidence interval indicated a high expected frequency of false negatives (42.3% to 75.3%) for the Leg Lift when feigning weakness.
• Similarly, the Arm Lift test showed a high expected frequency of false negatives (51.9% to 83.3%) under feigned weakness conditions.

Conclusions:

• Neither the Leg Lift nor the Arm Lift isometric tests are appropriate for accurately classifying the validity of effort.
• The high rate of false negatives suggests these tests cannot reliably differentiate between genuine maximal effort and simulated weakness.
• Clinicians should discontinue the use of these specific isometric lifts for assessing effort validity due to their inherent limitations.