Seven-day test-retest reliability of knee-extension strength using magnetic stimulation of the femoral nerve in critically-ill veterans

Background: Loss of muscle mass is common in critically ill patients, often predicting worse outcome.  Loss primarily occurs in first week of critical illness.   Voluntary contractions needed to gauge muscle strength are impossible in many critically ill patients as they often require sedation and are unable to follow commands. This study examined the test-retest reliability of the force of knee-extension elicited by single and paired magnetic stimulations of the femoral nerve over a week-long period in 15 ambulatory subjects. 

Method: A knee-holder device was developed to facilitate recording of knee-extension strength elicited by magnetic stimulation of the femoral nerve independent from patients’ volition.  Study subjects were measured at baseline and then at follow-up seven days later in order to determine whether these measurements remain reliable.

Results: Initial knee-extension forces elicited by single stimulation and by paired stimulations delivered at 100Hz were 8.54±0.53 and 20.03±0.93 kg (mean±SE), respectively. Repeat testing at one week revealed forces of 8.33±0.74 and 20.59±0.98 kg.  Intraclass-correlation coefficients of 0.94 and 0.96 (p>0.68), with respective typical percentage errors of 6.2% and 5.8% demonstrate high test-retest reliability of stimulations elicited by single and paired stimulations. 

Conclusion: Measurements of knee-extension strength elicited by single and paired magnetic stimulations of the femoral nerve delivered while the leg rests upon a knee-holder device are reliable, making this technique suitable for cross-sectional and longitudinal studies. These findings help improve diagnosis of muscle strength in critically ill patients, and provide a cutting-edge, necessary resource to aid intensive care clinicians in providing optimum care.