Objective: The relationship between muscle force production in ALS SOD1G93A mice and single and modeled multifrequency electrical impedance myography (EIM) parameters is unknown. We evaluated the relationship between multifrequency EIM data and paw grip and in situ force measurements, as well to standard measures including body weight and compound motor action potential (CMAP) amplitude.
Methods: Twenty-nine SOD1 G93A mice aged 13-18 weeks (approximately 4-5 per week) and a group of similarly aged wild-type mice (N = 7) were studied with single and multifrequency EIM, CMAP, front and hind-limb paw grip measures, and in situ force measurements of the gastrocnemius.
Results: Significant differences among WT, presymptomatic, and symptomatic ALS animals were identified for all standard measures and single 50 kHz frequency EIM parameters. Of the modeled multifrequency measures, the center frequency, fc , an index of cell size, showed the strongest relationship to force output. The two other multifrequency parameters corresponding to cell size distribution and cell density showed consistent although mostly non-significant differences.
Conclusion: Reductions in force are reflected in single 50 kHz impedance values and in the fc. These data support the construct validity of EIM as an assessment tool of muscle dysfunction in diseases associated with motor neuron loss.
Keywords: Motor neuron disease; amyotrophic lateral sclerosis; compound motor action potential amplitude; electrical impedance; force.