Handgrip Maximal Voluntary Isometric Contraction Does Not Correlate with Thenar Motor Unit Number Estimation

In slowly progressive conditions, such as motor neurone disease (MND), 50–80% of motor units may be lost before weakness becomes clinically apparent. Despite this, maximal voluntary isometric contraction (MVIC) has been reported as a clinically useful, reliable, and reproducible measure for monitoring disease progression in MND. We performed a study on a group of asymptomatic subjects that showed a lack of correlation between isometric grip strength and thenar MUNE. Motor unit number estimation (MUNE) estimates the number of functioning lower motor neurones innervating a muscle or a group of muscles. We used the statistical electrophysiological technique of MUNE to estimate the number of motor units in thenar group of muscles in 69 subjects: 19 asymptomatic Cu, Zn superoxide dismutase 1 (SOD 1) mutation carriers, 34 family controls, and 16 population controls. The Jamar hand dynamometer was used to measure isometric grip strength. This study suggests that MUNE is more sensitive for monitoring disease progression than maximal voluntary isometric contraction (MVIC), as MUNE correlates with the number of functional motor neurones. This supports the observation that patients with substantial chronic denervation can maintain normal muscle twitch tension until 50–80% of motor units are lost and weakness is detectable. 1. Introduction Motor neurone disease (MND) is considered to be a group of generally fatal, progressive neurodegenerative disorders. The disease is characterised pathologically by progressive degeneration and loss of motor neurones in the anterior horn cells of the spinal cord, motor nuclei of the brainstem, and the descending pathways within the corticospinal tracts [1]. It is primarily a condition of middle to late life, with onset of symptoms between the ages of 50 and 70. The overall median survival is 4.0 years from the onset of symptoms [2]. When disorders are recent or rapidly progressive, motor neurone loss results in weakness and wasting. In slowly progressive denervating diseases such as MND, loss of more than 50–80% of motor units may occur before weakness becomes clinically apparent [3]. In MND, needle electromyography may reveal changes of chronic reinnervation, but provides little direct evidence to the extent of motor neurone and axonal loss. The supramaximal compound muscle action potential (CMAP) amplitude also provides little direct evidence of the extent of motor neurone loss as a normal CMAP amplitude might mistakenly be thought to indicate that motor neurone loss has not occurred [4]. Motor unit number estimation (MUNE) has