Age-related changes in muscle fatigue resistance in humans.

OBJECTIVE: The goal of this study was to compare the relative contributions from the muscle and the central nervous system to muscle fatigue resistance in aging. METHODS: Each subject carried out 90 s of sustained maximal voluntary isometric contraction (MVC) of the thumb using the thenar and forearm thumb muscles. Contractile capacity of the thenar muscles was assessed through tetanic stimulation of the median nerve. Interpolated doublets delivered during an MVC represented the overall voluntary activation level while transcranial cortical stimulation with an electromagnetic stimulator was used to assess motor output upstream from the corticomotoneuronal pathway. RESULTS: Nine elderly subjects [four females and five males, 70+/-9 years old (mean+/-SD)] and 10 younger subjects (five females and five males, 30+/-6 years old) were tested. After the fatiguing exercise, the elderly group's MVC declined by 29% as opposed to 47% in the younger group (p<0.01). The elderly group's greater fatigue resistance was accounted for by increased fatigue resistance at the muscle level as well as in the central nervous system. At least some of the decline in the central motor drive was upstream from the corticomotoneuronal pathway. CONCLUSION: The higher muscle fatigue resistance in the elderly group was attributable to differences in both the peripheral and central nervous systems.

Use of the collision technique to improve the accuracy of motor unit number estimation.

OBJECTIVE: To evaluate the risk of stimulating the same motor axon at different points along the median nerve when using the multiple point stimulation (MPS) technique and how this affects the accuracy of the motor unit number estimate (MUNE). METHODS: Using the MPS technique, MUNE in the median innervated intrinsic hand muscles was done on two normal subjects, a patient with carpal tunnel syndrome and one with prior poliomyelitis. The collision technique was then used to confirm whether two motor unit action potentials (MUAPs) with similar configurations and sizes were generated by the same motor unit. A new MUNE was recalculated after the repeated inclusion of the same motor unit had been excluded. RESULTS: While the risk of stimulating the same motor axon at widely separate locations of the median nerve was negligible in normal subjects, this risk was much higher in patients with a depleted motor neuron pool. This resulted in marked distortion of the calculated size of the average single MUAP and, consequently, inaccuracy of the MUNE. CONCLUSIONS: The inadvertent inclusion of the same motor units, if not recognized, can markedly affect the accuracy of the MUNE. The collision technique can be useful in minimizing this risk.