Dissociation between mechanical and myoelectrical manifestation of muscle fatigue in amyotrophic lateral sclerosis.

Motor fatigue, during 30 seconds of maximum voluntary isometric contraction (MVIC) was simultaneously evaluated by the decline in mechanical force output, and from the compression in the power spectrum obtained from surface electromyogram (sEMG). Measurements were performed in patients diagnosed with amyotrophic lateral sclerosis (ALS) and normal control (NC) in two muscle groups, elbow flexors (EF) and ankle dorsiflexors (DF). The decline in force output, as a manifestation of mechanical fatigue, was digitally calculated online by partitioning the force versus time curve to determine the percent of MVIC reduction over a 30 sec period and was expressed as force fatigue index (FFI). The compression in the sEMG power spectrum, as a manifestation of myoelectrical fatigue, was tracked by calculating the median frequency shift (MFS) from the first 5 sec to the last 5 sec of the 30 sec MVIC using digital Fast Fourier Transformation. In ALS patients, the significantly higher reduction in mechanical force output during the 30 sec MVIC (higher FFI) was accompanied with significantly less compression in the sEMG power spectrum (less MFS) as compared to NC (P < or =0.005) in the two muscle groups. This dissociation between the mechanical and myoelectrical manifestation of muscle fatigue in ALS indicates that a reduction in muscle fiber conduction velocity (MFCV) may be a contributing peripheral factor in the pathogenesis of muscle fatigue in ALS. Alterations in motor unit functionality, especially in type II fast motor unit muscle fibers, and structural damage in denervated muscle fibers may contribute to the lower MFCV during motor fatigue in ALS patients.

Quantitative assessment of motor fatigue in amyotrophic lateral sclerosis.

Motor fatigue is a common complaint in patients with amyotrophic lateral sclerosis (ALS), but is often excluded, unlike weakness, from the clinical assessment of these patients. This could be due to the complexity and often painful assessment techniques of this motor deficit. This study examines the feasibility of quantitative assessment of motor fatigue by modifying presently available force measurements. The relationship between weakness and fatigue in ALS patients was also examined. Fifty-four ALS patients and 39 normal control subjects performed 30 s of sustained maximal voluntary isometric contraction (MVIC) of elbow flexors (EF), knee extensors (NE), and ankle dorsiflexors (DF), using a computerized force measurement system and standardized testing procedures. Fatigue index (FI) was digitally calculated, from the force-time curve, as the percentage of MVIC unable to be sustained over the 30-s period. Fatigue was greater in ALS patients than in normal control (mean=23% vs. 15%) in all muscles including muscles that were not clearly weak. Weakness and fatigue were poorly correlated in ALS patients and may be independent measures of the pathogeneses of ALS.