Article

Secondary-progressive multiple sclerosis (SPMS) patients have
structural cortical damage resulting in increased compensatory cortical activity
during (submaximal) performance. However, functional effects of changed cortical
output are difficult to measure. The interpolated-twitch technique allows for
measurement of voluntary activation (VA) necessary for force production. This
study aimed to determine VA, force, and muscle fatigue during brief and sustained
contractions in SPMS patients. Because fatigue effects are not confined to the
motor system, we additionally examined fatiguing effects on cognitive
performance. METHODS: Twenty-five SPMS and 25 sex-, age-, and education-matched
participants performed brief (5 seconds) and sustained (2 minutes) maximal index
finger abductions. To evaluate VA, double-pulse twitches were evoked before,
during, and after contractions. Additionally, data were compared with data
obtained in relapsing-remitting multiple sclerosis (RRMS) patients. Subjects also
performed choice-reaction time tasks before and after the sustained contraction.
RESULTS: During brief contractions, VA (85% vs 94%,P= .004) and force (25 N vs 32
N,P= .011) were lower for SPMS patients than controls. During sustained
contractions, VA (P= .001) was also lower, resulting in greater force decline
(73% vs 63%,P< .001) and reduced peripheral fatigue (19% vs 50%,P< .001).
Comparisons with RRMS resulted in lower VA, greater force decline, and greater
estimated central fatigue in SPMS. SPMS patients were slower (P< .001) and made
more errors (P< .001) than controls, but neither group reduced their performance
after the sustained contraction. CONCLUSION: SPMS patients had lower VA than RRMS
patients and controls. The importance of voluntary activation for muscle force
and fatigability warrants targeted rehabilitation strategies.
CI - (c) The Author(s) 2015.

Langue : ANGLAIS