Abstract
K+ channel blockers like 3,4-diaminopyridine (DAP) can double isometric muscle force. Functional movements require more complex concentric and eccentric contractions, however the effects of K+ channel blockade on these types of contractions in situ are unknown. Extensor digitorum longus (EDL) muscles were stimulated in situ with and without DAP in anesthetized rats and fatigability was addressed using a series of either concentric or eccentric contractions. During isotonic protocols (5–100% load), DAP significantly shifted shortening- and maximum shortening velocity-load curves upward and to the right and increased power and work. Maximum shortening, maximum shortening velocity, and power doubled while work increased by ∼250% during isotonic contraction at 50% load. During isotonic fatigue, DAP significantly augmented maximum shortening, work, shortening velocity, and power. During constant velocity eccentric protocols (2–12 mm/s), DAP increased muscle force during eccentric contractions at 6, 8, 10, and 12 mm/s. During eccentric contraction at a constant velocity of 6 mm/s while varying the stimulation frequency, DAP significantly increased muscle force during 20, 40, and 70 Hz. The effects of DAP on muscle contractile performance during eccentric fatigue varied with level of fatigue. DAP-induced contractile increases during isotonic contractions were similar to those produced during previously studied isometric contractions, while the DAP effect during eccentric contractions was more modest. These findings are especially important in attempting to optimize functional electrical stimulation parameters for spinal cord injury patients while also preventing rapid fatigue of those muscles.
Highlights
The fundamental strategy for successful functional neuromuscular stimulation of skeletal muscles is the use of stimulation paradigms which maximize muscle force yet minimize muscle fatigue
The extensor digitorum longus (EDL) tendon was attached to the lever arm of a dual-mode servocontrolled force transducer which was controlled by a computer using a data acquisition program [Dynamic Muscle Control (DMC) Software, Aurora Scientific]
During isotonic contractions, maximum shortening, maximum shortening velocity, and power doubled while work increased by ∼250%
Summary
The fundamental strategy for successful functional neuromuscular stimulation of skeletal muscles is the use of stimulation paradigms which maximize muscle force yet minimize muscle fatigue. One common strategy has been to use variable frequency train stimulation, in which a brief high frequency burst precedes a lower frequency stimulus train, thereby eliciting the catch-like property of muscle (Binder-Macleod and Barrish, 1992; Binder-Macleod et al, 1998; Bigland-Ritchie et al, 2000; van Lunteren and Sankey, 2000). This approach has not been very successful in clinical use due to only modest force increases. In previous in vitro studies, the maximum DAPinduced extensor digitorum longus (EDL) twitch force increase was 94 ± 12% (van Lunteren et al, 2006)
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