Abstract
BackgroundHigh-frequency trains of electrical stimulation applied over the lower limb muscles can generate forces higher than would be expected from a peripheral mechanism (i.e. by direct activation of motor axons). This phenomenon is presumably originated within the central nervous system by synaptic input from Ia afferents to motoneurons and is consistent with the development of plateau potentials. The first objective of this work was to investigate if vibration (sinusoidal or random) applied to the Achilles tendon is also able to generate large magnitude extra torques in the triceps surae muscle group. The second objective was to verify if the extra torques that were found were accompanied by increases in motoneuron excitability.MethodsSubjects (n = 6) were seated on a chair and the right foot was strapped to a pedal attached to a torque meter. The isometric ankle torque was measured in response to different patterns of coupled electrical (20-Hz, rectangular 1-ms pulses) and mechanical stimuli (either 100-Hz sinusoid or gaussian white noise) applied to the triceps surae muscle group. In an additional investigation, Mmax and F-waves were elicited at different times before or after the vibratory stimulation.ResultsThe vibratory bursts could generate substantial self-sustained extra torques, either with or without the background 20-Hz electrical stimulation applied simultaneously with the vibration. The extra torque generation was accompanied by increased motoneuron excitability, since an increase in the peak-to-peak amplitude of soleus F waves was observed. The delivery of electrical stimulation following the vibration was essential to keep the maintained extra torques and increased F-waves.ConclusionsThese results show that vibratory stimuli applied with a background electrical stimulation generate considerable force levels (up to about 50% MVC) due to the spinal recruitment of motoneurons. The association of vibration and electrical stimulation could be beneficial for many therapeutic interventions and vibration-based exercise programs. The command for the vibration-induced extra torques presumably activates spinal motoneurons following the size principle, which is a desirable feature for stimulation paradigms.
Highlights
High-frequency trains of electrical stimulation applied over the lower limb muscles can generate forces higher than would be expected from a peripheral mechanism
The results showed that vibration bursts delivered to the Achilles tendon can consistently increase the force generated by the triceps surae (TS) muscle group while a basal train of 20-Hz electrical stimuli is applied to the TS
We suggest that the underlying mechanisms of neuronal adaptations may be optimised by the use of stimulations techniques that favour the stimulation of sensory axons, leading to enhanced contractions mediated by a central mechanism, as obtained by the combination of vibratory and electrical stimulation
Summary
High-frequency trains of electrical stimulation applied over the lower limb muscles can generate forces higher than would be expected from a peripheral mechanism (i.e. by direct activation of motor axons). This phenomenon is presumably originated within the central nervous system by synaptic input from Ia afferents to motoneurons and is consistent with the development of plateau potentials. That is, during a burst-like pattern that alternated periods of 25 and 100 Hz stimulation, more force was generated after the high-frequency burst than before it, despite the similar stimulus frequency and intensity [2,5,6]. These sustained forces observed following the highfrequency-bursts could continue even after the end of the stimulation period (i.e. when any stimulus was already turned off ) [5]
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