Abstract
Even under isometric conditions, muscle contractions are associated with some degree of fiber shortening. The effects of muscle shortening on extracellular electromyographic potentials have not been characterized in detail. Moreover, the anatomical, biophysical, and detection factors influencing the muscle-shortening effects have been neither identified nor understood completely. Herein, we investigated the effects of muscle shortening on the amplitude and duration characteristics of single-fiber, motor unit, and compound muscle action potentials. We found that, at the single-fiber level, two main factors influenced the muscle-shortening effects: (1) the electrode position and distance relative to the myotendinous zone and (2) the electrode distance to the maxima of the dipole field arising from the stationary dipole created at the fiber-tendon junction. Besides, at the motor unit and muscle level, two additional factors were involved: (3) the overlapping between the propagating component of some fibers with the non-propagating component of other fibers and (4) the spatial spreading of the fiber-tendon junctions. The muscle-shortening effects depend critically on the electrode longitudinal distance to the myotendinous zone. When the electrode was placed far from the myotendinous zone, muscle shortening resulted in an enlargement and narrowing of the final (negative) phase of the potential, and this enlargement became less pronounced as the electrode approached the fiber endings. For electrode locations close to the myotendinous zone, muscle shortening caused a depression of both the main (positive) and final (negative) phases of the potential. Beyond the myotendinous zone, muscle shortening led to a decrease of the final (positive) phase. The present results provide reference information that will help to identify changes in MUPs and M waves due to muscle shortening, and thus to differentiate these changes from those caused by muscle fatigue.Graphical abstract
Highlights
During voluntary isometric contractions, muscles undergo important changes in their architecture, such as shortening of fascicle lengths [22, 26], increases in pennation angles [20], fascicle rotation [5], and alterations in the relative orientation of muscle fibers with respect to the detection system [10]
Electrode placed between the neuromuscular and fiber-tendon junctions, it can be seen that, for electrode locations far from the tendon junction (dIZ = 25 mm, Fig. 4(a)), fiber shortening caused an increase in the single-fiber action potentials (SFAPs) final phase, whereas the opposite occurred for electrode locations close to this junction (dIZ = 45 mm, Fig. 4(b))
Electrode placed beyond the fibertendon junction, it can be seen that, for electrode locations a little beyond tendon junction (dIZ = 70 mm, Fig. 4(c)), fiber shortening caused an increase in the SFAP final phase, whereas, for electrode locations far beyond this junction (dIZ = 80 mm, Fig. 4(d)), the SFAP final phase decreased when the fiber was shortened
Summary
Muscles undergo important changes in their architecture, such as shortening of fascicle lengths [22, 26], increases in pennation angles [20], fascicle rotation [5], and alterations in the relative orientation of muscle fibers with respect to the detection system [10]. Among the above architectural alterations, muscle shortening is relevant due to its mechanical, physiological, and electrical implications. In mechanical terms, shortening of fascicle lengths during an isometric. A reduction of the fiber length causes alterations in the shape of individual single-fiber action potentials (SFAPs) and in the pattern of summation of these SFAPs in motor unit potentials (MUPs) and compound muscle action potentials (M waves). Previous studies on muscle shortening have concentrated more on assessing how such shortening influences the spectral and amplitude characteristics of the interference surface EMG signal [24, 36], rather than on examining the effects on the shape of individual SFAPs, MUPs, and M waves.
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