Central and Peripheral Fatigue of the Knee Extensor Muscles Induced by Electromyostimulation

Abstract

The main purpose of this study was to characterise neuromuscular fatigue induced by 30 contractions of the knee extensor muscles evoked by electromyostimulation (EMS). Twelve healthy subjects were tested before and after a typical EMS session (frequency: 75 Hz, on-off ratio: 6.25 s on-20 s off) used for quadriceps femoris muscle strengthening. Surface electromyographic (EMG) activity and torque obtained during maximal voluntary and electrically evoked contractions were analysed to distinguish peripheral from central fatigue. Maximal voluntary torque of the knee extensor muscles decreased approximately 20 % (p < 0.001) following EMS. In the same way, peak torque associated to single (p < 0.05) and paired (p < 0.001) stimuli as well as M-wave amplitude (p < 0.05) significantly decreased as a result of EMS. The raw EMG activity of both vastus lateralis and rectus femoris muscle recorded during maximal voluntary isometric contraction significantly decreased after the session (-17.3 and -14.5 %, respectively) whereas no changes were observed when EMG signals were normalised to respective M-wave amplitudes. Similarly, voluntary activation estimated by using the twitch interpolation technique was unchanged following EMS. In conclusion, a typical session of EMS of the knee extensor muscles mainly induced neuromuscular propagation failure while excitation-contraction coupling and neural mechanisms were not significantly affected. It is recommended to interpret surface EMG data together with the corresponding M wave, at least for the knee extensor muscles, in order to distinguish peripheral from central causes of fatigue.

Less indication of muscle damage in the second than initial electrical muscle stimulation bout consisting of isometric contractions of the knee extensors

“…3). Zory et al (2005) reported that MVC decreased approximately 20% immediately after 30 isometric contractions of the knee extensors evoked by EMS (75 Hz, on-off ratio 6.25-20 s), and neuromuscular propagation failure was mainly responsible for the decrease. It seems that excitation-contraction coupling failure was at least partially responsible for the decreases in MVC (Ingalls et al 2004); however, ultrastructural changes in myofibrils as shown in the study by Mackey et al (2008) may also account for the long-lasting decreases in MVC after EMS.…”

Comparative effect of a 1 h session of electrical muscle stimulation and walking activity on energy expenditure and substrate oxidation in obese subjects

“…Thus a limitation is that it is not possible to investigate any correlation between stimulation intensity and other physiological variables in this study. However, issues such as pain and (or) discomfort that are associated with application of NMES mean that it is difficult to rapidly increase intensity to produce high levels of muscle contraction force early during a session (Theurel et al 2007;Zory et al 2005). A gradual increase in intensity is required to accommodate the current.…”

Muscle fatigue: from observations in humans to underlying mechanisms studied in intact single muscle fibres

“…The resultant compound muscle action potential (M wave) and the maximal mechanical twitch response (peak twitch force, Pt) are recorded. A reduction in M-wave amplitude is interpreted as evidence of impaired neuromuscular transmission or action potential propagation (BiglandRitchie et al 1982) and if Pt is reduced to a comparable extent, this probably indicates that the impaired force production is due to reduced muscle excitability (Zory et al 2005). However, it should be noted that action potentials in muscle cells might have a large safety margin so that their amplitude can be greatly reduced before Ca 2?…”