The surface mechanomyogram as a tool to describe the influence of fatigue on biceps brachii motor unit activation strategy. Historical basis and novel evidence Claudio Orizio, Massimiliano Gobbo, Bertrand Diemont, Fabio Esposito and Arsenio Veicsteinas Eur J Appl Physiol (2003) 90:

Introduction

Surface MMG MMG records movement of the muscle MMG records movement of the muscle recruited MUs taking up slack of the resting muscle recruited MUs taking up slack of the resting muscle mirrors the force-generation process mirrors the force-generation process

MMG at Low Firing Rates At low firing rates MMG is a linear sum of the transverse activity of motor units. At low firing rates MMG is a linear sum of the transverse activity of motor units.

MMG – Recruitment & Firing Rate When recruitment and firing rate increase, MMG increases up to the end of MU recruitment. When recruitment and firing rate increase, MMG increases up to the end of MU recruitment. MMG can be used for Motor Unit Activation Strategy. MMG can be used for Motor Unit Activation Strategy.

MMG & Activation Strategy MMG amplitude and frequency can be used to identify the end of recruitment. The muscle continues to generate additional force via increased firing rate (reflected by increased MF). MMG amplitude and frequency can be used to identify the end of recruitment. The muscle continues to generate additional force via increased firing rate (reflected by increased MF).

MMG Amplitude at Fatigue MMG-RMS increase (low level of effort) may be related to MMG-RMS increase (low level of effort) may be related to recruitment of new MUs. recruitment of new MUs. global MUs firing increment global MUs firing increment the synchronization and grouping of the active MUs the synchronization and grouping of the active MUs MMG-RMS decrease (high level of effort) may be related to MMG-RMS decrease (high level of effort) may be related to de-recruitment of fast fatiguing MUs de-recruitment of fast fatiguing MUs the reduction in the MUs FR the reduction in the MUs FR the prolongation of the mechanical twitch leading to better fusion of the mechanical events the prolongation of the mechanical twitch leading to better fusion of the mechanical events

MMG Frequency at Fatigue MMG spectral changes can be explained as MMG spectral changes can be explained as high frequency content reduction may reflect prolongation of the single mechanical events high frequency content reduction may reflect prolongation of the single mechanical events reduction of the global MUs FR reduction of the global MUs FR de-recruitment of fatigued fast twitch MUs de-recruitment of fatigued fast twitch MUs synchronization of MUs (resembles increased tremor in force of fatigued muscle). synchronization of MUs (resembles increased tremor in force of fatigued muscle). transient increase in the high frequency at some intensities could be due to the increase in MUs firing and/or faster mechanical events (possibly due to the potentiation) transient increase in the high frequency at some intensities could be due to the increase in MUs firing and/or faster mechanical events (possibly due to the potentiation)

Purpose of the Study (1) describing the possible changes in the dynamics of EMG and MMG time and frequency domain parameters from low to high intensity of contraction; (1) describing the possible changes in the dynamics of EMG and MMG time and frequency domain parameters from low to high intensity of contraction; (2) providing some contribution to the investigation of MUAS in fatigued biceps brachii. (2) providing some contribution to the investigation of MUAS in fatigued biceps brachii.

Methods -- EMG EMG was detected from the belly of the biceps brachii muscle by means of two silver bars (0.5×1.0 cm) spaced 1 cm apart and 1 cm distally from the motor point. EMG was detected from the belly of the biceps brachii muscle by means of two silver bars (0.5×1.0 cm) spaced 1 cm apart and 1 cm distally from the motor point. The signal was amplified and filtered (bandwidth: 3–500 Hz). The signal was amplified and filtered (bandwidth: 3–500 Hz).

Methods -- MMG MMG was detected by an accelerometer (Entran EGA 25 D, bandwidth 0–800 Hz, dimensions: 1x0.5x0.5 cm, mass 0.5 g, sensitivity 5 mV/g) MMG was detected by an accelerometer (Entran EGA 25 D, bandwidth 0–800 Hz, dimensions: 1x0.5x0.5 cm, mass 0.5 g, sensitivity 5 mV/g) Placed between the two silver electrodes and fixed to the skin using double adhesive tape. Placed between the two silver electrodes and fixed to the skin using double adhesive tape. The MMG signal was then amplified and filtered between 5 and 250 Hz. The MMG signal was then amplified and filtered between 5 and 250 Hz. The force signal (bandwidth: 0–128 Hz), the EMG and the MMG were recorded on computer after analogue-to-digital conversion (sampling rate: 1024 Hz The force signal (bandwidth: 0–128 Hz), the EMG and the MMG were recorded on computer after analogue-to-digital conversion (sampling rate: 1024 Hz

Fatigue Protocol After MVC After MVC first 6.75 s ramp first 6.75 s ramp followed by the intermittent series of 6 s on + 3 s off 50% MVC contractions. followed by the intermittent series of 6 s on + 3 s off 50% MVC contractions. The last contraction was the one in which the pre-fatigue 50% MVC could not be maintained within ± 5% of the target value, for the whole 6 s period. This effort was identified as the new MVC for the subject. The last contraction was the one in which the pre-fatigue 50% MVC could not be maintained within ± 5% of the target value, for the whole 6 s period. This effort was identified as the new MVC for the subject. Within 3 s a new 6.75 s ramp was administered. The 90% MVC of fatigued muscle corresponded to the 45% MVC of the fresh muscle Within 3 s a new 6.75 s ramp was administered. The 90% MVC of fatigued muscle corresponded to the 45% MVC of the fresh muscle

Signal Analysis Amplitude (RMS) and frequency (MF) of EMG and MMG. Amplitude (RMS) and frequency (MF) of EMG and MMG. Range from 10-90% MVC was analyzed Range from 10-90% MVC was analyzed windows covered the 15–85% MVC range effort with a step of 5%, yielding 15 windows. windows covered the 15–85% MVC range effort with a step of 5%, yielding 15 windows. Each window was centered on a multiple of 5% MVC. Each window was centered on a multiple of 5% MVC. STATISTICS: MMG and EMG were log transformed to normalize their distribution. STATISTICS: MMG and EMG were log transformed to normalize their distribution.

Fatigue Effects EMG, MMG and force during the two ramps. The most striking result is the different behaviour of MMG throughout the ramp before and after fatiguing intermittent contractions. EMG, MMG and force during the two ramps. The most striking result is the different behaviour of MMG throughout the ramp before and after fatiguing intermittent contractions.

EMG Amplitude & Frequency Average EMG in fresh and fatigued muscle during 15– 85% MVC ramps. Average EMG in fresh and fatigued muscle during 15– 85% MVC ramps. EMG–RMS does not present any appreciable differences in the two conditions. EMG–RMS does not present any appreciable differences in the two conditions. In fatigued muscle the EMG–MF trend is shifted about 25 Hz below the pre- fatigue values. In fatigued muscle the EMG–MF trend is shifted about 25 Hz below the pre- fatigue values. The vertical bars show large SDs The vertical bars show large SDs

MMG in fresh and fatigued muscle during the 15–85% MVC ramps. MMG in fresh and fatigued muscle during the 15–85% MVC ramps. MMG–RMS dynamics in fatigued muscle are completely different to those in fresh muscle. They do not increase in the 20–65% MVC range. MMG–RMS dynamics in fatigued muscle are completely different to those in fresh muscle. They do not increase in the 20–65% MVC range. On the contrary they show a continuous decrease beyond 25% MVC. On the contrary they show a continuous decrease beyond 25% MVC. As for the EMG–MF the MMG–MF trend in fatigued muscle is shifted towards lower values. As for the EMG–MF the MMG–MF trend in fatigued muscle is shifted towards lower values. Beyond 65% MVC the trend of MMG–MF does not present a steep increase when muscle is fatigued. Beyond 65% MVC the trend of MMG–MF does not present a steep increase when muscle is fatigued. The vertical bars large SDs. The vertical bars large SDs.

High FR & MMG–RMS decrease MMG–MF seems to be more sensitive to the global MUs FR can be explained by taking into account that the elementary event contributing to MMG, per MU activation, is much longer (about 100 ms) than the MUAP (about 10 ms) contributing to EMG. MMG–MF seems to be more sensitive to the global MUs FR can be explained by taking into account that the elementary event contributing to MMG, per MU activation, is much longer (about 100 ms) than the MUAP (about 10 ms) contributing to EMG. At the lowest firing frequency of about 10 Hz MMG may be regarded, because of a partial fusion of the single events, as the summation of more or less distorted sinusoids and not as the summation of trains of fast distinct MUAPs, as in the EMG. At the lowest firing frequency of about 10 Hz MMG may be regarded, because of a partial fusion of the single events, as the summation of more or less distorted sinusoids and not as the summation of trains of fast distinct MUAPs, as in the EMG. It can be concluded that the MUs global firing information (10–40 Hz range) may dominate the MMG spectrum It can be concluded that the MUs global firing information (10–40 Hz range) may dominate the MMG spectrum while the MUAPs shape information is mainly reflected in the EMG spectrum. while the MUAPs shape information is mainly reflected in the EMG spectrum.

MMG Amplitude & Fatigue The recruitment of new and more superficial MUs as well as the FR increase of the already active MUs in the 15–65% MVC may explain the MMG–RMS increment in the same force range. The recruitment of new and more superficial MUs as well as the FR increase of the already active MUs in the 15–65% MVC may explain the MMG–RMS increment in the same force range. Beyond 65% MVC the reduction of the MMG–RMS could be attributed to the high FR attained to increase force. Beyond 65% MVC the reduction of the MMG–RMS could be attributed to the high FR attained to increase force. The upper limit for the complete recruitment (REC-end) was 65% MVC The upper limit for the complete recruitment (REC-end) was 65% MVC

Rate of Force Development The production of different force rates may provide REC-end at different % MVC. The production of different force rates may provide REC-end at different % MVC. Akataki et al. (2001), strongly supports the hypothesis that the global MUs firing frequency is reflected in the MMG spectrum. Akataki et al. (2001), strongly supports the hypothesis that the global MUs firing frequency is reflected in the MMG spectrum.2001 Simultaneous MMG–RMS decrease and the MMG–MF increase in the 65–90% MVC range is new confirmation for the MUAS key point represented by the end of REC and the crucial use of FR increase to attain the highest contraction intensities. Simultaneous MMG–RMS decrease and the MMG–MF increase in the 65–90% MVC range is new confirmation for the MUAS key point represented by the end of REC and the crucial use of FR increase to attain the highest contraction intensities.