Muscle-related differences in mechanomyography–force relationships are model-dependent

Endurance training alters motor unit activation strategies for the vastus lateralis, yet sex-related differences and relationships with muscle size remain

PURPOSE

To examine the effects of 10 weeks of endurance cycling training on mechanomyographic amplitude (MMG_RMS)-torque relationships and muscle cross-sectional area (mCSA) of the vastus lateralis (VL) for 10 sedentary males (Age ± SD; 20.2 ± 1.9 years) and 14 sedentary females (21.9 ± 5.3 years).

METHODS

Participants performed maximal voluntary contractions (MVCs) and an isometric ramp up muscle action to 70% MVC of the knee extensors before (PRE) and after training at the same absolute pre-treatment submaximal torque (POST_ABS). MMG was recorded from the VL and b terms were calculated from the natural log-transformed MMG_RMS-torque relationships for each subject. mCSA was determined with ultrasonography.

RESULTS

Cycling decreased MVCs from pre- (168.10 ± 58.49 Nm) to post-training (160.78 ± 58.39 Nm; p = 0.005) without changes in mCSA. The b terms were greater for POST_ABS (0.623 ± 0.204) than PRE (0.540 ± 0.226; p = 0.012) and for males (0.717 ± 0.171) than females (0.484 ± 0.168; p = 0.003). mCSA was correlated with the b terms for PRE (p < 0.001, r = 0.674) and POST_ABS (p = 0.020, r = 0.471).

CONCLUSION

The decrease in MVC and increase in MMG_RMS (b terms) post-training suggests increased motor unit (MU) recruitment to match pre-training torques. The greater acceleration in the b terms by males may reflect sex-related differences in fiber-type area. MMG_RMS-torque relationships during a high-intensity contraction provided insight on MU activation strategies following endurance training and between sexes. Furthermore, the findings suggest a relationship between MMG_RMS and muscle size.

Association of anthropometric parameters with amplitude and crosstalk of mechanomyographic signals during forearm flexion, pronation and supination torque tasks

This study aimed to quantify the association of four anthropometric parameters of the human arm, namely, the arm circumference (CA), arm length (LA), skinfold thickness (ST) and inter-sensor distance (ISD), with amplitude (RMS) and crosstalk (CT) of mechanomyography (MMG) signals. Twenty-five young, healthy, male participants were recruited to perform forearm flexion, pronation and supination torque tasks. Three accelerometers were employed to record the MMG signals from the biceps brachii (BB), brachialis (BRA) and brachioradialis (BRD) at 80% maximal voluntary contraction (MVC). Signal RMS was used to quantify the amplitude of the MMG signals from a muscle, and cross-correlation coefficients were used to quantify the magnitude of the CT among muscle pairs (BB & BRA, BRA & BRD, and BB & BRD). For all investigated muscles and pairs, RMS and CT showed negligible to low negative correlations with CA, LA and ISD (r = −0.0001–−0.4611), and negligible to moderate positive correlations with ST (r = 0.004–0.511). However, almost all of these correlations were statistically insignificant (p > 0.05). These findings suggest that RMS and CT values for the elbow flexor muscles recorded and quantified using accelerometers appear invariant to anthropometric parameters.

Log-Transformed Electromyography Amplitude-Power Output Relationship: Single-Leg Knee-Extensor Versus Single-Leg Cycle Ergometry

Noble, EB, Pilarski, JM, Vora, HK, Zuniga, JM, and Malek, MH. Log-transformed electromyography amplitude-power output relationship: single-leg knee-extensor versus single-leg cycle ergometry. J Strength Cond Res 33(5): 1311-1319, 2019-Comparing and contrasting motor unit recruitment and activation for the same muscles for multiple versus single-joint exercise may provide a better understanding of neuromuscular fatigue. The purpose of this study, therefore, is to compare the slope and y-intercept terms for the 3 superficial quadriceps femoris (QF) (vastus lateralis, rectus femoris, and vastus medialis) muscles derived from the log-transformed electromyography (EMG) amplitude-power output relationship between the single-leg knee-extensor ergometry (KE) and the single-leg cycle ergometry (CE). Ten healthy college-aged men who engaged in regular physical activity visited the laboratory on 2 occasions separated by 7 days to perform either single-leg CE or single-leg KE in a randomized order. For each visit, subjects performed incremental exercise until voluntary fatigue. Electromyography electrodes were placed on the superficial QF muscles. The slope and y-intercept terms, for each muscle, for the EMG amplitude versus power output relationship was examined using the log-transformed model for each subject's data. The results indicated no significant (p > 0.05) mean differences for either slope or y-intercept terms between exercise modes and across muscles. In addition, separate repeated-measures analyses of variance (ANOVAs) were used to determine mean differences for the slope and y-intercept values between the 3 muscles. In addition, separate 2 (mode: CE or KE) × intensity (intensity: 30, 60, and 90% of maximal workload) repeated-measures ANOVAs were conducted for each muscle. There was a significant (p < 0.05) mode × exercise intensity interaction for each muscle. Follow-up testing indicated that in most cases, the normalized EMG amplitude was significantly higher for single-leg KE than single-leg CE. These results indicated that incremental single-leg KE activates the superficial QF muscles significantly greater than single-leg CE.

The influence of myosin heavy chain isoform content on mechanical behavior of the vastus lateralis in vivo

This study examined correlations between type I percent myosin heavy chain isoform content (%MHC) and mechanomyographic amplitude (MMGRMS) during isometric muscle actions. Fifteen (age=21.63±2.39) participants performed 40% and 70% maximal voluntary contractions (MVC) of the leg extensors that included increasing, steady force, and decreasing segments. Muscle biopsies were collected and MMG was recorded from the vastus lateralis. Linear regressions were fit to the natural-log transformed MMGRMS-force relationships (increasing and decreasing segments) and MMGRMS was selected at the targeted force level during the steady force segment. Correlations were calculated among type I%MHC and the b (slopes) terms from the MMGRMS-force relationships and MMGRMS at the targeted force. For the 40% MVC, correlations were significant (P<0.02) between type I%MHC and the b terms from the increasing (r=-0.804) and decreasing (r=-0.568) segments, and MMGRMS from the steady force segment (r=-0.606). Type I%MHC was only correlated with MMGRMS during the steady force segment (P=0.044, r=-0.525) during the 70% MVC. Higher type I%MHC reduced acceleration in MMGRMS (b terms) during the 40% MVC and the amplitude during the steady force segments. The surface MMG signal recorded during a moderate intensity contraction provided insight on the contractile properties of the VL in vivo.

Mechanomyographic mean power frequency during an isometric trapezoid muscle action at multiple contraction intensities

This study examined the mechanomyographic mean power frequency (MMGMPF)-force relationships for five (age = 19.20   ±   0.45 years) aerobically-trained (AT), five (age = 25   ±   4.53 years) resistance-trained (RT), and five (age = 21.20   ±   2.17 years) sedentary (SED) individuals. Participants performed isometric trapezoidal muscle actions at 50, 60, and 70% maximal voluntary contraction (MVC) of the leg extensors that included linearly increasing, steady force, and linearly decreasing segments. MMG was recorded from the vastus lateralis. Linear regressions were fit to the natural-log transformed MMGMPF versus natural log-transformed force relationships (linearly increasing and decreasing segments) with the b (slope) and a (y-intercept) terms used for comparisons. MMGMPF was averaged for the entire steady force segment. The b and a terms were not different among training statuses (P  >  0.05) or linearly increasing and decreasing segments (P  >  0.05). There were muscle action-related differences in the b terms as a function of training status from the 70% MVC. The SED had greater b terms during the linearly increasing than decreasing muscle action (P = 0.010), and the converse was true for the AT (P = 0.013), whereas the RT displayed no muscle action-related differences (P  >  0.05). The unique muscle action-related differences in the b terms as a function of training status may be the result of unique adaptations to motor unit activation and deactivation strategies.

Relationships between skinfold thickness and electromyographic and mechanomyographic amplitude recorded during voluntary and non-voluntary muscle actions

INTRODUCTION

The purpose of this study was to examine possible correlations between skinfold thicknesses and the a terms from the log-transformed electromyographic (EMGRMS) and mechanomyographic amplitude (MMGRMS)-force relationships, EMG M-Waves, and MMG gross lateral movements (GLM).

METHODS

Forty healthy subjects performed a 6-s isometric ramp contraction from 5% to 85% of their maximal voluntary contraction with EMG and MMG sensors placed on the vastus lateralis (VL) and rectus femoris (RF). A single electrical stimulus was applied to the femoral nerve to record the EMG M-waves and MMG GLMs. Skinfold thickness was assessed at the site of each electrode. Pearson's product correlation coefficients were calculated comparing skinfold thicknesses with the a terms from the log-transformed EMGRMS-and MMGRMS-force relationships, EMG M-waves, and MMG GLMs.

RESULTS

There were no significant cor1relations (p>0.05) between the a terms and skinfold thicknesses for the RF and VL from the EMGRMS and MMGRMS-force relationships. However, there were significant correlations (p<0.05) between skinfold thicknesses and the EMG M-waves and MMG GLMs for the RF (r=-0.521, -0.376) and VL (r=-0.479, -0.484).

DISCUSSION

Relationships were only present between skinfold thickness and the amplitudes of the EMG and MMG signals during the non-voluntary muscle actions.