Normalized Electromyography Amplitude Research Articles

Are performance and perceived fatigability dependent on the anchor scheme of fatiguing isometric tasks in men?

Ratings of perceived exertion (RPE) can be used to regulate exercise intensity. This study examined the effect of anchor scheme on performance fatigability and neuromuscular responses following fatiguing forearm flexion tasks. Twelve men (age 20.9±2.2 years; height 179.8±5.3 cm; body mass 80.2±9.9 kg) performed sustained, isometric forearm flexion tasks to failure anchored to RPE=6 (RPEFT) and the torque (TRQFT) that corresponded to RPE=6. Pre-test and post-test maximal voluntary isometric contractions (MVIC) were performed to quantify changes in the amplitude (AMP) and mean power frequency (MPF) of the electromyographic (EMG) and mechanomyographic (MMG) signals. Neuromuscular efficiency (NME) was calculated by dividing normalized torque by normalized EMG AMP. A dependent t-test was used to assess the mean difference for time to task failure (TTF). Repeated measures ANOVAs were used to compare mean differences for performance fatigability and normalized neuromuscular parameters. The RPEFT had a greater TTF than the TRQFT (P<0.001). MVIC and NME decreased from pre-test to post-test following the RPEFT and TRQFT (P<0.05) with no differences between anchor schemes. Following the TRQFT, normalized EMG MPF decreased from pre-test to post-test (P=0.004). Following the RPEFT, normalized MMG MPF increased from pre-test to post-test (P=0.021). There were no changes in normalized EMG AMP or MMG AMP (P>0.05). These findings indicated anchor scheme-specific neuromuscular responses and TTF, despite no difference in performance fatigability. Furthermore, performance fatigability was likely due to peripheral fatigue (based on normalized EMG MPF and NME) following the TRQFT, but peripheral and central fatigue (based on normalized MMG MPF and NME) following the RPEFT.

The Effects of Anchoring a Fatiguing Forearm Flexion Task to a High vs. Low Rating of Perceived Exertion on Torque and Neuromuscular Responses.

Ortega, DG, Housh, TJ, Smith, RW, Arnett, JE, Neltner, TJ, Schmidt, RJ, and Johnson, GO. The effects of anchoring a fatiguing forearm flexion task to a high versus low rating of perceived exertion on torque and neuromuscular responses. J Strength Cond Res 38(5): e219-e225, 2024-This study examined the torque and neuromuscular responses following sustained, isometric, forearm flexion tasks anchored to 2 ratings of perceived exertion (RPE). Nine men (mean ± SD: age = 21.0 ± 2.4 years; height = 179.5 ± 5.1 cm; body mass = 79.6 ± 11.4 kg) completed maximal voluntary isometric contractions (MVIC) before and after sustained, isometric, forearm flexion tasks to failure anchored to RPE = 2 and RPE = 8. The amplitude (AMP) and mean power frequency (MPF) of the electromyographic (EMG) signal were recorded from the biceps brachii. Normalized torque was divided by normalized EMG AMP to calculate neuromuscular efficiency (NME). A dependent t-test was used to assess the mean difference for time to task failure (TTF). Repeated-measures analysis of variances was used to compare mean differences for MVIC and normalized neuromuscular parameters. There was no significant difference in TTF between RPE = 2 and RPE = 8 (p = 0.713). The MVIC decreased from pretest to posttest at RPE = 2 (p = 0.009) and RPE = 8 (p = 0.003), and posttest MVIC at RPE = 8 was less than that at RPE = 2 (p < 0.001). In addition, NME decreased from pretest to posttest (p = 0.008). There was no change in normalized EMG AMP or EMG MPF (p > 0.05). The current findings indicated that torque responses were intensity specific, but TTF and neuromuscular responses were not. Furthermore, normalized EMG AMP and EMG MPF remained unchanged but NME decreased, likely due to peripheral fatigue and excitation-contraction coupling failure. Thus, this study provides information regarding the neuromuscular responses and mechanisms of fatigue associated with tasks anchored to RPE, which adds to the foundational understanding of the relationship between resistance exercise and the perception of fatigue.

Sex-related differences in motor unit behavior are influenced by myosin heavy chain during high- but not moderate-intensity contractions.

Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified. Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMGRMS ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAPAMP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies. There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAPAMP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMGRMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018-0.031) with the slopes for the MUAPAMP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585). Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMGRMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.

Low-Level Laser Therapy Facilitates Post-Contraction Recovery with Ischemic Preconditioning.

Despite early development of muscle fatigue, ischemic preconditioning is gaining popularity for strength training combined with low-load resistance exercise. This study investigated the effect of low-level laser (LLL) on post-contraction recovery with ischemic preconditioning. Forty healthy adults (22.9 ± 3.5 years) were allocated into sham (11 males, 9 females) and LLL (11 males, 9 females) groups. With ischemic preconditioning, they were trained with three bouts of intermittent wrist extension of 40% maximal voluntary contraction (MVC). During the recovery period, the LLL group received low-level laser (wavelength of 808 nm, 60 Joules) on the working muscle, whereas the sham group received no sham therapy. MVC, force fluctuations, and discharge variables of motor units (MU) for a trapezoidal contraction were compared between groups at baseline (T0), post-contraction (T1), and after-recovery (T2). At T2, the LLL group exhibited a higher normalized MVC (T2/T0) (86.22 ± 12.59%) than that of the sham group (71.70 ± 13.56%)(p = .001). The LLL group had smaller normalized force fluctuations (LLL: 94.76 ± 21.95%, Sham: 121.37 ± 29.02%, p = .002) with greater normalized EMG amplitude (LLL: 94.33 ± 14.69%, Sham: 73.57 ± 14.94%, p < .001) during trapezoidal contraction. In the LLL group, the smaller force fluctuations were associated with lower coefficients of variation of inter-spike intervals of MUs (LLL: .202 ± .053, Sham: .208 ± .048, p = .004) with higher recruitment thresholds (LLL: 11.61 ± 12.68 %MVC, Sham: 10.27 ± 12.73 %MVC, p = .003). Low-level laser expedites post-contraction recovery with ischemic preconditioning, manifesting as superior force generation capacity and force precision control for activation of MUs with a higher recruitment threshold and lower discharge variability.

Normalization of the electromyography amplitude during a multiple-set resistance training protocol: Reliability and differences between approaches.

The first aim was to investigate the impact of different electromyography (EMG) parameters as a reference to normalize the EMG amplitude of the superficial quadriceps femoris muscles across different sets of a knee extension exercise. The second aim is to examine the reliability between days of the EMG parameters used as a reference. Eleven young males attended the laboratory on 4 different days and performed one repetition maximum test, maximumvoluntary isometric contractions, and a resistance training protocol until failure. Surface EMG was placed over the rectus femoris, vastus lateralis, and vastus medialis muscles. Seven EMG parameters were calculated from the tasks and used to normalize EMG amplitude measured during the resistance training protocol. A repeated-measures two-way ANOVA was used (normalized EMG amplitude×set) to compare normalized EMG across sets, while an intraclass correlation coefficient, coefficient of variation, and Bland-Altman plots were used to calculate the intra-day reliability of the EMG parameters. The present investigation showed that normalized EMG amplitude of the superficial muscles of the quadriceps measured during a knee extension exercise is influenced by the EMG parameter and depends on the muscle. While rectus femoris and vastus lateralis normalized EMG amplitude presented one parameter among seven showing similar value to the other parameters, VM showed two. Lastly, all EMG parameters for all muscles presented an overall excellent reliability and agreement between days.

Snowboard Landings from Different Heights: Electroencephalography Activity in Motor Preparation and Lower Limb Electromyography Changes.

The neural strategies for movements of the lower extremities for landings from different landing heights in trained half-pipe snowboarders are not well known. We observed changes in brain activity as measured by electroencephalography (EEG) and lower limb muscle activity as measured by electromyography (EMG) in trained and untrained half-pipe snowboarders landing from different heights (30 and 60cm). There were 12 trained male half-pipe snowboarders (HS) and 12 untrained participants (UP). We recorded EEG signals during motor preparation prior to dropping and EMG signals from right lateral rectus femoris (RF), tibialis anterior (TA), and gastrocnemius lateralis (GL) muscles during landings. Generally, theta power in the frontal cortex significantly increased in the preparation period compared to the resting state, while the alpha 1 and alpha 2 power values in central and parietal cortical areas decreased as dropping heights increased. Additionally, the HS group displayed greater magnitudes of change in power values in three frequency bands compared to the UP group. The HS group (relative to UP group) also showed higher normalized EMG amplitudes for RF and GL during contact, especially at 60cm. The HS group (relative to the UP group) presented lower antagonist EMG activity and a higher GL/TA ratio at the 60cm dropping height. Long-term specialized training might lead to greater neural modulation of predictive sensorimotor control and specific neuromuscular activation patterns during landing.

Neuromuscular, Swelling, And Perceived Effort Responses Following Isotonic Versus Isokinetic Blood Flow Restriction Exercise

PURPOSE: The application of blood flow restriction (BFR) with resistance training has been applied to various exercise modalities, but no previous investigations have examined isotonic BFR vs isokinetic BFR. Therefore, the purpose of this study was to examine the neuromuscular, swelling, and rating of perceived exertion (RPE) responses following isotonic and isokinetic BFR exercise. METHODS: Fifteen (22 ± 2 years) women randomly performed (on separate days) 75 (1x30, 3x15) submaximal (30% of one repetition maximum [isotonic] or 30% maximal voluntary isometric contraction (MVIC) [isokinetic]) unilateral leg extensions with BFR (60% of total arterial occlusion pressure). Before and immediately after each exercise, subjects performed MVICs while electromyography (EMG) was simultaneously recorded from the vastus lateralis (VL) muscle. Ultrasound muscle thickness (MT) was assessed from the VL prior to and after the MVICs. RPE was recorded during all 4 sets. Separate 2 (Condition [Isotonic, Isokinetic]) × 2 (Time [pretest, posttest]) repeated measures ANOVAs were used to examine the EMG and MT responses. A separate 2 (Condition [Isotonic, Isokinetic]) × 4 (Time [set 1, set 2, set 3, set 4]) repeated measures ANOVAs was used to assess RPE. Paired samples t-tests were used to analyze percent change in MVIC torque. RESULTS: There was no significant (p = 0.294-0.823) interaction or main effect for normalized EMG amplitude. For EMG frequency and MT there were significant interactions (p = 0.008) that indicated greater decreases in EMG frequency following isotonic (13%) than isokinetic (6%) but similar increases in MT (22%). For RPE, there was no significant (p = 0.368) interaction, but there were significant (p = 0.004) main effects for Time (set 4 > set 3 > set 2 > set 1) and Condition (isotonic > isokinetic) (p = 0.018). Additionally, there was a greater (34.9%) decrease in MVIC following the isotonic than isokinetic (15.2%) condition. CONCLUSIONS: Isotonic BFR exercise elicited greater fatigue-induced changes in EMG frequency, MVIC, and perceived effort than isokinetic condition. Therefore, relative to isokinetic, isotonic BFR exercise may provide a more potent acute exercise response and discomfort, but this may not translate to chronic adaptations as there were similar EMG amplitude responses between conditions.

Is the Side Bridge Test Valid and Reliable for Assessing Trunk Lateral Flexor Endurance in Recreational Female Athletes?

Simple SummaryAlthough the side bridge test has been widely used for assessing trunk lateral flexor endurance in sport, clinical, and scientific settings, to the best of the authors’ knowledge, no study has analyzed its validity and reliability in an only female population. The surface electromyography of eight abdominal, back, shoulder and hip muscles was measured during the test to analyze its validity. A one-week test-retest design was performed to evaluate its reliability. No significant differences were found between the trunk lateral flexors and the deltoids. The study data showed that the test performance could be significantly predicted by external oblique and deltoid normalized median frequency slopes and by body mass and trunk height. Based on the results of this study, the shoulder muscle activation and fatigue and the individuals’ anthropometric characteristics, especially the mass, played an important role in the side bridge test performance, which questions the validity of this multi-joint test to specifically assess trunk lateral flexor endurance. In addition, although the side bridge test showed a good data consistency, its intra-subject variability was high, which reduces its utility when small intra-subject changes in muscle endurance are important (e.g., elite sport).The side bridge test (SBT) is one of the most popular tests to assess isometric trunk lateral flexor endurance. The aim of this study was to assess the validity and reliability of the SBT in healthy females. Twenty-four (24.58 ± 3.92 years) physically active (1–2 h of moderate physical activity, 2–3 times a week) females voluntarily participated in this study. The surface electromyography (EMG) of eight abdominal, back, shoulder and hip muscles was measured during the SBT. Normalized median frequency slopes (NMFslope) were calculated to analyze the muscle fatigue. The EMG amplitudes were normalized to maximum EMG values to assess muscle activity intensity. A one-week test-retest design was performed to evaluate the SBT reliability through the ICC3,1 and typical error. Higher NMFslopes and normalized EMG amplitudes were found in deltoids, abdominal obliques, rectus abdominis, and erector spinae in comparison to latissimus dorsi, gluteus medius, and rectus femoris. However, no significant differences were found between the trunk lateral flexors and the deltoids. Linear regression analysis showed that SBT performance could be significantly predicted by external oblique and deltoid NMFslope (adjusted R2 = 0.673) and by body mass and trunk height (adjusted R2 = 0.223). Consistency analysis showed a high intraclass correlation coefficient (0.81) and a relatively high typical error (10.95 s). Despite the good relative reliability of the SBT, its absolute reliability was low and its validity questionable, as the shoulder muscle activation and fatigue and the individuals’ anthropometric characteristics played an important role in SBT performance.

A Comparison of Double Poling Physiology and Kinematics Between Long-Distance and All-Round Cross-Country Skiers.

PurposeThe objective of this study was to compare physiological and kinematic responses to double poling (DP) between long-distance (LDS) and all-round (ARS) cross-country skiers.MethodsA number of five world-class LDS (28.8 ± 5.1 years, maximal oxygen uptake (VO2max): 70.4 ± 2.9 ml·kg−1·min−1) and seven ARS (22.3 ± 2.8 years, VO2max: 69.1 ± 4.2 ml·kg−1·min−1) athletes having similar training volumes and VO2max performed three identical tests; (1) submaximal and incremental tests to exhaustion while treadmill DP to determine gross efficiency (GE), peak oxygen uptake (DP-VO2peak), and peak speed; (2) submaximal and incremental running tests to exhaustion to determine GE, VO2max (RUN-VO2max), and peak speed; and (3) an upper-body pull-down exercise to determine one repetition maximum (1RM) and peak power. Physiological responses were determined during both DP and running, together with the assessments of kinematic responses and electromyography (EMG) of selected muscles during DP.ResultsCompared to ARS, LDS reached higher peak speed (22.1 ± 1.0 vs. 20.7 ± 0.9 km·h−1, p = 0.030), DP-VO2peak (68.3 ± 2.1 vs. 65.1 ± 2.7 ml·kg−1·min−1, p = 0.050), and DP-VO2peak/RUN-VO2max ratio (97 vs. 94%, p = 0.075) during incremental DP to exhaustion, as well as higher GE (17.2 vs. 15.9%, p = 0.029) during submaximal DP. There were no significant differences in cycle length or cycle rate between the groups during submaximal DP, although LDS displayed longer relative poling times (~2.4% points) at most speeds compared to ARS (p = 0.015). However, group × speed interaction effects (p < 0.05) were found for pole angle and vertical fluctuation of body center of mass, with LDS maintaining a more upright body position and more vertical pole angles at touchdown and lift-off at faster speeds. ARS displayed slightly higher normalized EMG amplitude than LDS in the muscles rectus abdominis (p = 0.074) and biceps femoris (p = 0.027). LDS performed slightly better on 1RM upper-body strength (122 vs. 114 kg, p = 0.198), with no group differences in power in the pull-down exercise.ConclusionsThe combination of better DP-specific aerobic energy delivery capacity, efficiency, and technical solutions seems to contribute to the superior DP performance found among specialized LDS in comparison with ARS.

Effects of continuous cycling training on motor unit firing rates, input excitation, and myosin heavy chain of the vastus lateralis in sedentary females.

This study examined the effects of continuous endurance training on motor unit (MU) mean firing rates (MFR), percent myosin heavy chain (%MHC) isoforms, and muscle cross-sectional area (mCSA) of the vastus lateralis (VL). Twelve females completed 5-weeks of continuous cycling-training (CYC), while 8 females were controls (CON). Participants performed maximal voluntary contractions (MVCs) and 40% MVCs of the knee extensors before (PRE) and after the 5-week treatment period at the same absolute pre-treatment submaximal torque (POSTABS) and relative to post-treatment MVCs (POSTREL). Surface electromyographic (EMG) signals were decomposed with the Precision Decomposition III algorithm. MU firing times and waveforms were validated with reconstruct-and-test and spike trigger average procedures. MFRs at steady torque, recruitment thresholds (RT), and normalized EMG amplitude (N-EMGRMS) were analyzed. Y-intercepts and slopes were calculated for the MFR vs. RT relationships. MHC isoforms and mCSA were determined with muscle biopsies and ultrasonography. CYC decreased MVCs and type IIX %MHC isoform without changes in mCSA. The slopes for the MFR vs. RT relationships decreased for CYC during POSTREL and POSTABS while N-EMGRMS increased for POSTABS with no differences between PRE and POSTREL. Type I %MHC isoform was correlated with the slope for the MFR vs. RT relationship during POSTABS and POSTREL for CYC. This study provides evidence that decreases in the MFRs of higher threshold MUs post-CYC is likely a function of changes in input excitation (POSTABS) and the firing frequency-excitation relationships (POSTREL). Evidence is provided that MHC isoforms influence the firing rate scheme of the muscle following short-term training.

Hip and Core Muscle Activation During High-Load Core Stabilization Exercises.

There is some evidence that high-load lumbar stabilization exercises, such as back bridge, can recruit both local and global muscles. Therapeutic exercises would optimize gluteus maximus (GMax), gluteus medius (GMed), multifidus (MF), and transversus abdominis (TrA) activation, while minimizing the activation of the tensor fascia latae (TFL) and erector spinae (ES) muscles in healthy individuals. Cross-sectional study. Research laboratory. Level 4. In this cross-sectional study, surface electromyography (EMG) of GMax, GMed, TFL, TrA, MF, and ES was used to quantify the gluteal-to-TFL muscle activation (GTA) index and a ratio of local to global (L/G) lumbar muscles during (1) the elbow-toe exercise in the prone position, (2) the elbow-toe with right left lifted, (3) the hand-knee with left arm and right leg lifted, (4) the back bridge, (5) the back bridge with right leg lifted, (6) the back bridge with left leg lifted, (7) the side bridge with left leg lifted, (8) the side bridge with right leg lifted, and (9) the elbow-toe with right leg horizontally lifted exercises in healthy individuals (20 men, 20 women; age, 25 ± 4 years). The back bridge exercise with left leg lift generated the highest L/G muscles activity ratio (L/G = 3.35) while the hand-knee exercise yielded the lowest L/G muscles activity ratio (L/G = 1.21). The side bridge exercise with left elbow and foot and lifting the right leg (GTA = 63.78), hand-knee exercise (GTA = 49.62), back bridge (GTA = 28.05), and elbow-toe exercise with left leg horizontally lifted (GTA = 23.02) generated the highest GTA indices, respectively. Meanwhile, the normalized EMG amplitude for GMax was significantly less than the TFL, for elbow-toe exercise (P < 0.001), back bridge with left leg lift (P = 0.001), side bridge exercise with the right elbow and foot and lifting the left leg (P = 0.002), and elbow-toe exercise with right leg horizontally lifted (P < 0.001). The highest GTA indexes were observed during (1) the side bridge lifting the dominant leg and (2) the hand-knee horizontally lifting dominant leg, respectively. The L/G ratio was highest during (1) the back bridge lifting nondominant leg, (2) back bridge, and (3) back bridge lifting dominant leg, respectively. This study supports the use of back bridge exercises to strengthen the MF and side bridges to improve gluteal muscle activation. The highest GTA index was observed in the side bridge lifting the right leg. Highest L/G ratio was in the back bridge with nondominant leg lifted. This study supports the use of back bridge exercises to strengthen the MF. This study supports the use of side bridges to improve gluteal muscle activation.

Gender Differences in the Activation and Co-activation of Lower Extremity Muscles During the Stair-to-Ground Descent Transition

The movement pattern during stair descent differs by gender, and this difference may be associated with gender differences in muscle strength or activation. The purpose of this study was to determine whether a gender difference in muscle activation exists by comparing the muscle activation amplitude and co-activation of lower extremity muscles during the stair-to-ground descent transition. Fifteen men and thirteen women descended stairs and the muscle activation was measured. Women showed a significantly lower peak normalized electromyography amplitude of the vastus lateralis (women: 171.7% peak EMG activation amplitude for level walking (PEALW), men: 297.0%PEALW, p < 0.05). Additionally, women showed significantly elevated co-activation of the vastus lateralis and biceps femoris compared to men (p < 0.05). The lower amplitude of the vastus lateralis in women indicates lower recruitment of knee extensor. In conclusion, the significantly higher co-activation of the vastus lateralis and biceps femoris in women may be a women’s stair descent strategy to safely descend stairs by increasing knee joint stability and stiffness through less recruitment of knee extensor muscle during stair-to-ground descent transition.

Effect of knee joint angle on the neuromuscular activation of the quadriceps femoris during repetitive fatiguing contractions

We assessed the effect of knee joint angle on the EMG amplitude and frequency of the four individual muscles in the quadriceps femoris during repetitive fatiguing maximum voluntary contractions (MVCs). Fifteen healthy men and women performed two fatiguing tasks consisting of 40 MVCs in flexion (80°) and extension (140˚) (full extension = 180˚). Neuromuscular activation of the vastus intermedius (VI), vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) was recorded using surface electrodes, and median frequency (MF) and root mean square (RMS) of electromyographic (EMG) signals (normalized by pre-test MVCs) were calculated. MVCs significantly decreased from the 10th to the 40th repetition in both flexion and extension. The MFs of VI and VM in flexion and that of RF flexion and extension were significantly decreased after the 10th repetition. There were no significant changes in normalized EMG amplitude in any muscles specific to knee angle. Stepwise regression analysis suggested that predictive synergistic action may occur in RF/VM and RF/VI in flexion and in RF/VM in extension. This suggest that EMG MF of RF/VM is independent, but that of RF/VI and RF/VL is dependent upon knee joint angle, which may, in part, explain joint angle-specific muscle fatigue.

Crossover Effects of Unilateral Static Stretching and Foam Rolling on Contralateral Hamstring Flexibility and Strength.

Static stretching (SS) and self-administered foam rolling (SAFR) are both effective techniques often used in rehabilitation settings to improve one's range of motion (ROM). However, their effects on nonintervened contralateral limb's performance remain equivocal. To examine the acute effects of unilateral hamstring's SS and SAFR on the contralateral hip-flexion passive ROM and the strength performance. Randomized crossover trial. Controlled laboratory. A total of 23 healthy young adults (13 males and 10 females) participated in this investigation. Ten sets of 30-second SS or SAFR were performed on the participants' dominant hamstring muscles. Before (pre) and after (post) the interventions, the contralateral hip-flexion passive ROM, the isometric strength of the contralateral hamstrings, and surface electromyography amplitude were measured. Separate 2-way (time × intervention) repeated measures analyses of variance were used to examine the changes in the dependent variables. Both interventions significantly increased the contralateral hip-flexion passive ROM. In addition, the post-ROM value was significantly greater (P = .03) for the SS (mean ± SE = 73.5° ± 4.7°) than that for the SAFR (mean ± SE = 70.3° ± 4.5°). There were also main effects for time (P = .03) and intervention (P = .02) for the contralateral hamstring strength. However, no significant interaction or main effects were found for the normalized electromyography amplitude of the knee flexor muscles. The increased contralateral hip-flexion passive ROM following both interventions was likely due to the enhanced stretch tolerance. However, the differential strength performance responses might be due to different neural mechanisms, which are proposed and discussed.

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.

Do individual differences in the distribution of activation between synergist muscles reflect individual strategies?

Individual differences in the distribution of activation between synergist muscles have been reported during a wide variety of tasks. Whether these differences represent actual individual strategies is unknown. The aims of this study were to: (i) test the between-day reliability of the distribution of activation between synergist muscles, (ii) to determine the robustness of these strategies between tasks, and to (iii) describe the inter-individual variability of activation strategies in a large sample size. Eighty-five volunteers performed a series of single-joint isometric tasks with their dominant leg [knee extension and plantarflexion at 25% of maximal voluntary contraction (MVC)] and locomotor tasks (pedalling and walking). Of these participants, 62 performed a second experimental session that included the isometric tasks. Myoelectrical activity of six lower limb muscles (thethree superficial heads of the quadriceps and the three heads of the triceps surae) was measured using surface electromyography (EMG) and normalized to that measured during MVC. When considering isometric contractions, distribution of normalized EMG amplitude among synergist muscles, considered here as activation strategies, was highly variable between individuals (15.8% < CV < 42.7%) and robust across days (0.57 < ICC < 0.82). In addition, individual strategies observed during simple single-joint tasks were correlated with those observed during locomotor tasks [0.37 < r < 0.76 for quadriceps (n = 83); 0.30 < r < 0.66 for triceps surae (n = 82); all P < 0.001]. Our results provide evidence that people who bias their activation to a particular muscle do so during multiple tasks. Even though inter-individual variability of EMG signals has been well described, it is often considered noise which complicates the interpretation of data. This study provides evidence that variability results from actual differences in activation strategies.

Coordination of hamstrings is individual specific and is related to motor performance.

The torque-sharing strategies between synergistic muscles may have important functional consequences. This study involved two experiments. The first experiment ( n = 22) aimed 1) to determine the relationship between the distribution of activation and the distribution of torque-generating capacity among the heads of the hamstring, and 2) to describe individual torque-sharing strategies and to determine whether these strategies are similar between legs. The second experiment ( n = 35) aimed to determine whether the distribution of activation between the muscle heads affects endurance performance during a sustained submaximal knee flexion task. Surface electromyography (EMG) was recorded from biceps femoris (BF), semimembranosus (SM), and semitendinosus (ST) during submaximal isometric knee flexions. Torque-generating capacity was estimated by measuring muscle volume, fascicle length, pennation angle, and moment arm. The product of the normalized EMG amplitude and the torque-generating capacity was used as an index of muscle torque. The distributions of muscle activation and of torque-generating capacity were not correlated significantly (all P > 0.18). Thus, there was a torque imbalance between the muscle heads (ST torque > BF and SM torque; P < 0.001), the magnitude of which varied greatly between participants. A significant negative correlation was observed between the imbalance of activation across the hamstring muscles and the time to exhaustion ( P < 0.001); i.e., the larger the imbalance of activation across muscles, the lower the muscle endurance performance. Torque-sharing strategies between the heads of the hamstrings are individual specific and related to muscle endurance performance. Whether these individual strategies play a role in hamstring injury remains to be determined. NEW & NOTEWORTHY The distribution of activation among the heads of the hamstring is not related to the distribution of torque-generating capacity. The torque-sharing strategies within hamstring muscles vary greatly between individuals but are similar between legs. Hamstring coordination affects endurance performance; i.e., the larger the imbalance of activation across the muscle heads, the lower the muscle endurance.

Force direction and arm position affect contribution of clavicular and sternal parts of pectoralis major muscle during muscle strength testing

Study DesignCross-sectional study. Purpose of the StudyThe study aims to determine the effects of force direction and arm position in differentiating the clavicular (PMc) and sternal (PMs) parts of the pectoralis major (PM) muscle during maximal voluntary isometric contraction (MVIC) to provide basic evidence to support the clinical thinking behind muscle strength testing of PM. MethodsNine experimental conditions with 3 force directions of horizontal adduction (+30° oblique, horizontal, and −30° oblique to the transverse plane) and 3 arm rotation positions (0°, 45°, and 90° shoulder external rotation from the transverse plane) were randomly tested for 26 healthy male participants. The MVIC force level was monitored and measured with a fixed dynamometer, and the surface electromyographic (EMG) signals of the PMc, PMs, anterior deltoid, middle deltoid, and latissimus dorsi were collected during the test for each condition. The PMc/PMs EMG ratio and normalized EMG amplitude were used to quantify the contribution of the tested muscles. ResultsThe MVIC force level significantly declined when the arm's external rotation increased (P < .01; the grand mean decreased from 106.7 N ± 27.8 N to 89.5 N ± 22.6 N). The PMc/PMs EMG ratio showed that the best test condition to differentiate the PMc and PMs was the force direction of +30° oblique to the transverse plane and the 45° arm rotation position. Other muscles contributed less than 40% of their MVIC activity levels, with a higher activation level found in the anterior deltoid muscle (P < .01). ConclusionsArm rotation position should be considered as a predominant factor when clinically examining the strength of horizontal adduction movement. All tested conditions failed to fully separate PMc and PMs activation during MVIC and suggested that functional differentiation of the PM might not be applicable to maximal exertion. Level of EvidenceNA.

Does increased superficial neck flexor activity in the craniocervical flexion test reflect reduced deep flexor activity in people with neck pain?

BackgroundThe craniocervical flexion test assesses the deep cervical flexor muscles (longus capitis, longus colli). Ideally, electromyography (EMG) studies measure activity in both deep and superficial (sternocleidomastoid, anterior scalene) flexors during the test, but most studies confine recordings to superficial muscle activity as the technique to record the deep muscles is invasive. Higher activity of the superficial flexors has been interpreted as an indicator of reduced deep flexor activity in people with neck pain but how close the inverse relationship is during this test is unknown. MethodsEMG was recorded from the sternocleidomastoid, anterior scalene and deep cervical flexor muscles to quantify their relationship during the craniocervical flexion test, from 32 women (age: 38.0 ± 11.6 yrs) with a history of chronic non-specific neck pain. The range of craniocervical flexion at each of the five test stages was also measured. ResultsA moderate negative correlation was identified (r = −0.45; P < 0.01) between the average normalized EMG amplitude of the deep cervical flexors and sternocleidomastoid across all stages of the craniocervical flexion test. There was a moderate although weaker and non-significant negative correlation between deep cervical flexors and anterior scalene activity (r = −0.34; P = 0.053). ConclusionsThe results affirm the interpretation that higher levels of activity of the superficial flexor muscles are an indicator of reduced deep cervical flexor activity in the craniocervical flexion test. Further studies of neuromuscular and movement strategies used by people with neck pain to compensate for poorer activation of the deep cervical flexors will inform best clinical assessment.

Immediate Effect of Patterned Electrical Neuromuscular Stimulation on Pain and Muscle Activation in Individuals With Patellofemoral Pain.

For individuals with patellofemoral pain (PFP), altered muscle activity and pain are common during functional tasks. Clinicians often seek interventions to improve muscle activity and reduce impairments. One intervention that has not been examined in great detail is electrical stimulation. To determine whether a single patterned electrical neuromuscular stimulation (PENS) treatment would alter muscle activity and pain in individuals with PFP during 2 functional tasks, a single-legged squat and a lateral step down. Cohort study. Sports medicine research laboratory. PATIENTS OF OTHER PARTICIPANTS: A total of 22 individuals with PFP (15 women, 7 men; age = 26.0 ± 7.9 years, height = 173.8 ± 8.1 cm, mass = 75.1 ± 17.9 kg). Participants were randomized into 2 intervention groups: a 15-minute PENS treatment that produced a strong motor response or a 15-minute 1-mA subsensory (sham) treatment. Before and immediately after the intervention, we assessed normalized electromyography amplitude, percentage of activation time across functional tasks, and onset of activation for the vastus medialis oblique, vastus lateralis, gluteus medius, adductor longus, biceps femoris, and medial gastrocnemius muscles during a single-legged squat and a lateral step down. Scores on the visual analog scale for pain were recorded before and after the intervention. After a single treatment of PENS, the percentage of gluteus medius activation increased (0.024) during the lateral step down. Visual analog scores decreased during both the single-legged squat (PENS: preintervention = 2.7 ± 1.9, postintervention = 0.9 ± 0.7; sham: preintervention = 3.2 ± 1.6, postintervention = 2.8 ± 1.9; group × time interaction: P = .041) and lateral step down (PENS: preintervention = 3.4 ± 2.4, postintervention = 1.1 ± 0.8; sham: preintervention = 3.9 ± 1.7, postintervention = 3.3 ± 2.0; group × time interaction: P = .023). No changes in electromyography or pain measures were noted in the sham group. The PFP participants who received PENS had immediate improvement in gluteus medius activation and a reduction in pain during functional tasks.