Isometric Muscle Actions Research Articles

Improving real-life, heart rate based estimates of emotion by taking metabolic heart rate into account – a perspective and an example in cooking

Physiological variables such as heart rate carry information about mental state (cognitive and emotional state - here we simply refer to ‘emotion’). An important advantage over verbal methods as a way to probe emotion, is that heart rate is an implicit measure that can be measured continuously. Extracting information about emotion from heart rate in real life is challenged by the concurrent effect of physical activity on heart rate caused by metabolic need. “Non-metabolic heart rate”, which refers to the heart rate that is caused by factors other than physical activity, would be a more sensitive and more universally applicable correlate of emotion than heart rate itself. To precisely determine non-metabolic heart rate is a challenge. We explored evidence from the literature that non-metabolic heart rate, as it has been determined up until now, indeed reflects emotion. We focused on methods using accelerometry since these sensors are readily available in devices suitable for daily life usage. We found no convincing evidence that these existing methods lead to estimates of non-metabolic heart rate that reflect emotion. This is probably caused by the fact that intensity of motion signals as recorded from accelerometers does not correspond one-on-one with muscle activity in cases that forces are exerted without much movement (isometric muscle activity). In addition, the placement of the sensors do not always match the currently moving body parts, leading to invalid estimates. Studies in the field of energy expenditure as measured through variables extracted from respiratory gas exchange, show that energy expenditure can be well estimated using accelerometry by first identifying the type of action that is performed, rather than (only) using the intensity of motion signals. We therefore suggest that for real-life cases, estimating the type and intensity of activities based on accelerometry (and other information), and in turn use those to determine the non-metabolic heart rate, is the most promising route to determining non-metabolic heart rate precisely enough in order for it to be useful in estimating emotional state. In a study on estimating emotion during real-life cooking, we determined non-metabolic heart rate by correcting heart rate in an activity specific way. Rather than using a model that estimates specific activities based on accelerometry, we made an effort to estimate a baseline metabolic heart rate from participants performing movements that occur during cooking, without the emotion that occurs during cooking. The aim was to investigate the potential of non-metabolic heart rate to reflect emotionally salient phases during the process of cooking a dish.

Acute effects of maximal isometric muscle action of the elbow extensors on contralateral dynamic task of the elbow flexors: a pilot study

Acute effects of maximal isometric muscle action of the elbow extensors on contralateral dynamic task of the elbow flexors: a pilot study

Time Course of Changes in Neuromuscular Parameters During Sustained Isometric Muscle Actions: Erratum.

Time Course of Changes in Neuromuscular Parameters During Sustained Isometric Muscle Actions: Erratum.

Isometric parameters in the monitoring of maximal strength, power, and hypertrophic resistance-training.

This study monitored strength-training adaptations via isometric parameters throughout 2 × 10 weeks of hypertrophic (HYP I-II) or 10 weeks maximum strength (MS) followed by 10 weeks power (P) training with untrained controls. Trainees performed bilateral isometric leg press tests analyzed for peak force (maximal voluntary contraction (MVC)) and rate of force development (RFD) every 3.5 weeks. These parameters were compared with dynamic performance, voluntary and electrically induced isometric contractions, muscle activity, and cross-sectional area (CSA) in the laboratory before and after 10 and 20 weeks. RFD increased similarly during the first 7 weeks (HYP I, 44% ± 53%; MS, 48% ± 55%, P < 0.05), but RFD continued to increase up to 65% ± 61% from baseline (P < 0.01) only during P. These increases were concomitant with enhanced dynamic performances of 1-repetition maximum (1RM) (HYP I, 8% ± 6%; MS, 11% ± 6%, P < 0.001), and explosive repetitions during P (11% ± 15%, P < 0.05). Time to reach peak RFD differed (P < 0.001) between HYP (mean 42 ± 20 ms) and MS-P (mean 31 ± 12 ms) groups because of training. The changes in MVC correlated with the changes in CSA during weeks 1-20 (HYP I-II, r = 0.664; MS-P, r = 0.595, P ≤ 0.05), as well as changes in 1RM (r = 0.724, P < 0.05) during weeks 11-20 (HYP II). Muscle activity increased during MS and P only. Both MVC and RFD improvements reflected combinations of central and peripheral adaptations. RFD parameters may be effective tools to evaluate adaptations, particularly during maximal strength/power training, while MVC cannot distinguish between strength or muscle mass changes. Monitoring RFD provided important information regarding plateaus in RFD improvement, which were observed in dynamic explosive performances after HYP II compared with P.

Chronic Effects of Different Rest Intervals Between Sets on Dynamic and Isometric Muscle Strength and Muscle Activity in Trained Older Women.

This study investigated the chronic effects of different rest intervals (RIs) between sets on dynamic and isometric muscle strength and muscle activity. We used a repeated-measures design (pretraining and posttraining) with independent groups (different RI). Twenty-one resistance-trained older women (66.4 ± 4.4 years) were randomly assigned to either a 1-minute RI group (G-1 min; n = 10) or 3-minute RI group (G-3 min; n = 11). Both groups completed 3 supervised sessions per week during 8 weeks. In each session, participants performed 3 sets of 15 repetitions of leg press exercise, with a load that elicited muscle failure in the third set. Fifteen maximum repetitions, maximal voluntary contraction, peak rate of force development, and integrated electromyography activity of the vastus lateralis and vastus medialis muscles were assessed pretraining and posttraining. There was a significant increase in load of 15 maximum repetitions posttraining for G-3 min only (3.6%; P < 0.05). However, posttraining results showed no significant differences between G-1 min and G-3 min groups for all dependent variables (P > 0.05). The findings suggest that different RIs between sets did not influence dynamic and isometric muscle strength and muscle activity in resistance-trained older women.

Are there mode-specific and fatigue-related electromechanical delay responses for maximal isokinetic and isometric muscle actions?

This study used a combined electromyographic, mechanomyographic, and force approach to identify electromechanical delay (EMD) from the onsets of the electromyographic to force signals (EMDE-F), onsets of the electromyographic to mechanomyogrpahic signals (EMDE-M), and onsets of mechanomyographic to force signals (EMDM−F). The purposes of the current study were to examine: (1) differences in EMDE-M, EMDM−F, and EMDE-F from the vastus lateralis between maximal isokinetic and maximal concentric isometric leg extensions; and (2) the effects of fatigue and recovery on EMDE-M, EMDM−F, and EMDE-F. These EMD measures were obtained from twelve men during maximal concentric isokinetic and isometric leg extensions pretest, posttest, and after 3-min and 5-min of recovery from 25 maximal isokinetic leg extensions at 60°s−1. The results indicated no differences between maximal isokinetic and isometric muscle actions for EMDE-M, EMDM-F, or EMDE-F during the pretest, posttest, 3-min recovery, and 5-min recovery measurements. These findings support the comparison of voluntary EMD measures between studies with different modes of exercise as long as the methodology for the determination of EMD are consistent. There were, however, fatigue-induced pretest to posttest increases in EMDE-M, EMDM-F, and EMDE-F which remained elongated after 3-min of recovery, but returned to pretest values after 5-min of recovery.

Effects of intensity on muscle-specific voluntary electromechanical delay and relaxation electromechanical delay

ABSTRACTThe purposes of this study were to examine: 1) the potential muscle-specific differences in voluntary electromechanical delay (EMD) and relaxation electromechanical delay (R-EMD), and 2) the effects of intensity on EMD and R-EMD during step incremental isometric muscle actions from 10 to 100% maximal voluntary isometric contraction (MVIC). EMD and R-EMD measures were calculated from the simultaneous assessments of electromyography, mechanomyography, and force production from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) during step isometric muscle actions. There were no differences between the VL, VM, and RF for the voluntary EMDE-M (onsets of the electromyographic to mechanomyographic signals), EMDM-F (onsets the mechanomyographic to force production), or EMDE-F (onsets of the electromyographic signal to force production) as well as R-EMDE-M (cessation of electromyographic to mechanomyographic signal), R-EMDM-F (cessation of mechanomyographic signal to force cessation), or R-EMDE-F (cessation of electromyorgraphic signal to force cessation) at any intensity. There were decreases in all EMD and R-EMD measures with increases in intensity. The relative contributions from EMDE-M and EMDM-F to EMDE-F as well as R-EMDE-M and R-EMDM-F to R-EMDE-F remained similar across all intensities. The superficial muscles of the quadriceps femoris shared similar EMD and R-EMD measurements.

Neural and Muscular Contributions to the Age-Related Reductions in Rapid Strength.

The purposes of this study were to investigate the age-related differences in absolute and normalized plantarflexion rate of torque development (RTD) at early (0-50 ms) and late (100-200 ms) time intervals and to examine specific neural and muscular mechanisms contributing to these differences. Thirty-two young (20.0 ± 2.1 yr) and 20 older (69.5 ± 3.3 yr) recreationally active men performed rapid plantarflexion isometric muscle actions to examine absolute and normalized RTD and muscle activation using EMG at early and late time intervals. Ultrasonography was used to examine medial gastrocnemius muscle size, echo intensity (EI), and muscle architecture (fascicle length [FL] and pennation angle [PA]). The older men were weaker (23.9%, P < 0.001) and had lower later absolute and normalized RTD (P = 0.001-0.034) variables when compared with the young men. The older men also had higher EI (P < 0.001), smaller PA (P = 0.004), and lower later EMG amplitude values (P = 0.009-0.046). However, there were no differences in early RTD and EMG amplitude values, muscle size, or FL between groups (P = 0.097-0.914). Lower late RTD values were related to higher EI, smaller PA, and lower EMG amplitude values (r = -0.28-0.59, P = 0.001-0.044); however, late RTD values were no longer related to PA after normalizing to peak torque. Age-related alterations in muscle quality (EI), architecture, and muscle activation may influence rapid torque production at late time intervals (≥100 ms) from contraction onset. These findings highlight specific neuromuscular factors that influence the age-related reductions in RTD, which has been shown to significantly influence function and performance in older adults.

Efeito da potencialização pós-ativação no torque isométrico do tríceps braquial em adultos

Objetivo: Analisar o efeito de contraes voluntrias isomtricas mximas e da eletroestimulao no torque isomtrico e na ativao muscular do trceps braquial em adultos fisicamente ativos. Mtodos: Dezenove homens foram submetidos a testes de fora isomtrica (FI) como pr-teste, por meio de contrao voluntria isomtrica mxima durante 10 segundos e atividades condicionantes (ACs) randomizadas: isometrias durante 7, 10 e 12 segundos e eletroestimulao em 70 e 120Hz por 10 segundos cada. Aps as ACs, foram realizadas duas contraes voluntrias isomtricas mximas de 10 segundos com intervalo de 4 e 8 minutos, respectivamente. A atividade eltrica do trceps braquial (pores medial e lateral) foi registrada. Resultados: No foi verificado efeito em nenhuma das condies para o torque mximo ou mdio, bem como para atividade eltrica do trceps braquial (P0,05). Concluso: O intervalo adotado entre as ACs e o teste de FI no foi suficiente para induzir a melhoria do torque isomtrico.

Greater Neural Adaptations following High- vs. Low-Load Resistance Training.

We examined the neuromuscular adaptations following 3 and 6 weeks of 80 vs. 30% one repetition maximum (1RM) resistance training to failure in the leg extensors. Twenty-six men (age = 23.1 ± 4.7 years) were randomly assigned to a high- (80% 1RM; n = 13) or low-load (30% 1RM; n = 13) resistance training group and completed leg extension resistance training to failure 3 times per week for 6 weeks. Testing was completed at baseline, 3, and 6 weeks of training. During each testing session, ultrasound muscle thickness and echo intensity, 1RM strength, maximal voluntary isometric contraction (MVIC) strength, and contractile properties of the quadriceps femoris were measured. Percent voluntary activation (VA) and electromyographic (EMG) amplitude were measured during MVIC, and during randomly ordered isometric step muscle actions at 10–100% of baseline MVIC. There were similar increases in muscle thickness from Baseline to Week 3 and 6 in the 80 and 30% 1RM groups. However, both 1RM and MVIC strength increased from Baseline to Week 3 and 6 to a greater degree in the 80% than 30% 1RM group. VA during MVIC was also greater in the 80 vs. 30% 1RM group at Week 6, and only training at 80% 1RM elicited a significant increase in EMG amplitude during MVIC. The peak twitch torque to MVIC ratio was also significantly reduced in the 80%, but not 30% 1RM group, at Week 3 and 6. Finally, VA and EMG amplitude were reduced during submaximal torque production as a result of training at 80% 1RM, but not 30% 1RM. Despite eliciting similar hypertrophy, 80% 1RM improved muscle strength more than 30% 1RM, and was accompanied by increases in VA and EMG amplitude during maximal force production. Furthermore, training at 80% 1RM resulted in a decreased neural cost to produce the same relative submaximal torques after training, whereas training at 30% 1RM did not. Therefore, our data suggest that high-load training results in greater neural adaptations that may explain the disparate increases in muscle strength despite similar hypertrophy following high- and low-load training programs.

Are there two forms of isometric muscle action? Results of the experimental study support a distinction between a holding and a pushing isometric muscle function

BackgroundIn isometric muscle function, there are subjectively two different modes of performance: one can either hold isometrically – thus resist an impacting force – or push isometrically – therefore work against a stable resistance. The purpose of this study is to investigate whether or not two different isometric muscle actions – the holding vs. pushing one (HIMA vs PIMA) – can be distinguished by objective parameters.MethodsTen subjects performed two different measuring modes at 80% of MVC realized by a special pneumatic system. During HIMA the subject had to resist the defined impacting force of the pneumatic system in an isometric position, whereby the force of the cylinder works in direction of elbow flexion against the subject. During PIMA the subject worked isometrically in direction of elbow extension against a stable position of the system. The signals of pressure, force, acceleration and mechanomyography/-tendography (MMG/MTG) of the elbow extensor (MMGtri/MTGtri) and the abdominal muscle (MMGobl) were recorded and evaluated concerning the duration of maintaining the force level (force endurance) and the characteristics of MMG-/MTG-signals. Statistical group differences comparing HIMA vs. PIMA were estimated using SPSS.ResultsSignificant differences between HIMA and PIMA were especially apparent regarding the force endurance: During HIMA the subjects showed a decisively shorter time of stable isometric position (19 ± 8 s) in comparison with PIMA (41 ± 24 s; p = .005). In addition, during PIMA the longest isometric plateau amounted to 59.4% of the overall duration time of isometric measuring, during HIMA it lasted 31.6% (p = .000). The frequency of MMG/MTG did not show significant differences. The power in the frequency ranges of 8–15 Hz and 10–29 Hz was significantly higher in the MTGtri performing HIMA compared to PIMA (but not for the MMGs). The amplitude of MMG/MTG did not show any significant difference considering the whole measurement. However, looking only at the last 10% of duration time (exhaustion), the MMGtri showed significantly higher amplitudes during PIMA.ConclusionThe results suggest that under holding isometric conditions muscles exhaust earlier. That means that there are probably two forms of isometric muscle action. We hypothesize two potential reasons for faster yielding during HIMA: (1) earlier metabolic fatigue of the muscle fibers and (2) the complexity of neural control strategies.

Motor Unit Action Potential Sizes of the First Dorsal Interosseous in Young and Older Individuals

Slopes from the motor unit action potential size (MUAPsize) vs. recruitment threshold (RT) relationships provides a non-invasive measurement of MU sizes. As such, the slopes (APslopes) may be able to quantify age-related MU atrophy. PURPOSE: To determine if APslopes differed between young and old individuals. METHODS: Twenty two young (YG, age = 22.5±2.7 yrs) and ten aged (OG, 61.0±2.0 yrs) subjects completed the investigation. Surface electromyography signals were recorded from the first dorsal interosseous (FDI) during an isometric trapezoidal muscle action with the steady force plateau set at 50% maximal voluntary contraction (MVC). The signals were decomposed to yield MUAPsizes and RTs for each MU. The APslope was calculated via the MUAPsizes vs. RT relationships for MUs recruited between 10 and 50% MVC for each subject. Few MUs with RTs < 10% MVC were observed in young or old and, thus, these MUs were excluded from the slope calculation. FDI cross-sectional area (CSA) and echo intensity (EI) were quantified using ultrasonography. Possible differences in APslope, CSA and EI between groups were examined with independent samples t-tests. RESULTS: OG demonstrated significantly reduced APslopes (OG: 0.033 ± 0.010; YG: 0.048 ± 0.020; p = 0.048) and greater EI (OG: 41.3 ± 7.0 AU; YG: 50.6 ± 7.5 AU; p = 0.002), however, CSA was similar between YG and OG (OG: 2.22 ± 0.47 cm2; YG: 2.09 ± 0.31 cm2; p = 0.438). CONCLUSION: The APslopes suggested non-uniform differences in MUAPsizes in relation to RT, likely due to reduced sizes of higher threshold MUs. Higher threshold MUs have been suggested to contain a greater proportion of type II muscle fibers, which demonstrate greater age induced atrophy than type I fibers commonly associated with lower threshold MUs. As such, the difference in APslopes suggested atrophy of higher threshold MUs in OG. In addition, OG’s elevated EI indicates greater infiltration of adipose and connective tissue into the muscle. Greater EI in combination with similar CSA may indicate a reduced contractile tissue volume in the OG, supporting the speculation that the APslope differences were due to MU atrophy.

Mechanomyographic Responses to Blood Flow Restricted, Fatiguing Isometric Muscle Actions

Blood flow restriction (BFR) during low intensity resistance exercise (LIRE) has been shown to increase activation of lower body musculature. It is unclear if this is due to increases in motor unit recruitment (MU) or firing rate. Mechanomyography (MMG) has been used to investigate motor unit recruitment (MMG amplitude) and motor unit firing rate (MMG frequency). PURPOSE: To use MMG to investigate the neuromuscular effects of BFR in the lower body during LIRE. METHODS: Ten males (age = 27.1 ± 3.1y; height = 177.6 ± 6.2 cm; mass = 86.9 ± 13.0 kg) performed 3 knee extensor maximal voluntary isometric contractions (MVIC) with the right limb. Occlusion (OCC) was then applied to the right thigh using a KAATSU master unit while two sets (set 1 = 30 ± 7.96 repetitions; set 2 = 13.7 ± 11.29 repetitions) of repeated 5-second isometric knee extensions at 30% of peak torque were performed. Each set was performed to fatigue, defined as the inability to reach 30% of MVIC torque, with 2-seconds rest between repetitions. The same procedure was repeated during a second visit without occlusion (NONOCC), while matching the number of repetitions per set to the first visit. An accelerometer was used to collect MMG data from the vastus lateralis during the MVICs and both sets. Prior to data analysis, repetitions were normalized as percentages (25%, 50%, 75% and 100%) of total repetitions completed, and MMG data were normalized to the values from MVIC testing. RESULTS: For MMG frequency, a significant 3-way interaction between set (2), condition (2) and percent repetitions (4) (p=0.046) was found. Follow-up tests revealed no interaction or main effects for set 1 (p>0.05), and no interactions for set 2. When collapsed across percent repetitions, MMG frequency was lower for OCC (0.837 ± 0.045) than for NONOCC (1.116 ± 0.099) during set 2 (p=0.046). For MMG amplitude, there was a significant condition by set interaction (p=0.023). MMG amplitude was higher for the OCC condition (0.754 ± 0.187) than for the NONOCC condition during set 2 (0.481 ± 0.163; p=0.001). MMG amplitude during set 1 (0.599 ± 0.179) was lower than set 2 (0.754 ± 0.187; p=0.042) for OCC. CONCLUSION: OCC exhibited lower levels of MMG frequency and higher values for MMG amplitude. This suggests that higher MU recruitment, rather than firing rate, was responsible for increased muscle activation during BFR.

Myosin Heavy Chain Influences Firing Rate Behavior from Moderate to High Intensity Targeted Forces

PURPOSE: It is suggested the ratio of myosin heavy chain (MHC) isoforms and their influence on twitch forces and fatigability effect motor unit (MU) behavior. However, no study has correlated changes in MU firing rates among moderate- to high-intensity contractions with MHC area in vivo. METHODS: Twelve individuals (age=20.89±2.52 yrs, weight=72.03±14.02 kg) performed 3 isometric maximal voluntary contractions (MVC) of the leg extensors on an isokinetic dynamometer followed by randomly ordered isometric muscle actions at 50, 70, and 90% MVC. An electromyographic (EMG) sensor was placed over the VL. EMG signals were decomposed to extract action potentials and firing events of single MUs. Only MUs with > 90% accuracy were used for analysis. Recruitment thresholds (REC Thresh) and mean firing rates (MFR) were calculated for each MU. MFR was calculated as the average value of the MFR trajectory during steady force. Subjects gave a muscle biopsy of the VL. Type I %MHC area was determined by SDS-PAGE. Linear regressions were performed for the 50% MVC, whereas inverse exponential regressions were performed on the 70% and 90% MVC to determine the slopes and y-intercepts for the MFR vs REC Thresh relationships. Predicted firing rates at target force were calculated from the regression equations for each subject for MUs with a REC threshold of 5-45% MVC in 5% increments. To examine changes in MU firing rates in relation to REC Thresh, predicted firing rates from the 70% and 90% MVC were normalized to the predicted firing rate value for the 50% MVC. For each REC Thresh, linear regressions were performed on the normalized firing rate values vs contraction intensity. Pearson’s product moment correlations were calculated comparing the slopes and type I %MHC area. Alpha was set at 0.05. RESULTS: Pearson’s product moment correlations were significant among type I %MHC area and the slopes of the change in predicted firing rates vs contraction intensity for MUs with a REC thresh of 35, 40, and 45% MVC (P < 0.05; R = -0.754 to -0.669). CONCLUSIONS: Individuals with lower percentages of type I MHC area had greater increases in MU firing rates with increments in targeted forces for MUs with REC Thresh of 35 - 45% MVC. This may indicate that MU firing rate and recruitment patterns differ as a function of MHC area.

FMS: Relationship between muscul activity in deep squat ans trunk stability pushup

Aims and Objectives:Motor control is described as the internal control and regulation processes which facilitate movements. Scientists divide motor control into the function of stability and the movement to maintain balance in locomotion. The FMS is used to analyze the motor behavior of muscle slings on the one hand, and to estimate injury risk on the other hand. Yet, the true extent of the FMS’ diagnostic information remains unclear. The low intensity level of the FMS items causes low tension within the motor network. Based on Hennemen’s size principle, this is a clear indicator for the activation of slow-twitch fibers in the muscle. A possible relationship between movement and stability function of muscle slings has yet to be examined. Therefore, the aim of this investigation was to shed light on the motor behavior of dynamic and isometric muscle activity as well as the examination of average muscle activity when performing the items deep squat (DS) and trunk stability pushup (TSPU).Materials and Methods:Data from 16 collegiate athletes (22.1±1.5yrs) with various sports histories was analyzed. The data analysis was based on Kendall’s tau correlation coefficient. The items DS and TSPU were rated via an ordinal ranking system ranging from 0 (pain) to 3 (adequate). Muscle activity was recorded with a new EMG device at a rate of 25 Hz (Myontec Ltd, Kuopio, Finland). In a concurrent pilot study the reliability of this device was examined with a progressive loaded squat protocol (50 up to 125% BW). It indicated almost perfect reliability for the quad/ham balance, quadriceps distribution (r< 0.9, CV:3-5%) and high reliability for the hamstrings (r >0.8, CV:<10%).Results:Strong relationships between the quad/ham balance (r=0.7, p<0.01) and quadriceps distribution (r=0.6, p<0.03) in DS and TSPU were identified. The connection between hamstring muscle activity balance was low (r< 0.2, p>0.5). In both items, the muscle contractions were dominated by the activation of the quadriceps (79% vs. 21% hamstrings). Additionally, the average muscle activity was 4 times higher in a loaded squat (100% BW) than it was in DS. Generally, DS was rated as follows: score 3 (3 times) and score 2 (13 times). For the item TSPU score 3 was given 14 times and score 2 was given 2 times.Conclusion:The relationship between dynamic and static motor behavior of the quadriceps supports the connection between the stabilizing and movement function of muscle slings. The low intensity corresponds with the slow EMG activity in DS, which is an indicator for the activation of mainly slow twitch muscle fibers. This scientific innovation is relevant for practitioners as it establishes the diagnostic value of the FMS. Based on this evidence, the FMS provides information with regards to low tension motor behavior. It is unlikely that this information is specific enough to estimate injury risk or performance because when it comes to return to sport decisions, it is also important to obtain information of motor control associated with fast-twitch fibers.

Isometric thermogenesis at rest and during movement: a neglected variable in energy expenditure and obesity predisposition.

Isometric thermogenesis as applied to human energy expenditure refers to heat production resulting from increased muscle tension. While most physical activities consist of both dynamic and static (isometric) muscle actions, the isometric component is very often essential for the optimal performance of dynamic work given its role in coordinating posture during standing, walking and most physical activities of everyday life. Over the past 75years, there has been sporadic interest into the relevance of isometric work to thermoregulatory thermogenesis and to adaptive thermogenesis pertaining to body-weight regulation. This has been in relation to (i) a role for skeletal muscle minor tremor or microvibration - nowadays referred to as 'resting muscle mechanical activity' - in maintaining body temperature in response to mild cooling; (ii) a role for slowed skeletal muscle isometric contraction-relaxation cycle as a mechanism for energy conservation in response to caloric restriction and weight loss and (iii) a role for spontaneous physical activity (which is contributed importantly by isometric work for posture maintenance and fidgeting behaviours) in adaptive thermogenesis pertaining to weight regulation. This paper reviews the evidence underlying these proposed roles for isometric work in adaptive thermogenesis and highlights the contention that variability in this neglected component of energy expenditure could contribute to human predisposition to obesity.

Isometric muscle activation of the serratus anterior and trapezius muscles varies by arm position: a pilot study with healthy volunteers with implications for rehabilitation

This study aimed to determine the most appropriate angle and moving direction of the arm for improving coordination of the periscapular muscles, including the serratus anterior (SA), upper trapezius (UT), middle trapezius (MT), and lower trapezius (LT). Muscle activation amplitudes were evaluated in the SA, UT, MT, and LT in 11 healthy subjects by use of surface electromyography. The subjects were asked to maintain the arm position at 5 elevated positions with maximal effort against applied manual forces, which were directed from upper to lower (test 1), lower to upper (test 2), posterior to anterior in the frontal plane and lateral to medial in the sagittal plane (test 3), and anterior to posterior in the frontal plane and medial to lateral in the sagittal plane (test 4). The relative activity of the UT with respect to the SA, MT, and LT was calculated, resulting in the UT/SA, UT/MT, and UT/LT ratios. Test 4 in all positions but 150° of elevation in the frontal plane showed high activity of the SA with a low UT/SA ratio. High MT activity with a low UT/MT ratio was observed during test 3 at the 90° elevated position, whereas high LT activity without UT hyperactivation was not found. To strengthen the periscapular muscles in the balanced condition, horizontal adduction is recommended for the SA. Horizontal abduction at the 90° elevated position should be effective for the MT. Because no technique in this study was effective for the LT, further studies are needed.

Influences of Interelectrode Distance and Innervation Zone on Electromyographic Signals

The purpose of the present study was to examine the effects of electrode placements centered over and offset from the innervation zone (IZ) with different interelectrode distances (IED) on the time and frequency domain parameters of the electromyographic (EMG) signal during a fatiguing submaximal, isometric workout. 11 adults performed an isometric leg extension muscle action at 50% maximal voluntary isometric contraction (MVIC) to exhaustion. Electromyographic amplitude and frequency parameters were determined from electrode placements with different IED centered over, at proximal offset, at distal offset, and away from the IZ at 10, 50, and 100% of the time to exhaustion using an electrode array. There were greater absolute EMG amplitude and lower absolute EMG frequency for electrode placements over and offset from the IZ, but lower absolute EMG amplitude over than offset from the IZ regardless of IED at each time-point during the time to exhaustion. The absolute EMG frequency values were affected by the location relative to the IZ and IED of the electrode placements, and were greater for distal offset vs. proximal offset electrode placements at each time-point. Normalization of the EMG amplitude and EMG frequency values to MVIC eliminated differences due to IED and electrode placements during the fatiguing workout.

Reduced activation in isometric muscle action after lengthening contractions is not accompanied by reduced performance fatigability.

After active lengthening contractions, a given amount of force can be maintained with less muscle activation compared to pure isometric contractions at the same muscle length and intensity. This increase in neuromuscular efficiency is associated with mechanisms of stretch-induced residual force enhancement. We hypothesized that stretch-related increase in neuromuscular efficiency reduces fatigability of a muscle during submaximal contractions. 13 subjects performed 60 s isometric knee extensions at 60% of maximum voluntary contraction (MVC) with and without prior stretch (60°/s, 20°). Each 60 s trial was preceded and followed by neuromuscular tests consisting of MVCs, voluntary activation (VA) and resting twitches (RT), and there was 4 h rest between sets. We found a significant (p = 0.036) 10% reduction of quadriceps net-EMG after lengthening compared to pure isometric trials. However, increase in neuromuscular efficiency did not influence the development of fatigue. Albeit we found severe reduction of MVC (30%), RT (30%) and VA (5%) after fatiguing trials, there were no differences between conditions with and without lengthening. As the number of subjects showing no activation reduction increased with increasing contraction time, intensity may have been too strenuous in both types of contractions, such that a distinction between different states of fatigue was not possible anymore.

Combining regression and mean comparisons to identify the time course of changes in neuromuscular responses during the process of fatigue

The purposes of the present study were to apply a unique method for the identification of the time course of changes in neuromuscular responses and to infer the motor unit activation strategies used to maintain force during a fatiguing, intermittent isometric workbout.Eleven men performed 50, 6 s intermittent isometric muscle actions of the leg extensors, each separated by 2 s of rest at 60% maximal voluntary isometric contraction (MVIC). Electromyographic (EMG) and mechanomyographic (MMG) amplitude (root mean square; RMS) and frequency (mean power frequency; MPF) were obtained from the vastus lateralis (VL) every 5 of the 50 repetitions and normalized as a percent of the initial repetition. Polynomial regression analyses were used to determine the model of best fit for the normalized EMG RMS, EMG MPF, MMG RMS, and MMG MPF versus repetition relationships and one-way repeated measures ANOVAs with post-hoc Student Newman–Keuls were used to identify when these neuromuscular parameters changed from the initial repetition. The findings of the present study indicated two unique phases of neuromuscular responses (repetitions 1–20 and 20–50) during the fatiguing workbout. The time course of changes in these four neuromuscular responses suggested that the after-hyperpolarization theory could not explain the maintenance of force production, but Muscle Wisdom and the Onion Skin Scheme could. The findings of the current study suggested that the time course of changes in neuromuscular responses can provide insight in to the motor unit activation strategies used to maintain force production and allow for a greater understanding of the fatiguing process by identifying the time-points at which these neuromuscular parameters changed.