Maximal Voluntary Contraction Test Research Articles

The Effects of Experimenter Gender on State Social Physique Anxiety and Strength in a Testing Environment

Social influences can impact self-presentational concerns such as social physique anxiety (SPA), concerns over one's body being evaluated by others. In addition, social influences may also impact performance on a physical test. In a physical testing environment, one social factor that influences SPA and which may also influence the outcomes of a physical test is experimenter gender. The present study examined the influence of experimenter gender on SPA and actual muscle strength in an experimental testing environment. Male (n = 50) and female (n = 50) university students were randomly assigned to either a male or female experimenter. Before strength testing, state SPA (SPA-S) was assessed. Actual strength was represented by the score obtained during the maximum voluntary contraction (MVC) test. Two 2 × 2 (participant gender × gender of the experimenter) analyses of variance were conducted with SPA-S and strength as the dependent variables. For SPA-S, a significant main effect was found only for participant gender (F(1,95) = 14.08, p < 0.01, η² = 0.13), with women scoring significantly higher than men. For MVC, there was a significant effect for participant gender (F(1,96) = 48.08, p < 0.001, η² = 0.33), with men, as expected, having significantly higher strength values than women. Although the gender of the experimenter did not influence SPA-S or muscle strength, other forms of anxiety (e.g., fitness anxiety) may be relevant in this setting. Future research should also investigate other factors in the testing environment (e.g., type of task) that may be more influential on psychological or performance outcomes.

Efeitos de dois parâmetros antropométricos no comportamento do sinal mecanomiográfico em testes de força muscular

Apesar da mecanomiografia (MMG) ser uma técnica com crescente destaque em investigações relativas ao fenômeno da contração muscular, poucos trabalhos se dedicaram a entender os possíveis efeitos de variáveis antropométricas no sinal de MMG. Portanto, o objetivo do estudo foi avaliar os efeitos da dobra cutânea e do perímetro do braço no comportamento do sinal de MMG nos domínios do tempo e da freqüência em diferentes níveis de contração isométrica. Participaram do estudo 21 indivíduos do sexo masculino (24,9 ± 7,8 anos) e 21 do feminino (20,7 ± 2,5 anos), todos destros. O protocolo experimental constou de um teste de contração voluntária máxima (CVM) que permitiu determinar cinco cargas percentuais administradas durante os testes de força (20%, 40%, 60%, 80% e 100% da CVM). Um acelerômetro biaxial foi colocado sobre o ventre muscular do bíceps braquial direito. O sinal de MMG foi analisado nos domínios do tempo, mediante o comportamento da amplitude do sinal, por meio do cálculo da raiz média quadrática (valor RMS), e da freqüência, através da freqüência mediana (FM) calculada a partir do seu espectro de potência. Estes parâmetros foram extraídos a partir dos sinais gerados na direção X, perpendicular às fibras musculares. Foram também medidos os parâmetros antropométricos dobra cutânea biciptal (BD_DC_B) e perímetros de braço relaxado (BD_PR) e contraído (BD_PC) no terço médio do braço direito. Ambos os grupos apresentaram um comportamento decrescente da FM com o nível de contração. O valor RMS apresentou comportamento crescente com a carga para ambos os grupos. Houve diferenças estatisticamente significativas (p &lt; 0,05) entre as variáveis antropométricas quando comparados os grupos. Sugere-se que o valor RMS, ao contrário da FM, que poderá sofrer atenuações por parte dos tecidos presentes na interface entre o músculo e o transdutor, seja mais consistente na discriminação da força muscular.

과제지향성 훈련이 뇌졸중 환자의 운동역학적 변인 및 근활성도에 미치는 영향

The purpose of this study was to examine the effects of Task-oriented training on the balance, lapse time and MVC(Maximum Voluntary Contraction) of CVA(cerebral vascular accident) patients. The active balance equipment was used to measure of the static balance, lapse time and task-oriented training. The EMG technique was used to record muscle activitie of affected side of gluteus medius and vastus medialis. The raw EMG data were filtered with band pass filter (60Hz) to remove artifacts and then low pass filtered (20Hz) to find the linear envelope which resemble muscle tension curve. The experiment had been conducted at the department of physical therapy in J hospital in M city during 8weeks. The thirty patients experienced the stroke were participated: training group (15), control group (15). They were ambulatory with or without an assistive device. They were assessed on central perturbation(mm) in the static balance, lapse time(s) and MVC test(mV). The data were analyzed using repeated measured ANOVA. The results were as follows: After Task-oriented training, central perturbation and lapse time was significantly differences in both groups(p<.001), and MVC in gluteus medius were no significant differences in both groups, but vastus medialis was significant differences in both groups (p<.001).

Influences of torque and joint angle on heart rate responses during isometric exercise in young men

Objective: To evaluate the influence of joint angle on heart rate (HR) responses induced by isometric exercise. Method: Ten healthy men (23.8 ± 2.5 years old) underwent isometric maximum voluntary contraction (MVC) tests lasting 10 sec using an electronic dynamometer under the following experimental conditions: knee extension at angles of 60° and 90° and knee flexion at angles of 30° and 90° angles. Their HR was recorded at rest (65 sec), during MVC (10 sec) and during the recovery period (120 sec). The data on mean maximum torque (MMT) and HR variation (ΔHR) were analyzed using the Friedman test with the Dunn post-hoc test, and their correlation was analyzed using the Spearman test (α= 0.05). Results: 1) MMT was significantly higher at 60° and 90° knee extension than at 30° and 90° knee flexion (p< 0.05), while no significant differences were found between the two extension angles or between the two flexion angles; 2) ΔHR was similar under all the experimental conditions; 3) No correlation was found between MMT and ΔHR. Conclusion: The data suggest that the rapid increase in HR during the 10 sec of isometric MVC does not depend on the joint angle or the morphofunctional differences between the two muscle groups studied.

Submaximal force production during perceptually guided isometric exercise

The aim of this study was to examine submaximal isometric force production guided by perceptual feelings of exertion. Thirty young adults performed isometric knee extensions on an isokinetic dynamometer. Subjects performed five different tests; the first test was the same for all subjects (standard naïve test). During the standard naïve test, subjects were asked to randomly produce force at perceived contraction intensities (25%, 50% and 75% of their maximum voluntary contraction (MVC)), with 100% MVC performed as the final intensity. All intensities, including the 100% MVC, were randomly performed in the other four tests (control tests 1 and 2, post 20% MVC and post 100% MVC tests). Post 20% MVC and post 100% MVC tests included fatiguing isometric exercise at 20% and 100% MVC respectively, which were performed prior to the test protocol. Results show that absolute peak force increased with increasing intensity (P<0.001) during all tests. During the standard naïve test, absolute peak force at 25% and 50% MVC was significantly lower (P=0.009) compared to control test 2, post 20% MVC and 100% MVC tests, and relative peak force was lower at all intensities compared to all other tests (P<0.001). Absolute and relative peak force was most accurate at 50% MVC (-12.06 N and -2.42%, respectively). Prior fatiguing isometric exercise did not affect the subsequent perceptual response range. In conclusion, isometric force was most accurate at 25% MVC but under-produced (perceptually overestimated) during higher contraction intensities preceding a maximal voluntary contraction (100% MVC). The ability to match absolute force with target contraction intensities was most accurate at 50% MVC during all five experimental conditions and poor at opposite ends of the force domain. Furthermore, prior fatiguing isometric exercise did not have an effect on the subsequent perceptual response range.

Skin Cooling Alters the Activation Patterns of Different Heads of the Quadriceps

The purpose of this study was to examine the effect of selective skin cooling over m. vastus lateralis (VL) on the activation patterns of quadriceps femoris muscle during knee extension exercise (KEE) using muscle function magnetic resonance imaging (mfMRI). The isometric force production of the right thigh was tested in 7 healthy young men at maximum voluntary contraction (MVC), and the transverse relaxation time (T2) value was taken from mfMR images at rest and immediately after KEE with 4 sets of 10 repetitions at a load equal to 60% of their 10-rep maximum, with and without skin cooling. The cooling was carried out by ice pack on the surface of the skin of the VL for 3 min before resting mfMRI and MVC tests, and before KEE, during KEE, and during the KEE rest intervals. The percent change in T2 of the m. vastus intermedius was significantly increased by skin cooling in comparison to the change without skin cooling, p < 0.05. This result suggests that skin cooling alters the activation pattern of the different heads of the quadriceps.

Intramuscular pressure and tissue oxygenation during low‐force static contraction do not underlie muscle fatigue

To test the hypothesis that time-wise increase in intramuscular pressure (IMP) and subsequent decrease in muscle tissue oxygenation (TO(2)) results in muscle fatigue development during a non-exhaustive, low-force contraction evidenced by changes in electromyogram (EMG) and particular mechanomyogram (MMG). Seven subjects performed static elbow flexion at 10% maximal voluntary contraction (MVC) for 10 min (10% MVC(10 min)). Surface EMG, MMG, IMP and TO(2) measured by near-infrared spectroscopy was recorded from m. biceps brachii during 10% MVC(10 min) and during 5% MVC test contractions of 1 min duration performed before 10% MVC(10 min), 10 and 30 min post-exercise. EMG and MMG were analysed for root mean square (rms) and mean power frequency (mpf). During 10% MVC(10 min) MMGrms increased from initial level of 0.04 +/- 0.01 to 0.11 +/- 0.07 m s(-2) in the last minute and MMGmpf and EMGmpf decreased from 34.9 +/- 8.2 to 21.3 +/- 3.8 Hz and from 71.7 +/- 10.9 to 61.7 +/- 10.0 Hz respectively. Similar changes were present in 5% MVC test contractions 30 min post-exercise. Initially, TO(2) decreased by 6.9 +/- 6.5% of resting level but returned to rest within 1 min. IMP remained constant during the contraction after an initial fourfold increase from resting level of 12.2 +/- 10.4 mmHg. IMP was anticipated to increase with time of contraction due to e.g. increased muscle water content; but this was not confirmed. Consequently, muscle blood flow was unlikely to be impeded with contraction time, which may account for the maintenance of TO(2). Thus, decreased TO(2) did not underlie either acute or long-term muscle fatigue development evidenced by changes in EMG and particular MMG variables.

Voluntary low-force contraction elicits prolonged low-frequency fatigue and changes in surface electromyography and mechanomyography

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC 10min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC 10min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC 10min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067 ± 0.028 mV before 10%MVC 10min to 0.107 ± 0.049 and 0.087 ± 0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054 ± 0.039 m s −2 to 0.133 ± 0.104 and 0.127 ± 0.099 m s −2, respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.

Activation characteristics of shoulder muscles during maximal and submaximal efforts

Determination of individual maximum voluntary contraction (MVC) force is used as the gold standard for normalising surface EMG (SEMG) data. Assuming a linear amplitude-force relationship, individual strain levels are defined according to percentage rates of the measured MVC levels. The purpose of the study was to investigate if the assumed force-strain relationship can be applied without qualification. Therefore, healthy volunteers (nine men, ten women) were investigated during isometric exercises of shoulder muscles at MVC and 50% levels. Tasks were performed at four different angular positions in frontal, sagittal and horizontal planes. In each plane, both possible force directions were investigated. SEMG was taken simultaneously from 13 muscles of the shoulder and upper arms from both sides of the body. At a force level of 50% MVC, SEMG amplitude levels were compared to the expected 50% level. Differences in muscular co-ordination patterns were also determined. During retroversion and horizontal flexion, amplitude levels significantly remained at levels below 50%. This was seen for all the muscles investigated, independent of relative contribution to force production. During horizontal extension and abduction, the main force-producing muscles showed amplitude levels that significantly exceeded the expected 50% level. Co-ordination patterns differed significantly between MVC and submaximal conditions for anteversion, retroversion and horizontal extension. Specifically, four shoulder muscles showed higher proportions at the 50% level compared to MVC. Therefore, certain percentage rates of MVC force levels exhibit quite different strain rates, as identified by SEMG. Depending on force direction, differences in co-ordination patterns exist between MVC and submaximal test conditions. Both findings have implications for therapeutic and training applications.

Selective fatigue of fast motor units after electrically elicited muscle contractions

The aim of the present study was to elucidate the electrophysiological manifestations of selective fast motor unit (MU) activation by electrical stimulation (ES) of knee extensor muscles. In six male subjects, test contraction measurement at 40% maximal voluntary contraction (MVC) was performed before and at every 5 min (5, 10, 15 and 20 min) during 20-min low intensity intermittent exercise of either ES or voluntary contractions (VC) at 10% MVC (5-s isometric contraction and 5-s rest cycles). Both isolated intramuscular MU spikes obtained from three sets of bipolar fine-wire electrodes and surface electromyogram (EMG) were simultaneously recorded and were analyzed by means of a computer-aided intramuscular spike amplitude–frequency analysis and frequency power spectral analysis, respectively. Results indicated that mean MU spike amplitude, particularly those MUs with relatively large amplitude, was significantly reduced while those MUs with small spike amplitude increased their firing rate during the 40% MVC test contraction after the ES. This was accompanied by the increased amplitude of surface EMG (rmsEMG). However, no such significant changes in the intramuscular and surface EMGs were observed after VC. These findings indicated differential MU activation patterns in terms of MU recruitment and rate coding characteristics during ES and VC, respectively. Our data strongly suggest the possibility of “an inverse size principle” of MU recruitment during ES.

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation–contraction coupling.

Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions.

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol (P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.

Muscular performance modeling of the upper limb in static postures.

The purpose of the present paper is to describe and evaluate the polynomial models for predicting the muscular work capacity of the upper limb during sustained holding tasks. This research was concerned with the relationship between indicators of performance, i.e., specific posture or specific level of maximum voluntary contraction (MVC), and then modeling the functional data based on experimental results to estimate factors that may have an effect on task performance. To this end, we designed an experiment using 10 subjects in which each subject performed sustained isometric shoulder and elbow flexion endurance exercise under 27 conditions [3 shoulder angles (SA)x3 elbow angles (EA)x3 levels of %MVC]. Throughout all experiments, subjective perception of effort was assessed using the Borg scale, every 60, 30, and 10 s during the 20%, 40%, and 60% MVC tests, respectively. Proposal models were represented by three approaches: model A: estimation of endurance time (ET), with input variables such as SA, EA, and %MVC; model B: estimation of recommendation time (RT, the time during which the operator was able to maintain a position under the desired condition), with input variables such as SA, EA, %MVC, and required rate on the Borg scale; and model C: estimation of limit strength or %MVC, with input variables such as SA, EA, request limit time for work (LT), and required rate on the Borg scale. Statistical analysis indicated that the three proposal estimation models based on polynomial regression functions showed high significance (p<0.0001). The proposal models suggested and recommended the possibility of finding the best positions entailing the reduction and minimization of total muscular strain from manual material handling tasks in different work situations, with the consequent increase in work efficiency.

Mechanomyogram and electromyogram responses of upper limb during sustained isometric fatigue with varying shoulder and elbow postures.

To investigate the behavior of mechanomyogram (MMG) and electromyogram (EMG) signals in the time and frequency domains during sustained isometric contraction, MMG and surface EMG were obtained simultaneously from four muscles: upper trapezius (TP), anterior deltoid (DL), biceps brachii (BB), and brachioradialis (BR) of 10 healthy male subjects. Experimental conditions consisted of 27 combinations of 9 postures [3 shoulder angles (SA): 0 degree, 30 degrees, 60 degrees and 3 elbow angles (EA): 120 degrees, 90 degrees, 60 degrees] and 3 contraction levels: 20%, 40%, and 60% of maximum voluntary contraction (MVC). Subjective evaluations of fatigue were also assessed using the Borg scale at intervals of 60, 30, and 10 sec at 20%, 40%, and 60% MVC tests, respectively. The mean power frequency (MPF) and root mean square (RMS) of both signals were calculated. The current study found clear and significant relationships among physiological and psychological parameters on the one hand and SA and EA on the other. EA's effect on MVC was found to be significant. SA had a highly significant effect on both endurance time and Borg scale. In all experimental conditions, significant correlations were found between the changes in MPF and RMS of EMG in BB with SA and EA (or muscle length). In all four muscles, MMG frequency content was two or three times lower than EMG frequency content. During sustained isometric contraction, the EMG signal showed the well-known shift to lower frequencies (a continuous decrease from onset to completion of the contraction). In contrast, the MMG spectra did not show any shift, although its form changed (generally remaining about constant). Throughout the contraction, increased RMS of EMG was found for all tests, whereas in the MMG signal, a significant progressive increase in RMS was observed only at 20% MVC in all four muscles. This supports the hypothesis that the RMS amplitude of the MMG signal produced during contraction is highly correlated with force production. Possible explanations for this behavioral difference between the MMG and EMG signals are discussed.

Sensorimotor adaptations to microgravity in humans.

Motor function is altered by microgravity, but little detail is available as to what these changes are and how changes in the individual components of the sensorimotor system affect the control of movement. Further, there is little information on whether the changes in motor performance reflect immediate or chronic adaptations to changing gravitational environments. To determine the effects of microgravity on the neural control properties of selected motor pools, four male astronauts from the NASA STS-78 mission performed motor tasks requiring the maintenance of either ankle dorsiflexor or plantarflexor torque. Torques of 10 or 50% of a maximal voluntary contraction (MVC) were requested of the subjects during 10 degrees peak-to-peak sinusoidal movements at 0.5 Hz. When 10% MVC of the plantarflexors was requested, the actual torques generated in-flight were similar to pre-flight values. Post-flight torques were higher than pre- and in-flight torques. The actual torques when 50% MVC was requested were higher in- and post-flight than pre-flight. Soleus (Sol) electromyographic (EMG) amplitudes during plantarflexion were higher in-flight than pre- or post-flight for both the 10 and 50% MVC tasks. No differences in medial gastrocnemius (MG) EMG amplitudes were observed for either the 10 or 50% MVC tasks. The EMG amplitudes of the tibialis anterior (TA), an antagonist to plantarflexion, were higher in- and post-flight than pre-flight for the 50% MVC task. During the dorsiflexion tasks, the torques generated in both the 10 and 50% MVC tasks did not differ pre-, in- and post-flight. TA EMG amplitudes were significantly higher in- than pre-flight for both the 10 or 50% MVC tasks, and remained elevated post-flight for the 50% MVC test. Both the Sol and MG EMG amplitudes were significantly higher in-flight than either pre- or post-flight for both the 10 and 50% MVC tests. These data suggest that the most consistent response to space flight was an elevation in the level of contractions of agonists and antagonists when attempting to maintain constant torques at a given level of MVC. Also, the chronic levels of EMG activity in selected ankle flexor and extensor muscles during space flight and during routine activities on Earth were recorded. Compared with pre- and post-flight values, there was a marked increase in the total EMG activity of the TA and the Sol and no change in the MG EMG activity in-flight. These data indicate that space flight, as occurs on shuttle missions, is a model of elevated activation of both flexor and extensor muscles, probably reflecting the effects of programmed work schedules in flight rather than a direct effect of microgravity.

Metabolic heat production during fatigue from voluntary repetitive isometric contractions in humans.

The effect of repetitive isometric knee extensions on the energy cost of contraction was examined. The rate of temperature rise (dT/dt) was determined in test contractions at 30 and 50% of maximal voluntary contraction (MVC) force before and during 30% MVC repetitive isometric exercise (RIE) to exhaustion and regularly in a 30-min postexercise recovery period (n = 9). Pulmonary O2 uptake and muscle temperature (Tmus) were determined at regular intervals. During the 30% MVC test contractions, dT/dt was 5.6 +/- 0.6 mK/s in unfatigued muscle, increasing linearly by 68% during exercise. In the 50% MVC test contractions, dT/dt rose by 84% from 9.8 +/- 1.1 mK/s. dT/dt determined during test contractions at both force levels did not decrease significantly throughout the 30-min postexercise recovery period. The rise in dT/dt was paralleled by 76% increased in O2 uptake. In contrast, Tmus rose initially and then leveled off. The present data indicate that RIE induced a gradual rise in the rate of energy turnover associated with isometric force production. Neither increased Tmus nor recruitment of less economic type II fibers can fully explain the increased energy cost. We suggest that energetic changes may occur at the cellular level and argue that this may be associated with the changes in muscle mechanics occurring during fatigue from submaximal voluntary RIE.

Anthropometric determination of thigh volumes and thigh forces following acute training of increasing intensity in adult men.

The purpose of the present study was to investigate the influence of submaximal training of increasing intensity on the rate of fatigue and on the anthropometry of the quadriceps muscle. A group of 24 middle-aged male subjects trained three times a week for 12 weeks at incremental exercise intensities which elicited 66%-83% of the maximal heart rate reserve; 11 male subjects acted as controls. A purpose-built, isometric chair with an adjustable force transducer ensured a standard posture for each subject during a 20 s maximal voluntary contraction (MVC). The muscle plus bone mass of the thigh was calculated from a previously validated anthropometric method reinforced by measurements derived from water displacement and ultrasonics. Training-induced submaximal relationships were assessed by means of two ergometer tests of progressive intensity. Following training, the subjects' blood lactate concentrations had decreased from 5.5 (SD 1.7) to 3.9 (SD 1.4) mmol.1(-1) (P < 0.01) at the final exercise intensities. They increased thigh volumes from 10.7 (SD 1.7) to 11.0 (SD 1.7) 1 (P < 0.05) and thigh forces in 47.5% (P < 0.05) of the measurements. Heart rates and rates of perceived exertion were reduced (P < 0.01) following training; for the controls, all the above parameters remained constant. These results would seem to indicate that a submaximal type of training could enable a group of middle-aged men to increase the isometric forces of the thigh muscles by almost 50% when completing a MVC test following training. This suggests that this is a more suitable, safer form of maintaining and increasing maximal thigh forces for this, and older, age groups than specific isometric training alone.

Myo-electric fatigue and force failure from submaximal static elbow flexion sustained to exhaustion.

Static contraction to the limit of endurance was performed at 40% and 10% of the maximal voluntary contraction (MVC). A group of 11 men (10 in their twenties, one aged 44) had the surface electromyogram of the brachioradialis and the biceps brachii (BB) muscles of the right arm (elbow angle 135 degrees) recorded. Endurance times were 113 (SD 28) s (40% MVC) and 51 (SD 19) min (10% MVC). Prolonged contraction changed the root mean square (rms) amplitude, the median frequency (fm), and the average muscle fibre conduction velocity (CV, measured by cross-correlation) as follows: 40% MVC: rms amplitude, increase of 150%-200%; fm, decrease of 55%-60%; CV, decrease of 25%-40%; and 10% MVC: rms amplitude, increase of 400%; fm, decrease of 20%; CV, decrease of 0%-10%. Since the CV of 10% MVC changed little or not at all, the large rms amplitude increases were attributed to motor unit (MU) recruitment, i.e. a massively intensified central motor command. The relative fm decreases of both contraction levels surpassed the CV slowing to an extent increasing with the relative contraction time; the additional fm lowering was in part assumed to reflect central nervous system mediated regulation of the time dispersion of MU firing, principally synchronization/grouping of MU action potentials (AP). Electrical stimulation of the BB muscle and the performance of 100% MVC test contractions found uniform relative force failures due to the performance of 40% and 10% MVC contractions. From variations in amplitude and conduction time of compound action potentials (CAP), it seemed unlikely that reduced muscle fibre excitability/AP propagation failure was underlying the force losses at exhaustion. Rather, the well preserved CAP after 10% MVC and the recovery CAP of 40% MVC indicated excitation-contraction failure caused by sustained voluntary contractions.

Factors contributing to the frequency shift in the surface electromyogram during maximal voluntary contraction (MVC) : H. Kranz, A.M. Williams and D.J. Caddy (Melbourne, Australia)

The objectives of this study were to: (1) compare the sEMG recordings from maximal voluntary contractions (MVC), and (2) examine the reproducibility of sEMG recordings from MVCs for selected lower extremity muscles derived from manual muscle testing (MMT) on dry land, and in water prior to and following aquatic treadmill running. Twelve healthy recreational male runners participated. The selected muscles were: M. quadriceps–vastus medialis (VM) and rectus femoris (RF), M. biceps femoris (BF), M. tibialis anterior (TA) and the M. gastrocnemius caput mediale (GAS) of the right leg. The MVC testing conditions were: dry land, underwater prior to (Water 1) and following an aquatic exercise trial (Water 2). For each muscle, a one-way analysis of variance with repeated measures was used to compare MVC scores between testing conditions, and the intra-class correlation coefficient (ICC) and typical error (CV%) were calculated to determine the reproducibility and precision of MVC scores, respectively, between conditions. For all muscles, no significant differences were observed between land and water MVC scores (p = 0.88–0.97), and high reliability (ICC = 0.96–0.98) and precision (CV% = 7.4–12.6%) were observed between MVC conditions. Under MMT conditions it appears that comparable MVC sEMG values were achieved on land and in water and the integrity of the EMG recordings were maintained during water immersion. Future studies using sEMG waterproofing procedures should conduct MVC testing in water for data normalization and perform post-exercise verification of sEMG signal integrity.