Exertion dependent alternations in force fluctuation and limb acceleration during sustained fatiguing contraction

The study was conducted to contrast exertion-dependent changes in electromyography (EMG), force fluctuation (FF), and limb acceleration (LA) during isometric contraction to attain a versatile picture of muscle fatigue. Fifteen volunteers performed sustained index abduction at 25 and 75% maximal voluntary contractions (MVC) until failure at the tasks; meanwhile, changes in temporal/spectral features of force, muscle activity of the first dorsal interosseous (FDI), and acceleration of the index and hand were monitored. The results showed a manifest increase in all recorded signals for the 25% MVC paradigm, especially for LA, which demonstrated the largest increment in amplitude. In addition to progressive enhancement of the mechanical coupling of the metacarpophalangeal (MCP) joint, the 25% MVC paradigm added to EMG–FF and EMG–LA coherences (Coh_EMG–FF and Coh_EMG–LA) at 8–12 Hz and the shift of the spectral peak of the LA to higher frequencies. In contrast, the 75% MVC paradigm did not modulate significantly the spectral peak of LA. Also, Coh_EMG–FF, Coh_EMG–LA at 8–12 Hz, and the mechanical coupling of the MCP joint, were conversely undermined consequent to the high exertion paradigm. The present study suggests that LA was most susceptible to muscle fatigue following sustained contraction at a lower exertion level, and diverging alternations among various physiological signals ascribed to exertion-dependent contributions of central and peripheral origins to muscle fatigue.     

Role of limb movement in the modulation of motor unit discharge rate during fatiguing contractions

Motor unit firing rates of the triceps brachii muscle have been shown to decline during sustained isometric contractions, but not if the fatiguing contraction incorporates arm movements. The purpose of this study was to determine the impact of the actual physical displacement of the limb on the maintenance of motor unit discharge rate during dynamic muscle fatigue. An isometric force pulse paradigm was used to recreate the motor unit activity patterns that occur during a dynamic contraction. With this paradigm, the variable force output that would occur during a dynamic contraction remained intact, but the movement of the limb was eliminated. Motor unit firing rates declined in the isometric force pulse protocol. Thus, factors related to the actual movement of the limb appear to enable the maintenance of motor unit discharge rates during fatigue.     

Phonomyogram and electromyogram relationships with isometric force reinvestigated in man

Summary Contracting muscle generates sounds which can be recorded easily by means of a microphone. To determine if a phonomyogram (PMG) can be used to monitor muscle force, a comparison was made between simultaneous recordings of PMG and monopolar electromyogram (EMG) from the isometrically contracting biceps brachii muscle and the external flexion force. Locations of the monopolar electrode and microphone were identified in relation to the motor point. Whatever the recording site, PMG amplitude was proportional to EMG amplitude and both showed a quadratic relationship to muscle force. Changes in the PMG spectrum with force were similar to those in EMG, i.e. the mean power frequency increased up to about 30% maximal voluntary contraction and then reached a plateau. De spite a slightly higher variability, PMG was shown to be a valid index of muscular isometric force. At the same force, the amplitude of both PMG and EMG was lower in the prone than in the supine position of the hand. This result indicated a selective recording of biceps brachii muscle activity.     

Passive hyperthermia reduces voluntary activation and isometric force production

It has been suggested that a critically high body core temperature may impair central neuromuscular activation and cause fatigue. We investigated the effects of passive hyperthermia on maximal isometric force production (MVC) and voluntary activation (VA) to determine the relative roles of skin (T_sk) and body core temperature (T_c) on these factors. Twenty-two males [O_2max=64.2 (8.9) ml kg^–1 min^–1, body fat=8.2 (3.9)%] were seated in a knee-extension myograph, then passively heated from 37.4 to 39.4°C rectal temperature (T_re) and then cooled back to 37.4^oC using a liquid conditioning garment. Voluntary strength and VA (interpolated twitch) were examined during an isometric 10-s MVC at 0.5°C intervals during both heating and cooling. Passive heating to a T_c of 39.4^oC reduced VA by 11 (11)% and MVC by 13 (18)% (P<0.05), but rapid skin cooling, with a concomitant reduction in cardiovascular strain [percentage heart rate reserve decreased from 64 (11)% to 29 (11)%] and psychophysical strain did not restore either of these measures to baseline. Only when cooling lowered T_c back to normal did VA and MVC return to baseline (P<0.05). We conclude that an elevated T_c reduces VA during isometric MVC, and neither T_sk nor cardiovascular or psychophysical strain modulates this response. Results are given as mean (SD) unless otherwise stated.     

Muscular sound and force relationship during isometric contraction in man

Summary The contracting muscle generates a low frequency sound detectable at the belly surface, ranging from 11 to 40 Hz. To study the relationship between the muscular sound and the intensity of the contraction a sound myogram (SMG) was recorded by a contact sensor from the biceps brachii of seven young healthy males performing 4-s isometric contractions from 10% to 100% of the maximal voluntary contraction (MVC), in 10% steps. Simultaneously, the electromyogram (EMG) was recorded as an index of muscle activity. SMG and EMG were integrated by conventional methods (iSMG and iEMG). The relationship between iSMG and iEMG vs MVC% is described by parabolic functions up to 80% and 100% MVC respectively. Beyond 80% MVC the iSMG decreases, being about half of its maximal value at 100% MVC. Our results indicate that the motor unit recruitment and firing rate affect the iSMG and iEMG in the same way up to 80% MVC. From 80% to 100% MVC the high motor units' discharge rate and the muscular stiffness together limit the pressure waves generated by the dimensional changes of the active fibres. The muscular sound seems to reflect the intramuscular visco-elastic characteristics and the motor unit activation pattern of a contracting muscle.     

Reliability of maximal muscle force and voluntary activation as markers of exercise-induced muscle damage

The loss of the ability of skeletal muscle to generate force is one of the most appropriate and valid means to quantify muscle damage. Routine measurements of maximal muscle force, however, include many potential sources of error, the most important of which may be a possible lack of central drive to the muscles. The aim of the present study was to determine the reliability of maximal isometric quadriceps muscle force and voluntary activation over a timescale that is typically employed to examine the aetiology of exercise-induced muscle damage. We also attempted to characterise the reliability of several twitch interpolation variables including the size of the interpolated twitch and the state (i.e. unpotentiated vs potentiated) and size of the resting twitch. Over a 7-day period, eight healthy active males performed repeated maximal voluntary isometric contractions (MVC) of the quadriceps (baseline and 2 h, 6 h, 24 h, 48 h, 72 h and 7 days post). Systematic variations in maximal muscle force, voluntary activation, interpolated twitch, unpotentiated twitch and potentiated twitch were not statistically significant (P>0.05) and 95% repeatability coefficients of ±76.03 N, ±4.42%, ± 8.44 N, ±25.92 N and ±43.58 N were observed, respectively. These data indicate that young healthy well-familiarised male subjects can reproduce their perceived maximal efforts both within and between days where activation levels of >90% are routinely achieved. Providing activation remains within these limits in the 7 days following an acute bout of exercise, the researcher would be 95% certain that exercise-induced muscle damage is present in individual subjects (taken from similar subject populations) if MVC force falls outside these limits.     

The effect of initial length on the shortening velocity of cat hind limb muscles

The effect of initial muscle length on the speed of shortening at different relative loads has been determined for the soleus and flexor digitorum longus (FDL) muscles of the cat. Isometric tetanic force-length relationships for both muscles were similar to those shown previously. The functional length range for FDL occurred at relatively long lengths, from optimum (100%) to 135% of optimum length; however, soleus worked at relatively short lengths from 60% to 110% of optimum length. In FDL the speed of shortening at any given load was relatively constant within the functional range, but at very short muscle lengths the speed of shortening declined. Soleus also showed a decline in the speed of shortening at all loads at short muscle lengths, which included the functional working range of the muscle. Speed of shortening at any given load was maximal at optimum length but tended to decline at low loads and long muscle lengths. It is concluded that in FDL even when the toes are at the extremity of their range, speed of muscle shortening is unaffected. Soleus may be relatively disadvantaged because its functional range extends over short muscle lengths. The results indicate that soleus is capable of making a significant contribution in standing and a slow walk, but that at faster gaits the contribution of soleus may be negligible.     

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.     

Muscle activation and time to task failure differ with load type and contraction intensity for a human hand muscle

Time to failure for sustained isometric contractions of the elbow flexors is briefer when maintaining a constant elbow angle while supporting an inertial load (position task) compared with exerting an equivalent torque against a rigid restraint (force task). Our primary purpose was to determine whether the effects of load type on time to task failure exist when motor unit recruitment cannot be enhanced during a sustained submaximal contraction of an intrinsic hand muscle. A second purpose was to determine whether a greater reserve remains in the muscle after early failure of the position task. Two groups of 10 strength-matched men performed the force and position tasks at either 20% or 60% of maximal force (MVC) with the first dorsal interosseus, followed by a second force task at the same relative intensity. The rate of increase in surface EMG was greater (P = 0.002) and time to failure was briefer (P = 0.005) for the position task (593±212 s) compared with the force task (983±328 s) at 20% MVC, whereas there were no task differences in these variables at 60% MVC (P ≥ 0.200). Time to failure for the second force tasks did not differ at either contraction intensity (P≥0.743). These results demonstrate that previously observed effects of load type generalize to a hand muscle, although only for low-intensity contractions. For the position task at low forces, muscle activity increased more rapidly and no additional reserve remained in the muscle at failure compared with the force task. We propose that the briefer time to failure for the position task during sustained, low-intensity contractions is due to earlier recruitment of the motor unit pool.     

Changes in strength and cross sectional area of the elbow flexors as a result of isometric strength training

Summary Changes in strength and size of the elbow flexor muscles have been compared during six weeks of isometric strength training in six male and six female subjects. Isometric training of one arm resulted in a significant increase in isometric force (14.5±5.1%, mean±SD,n = 12). No differences were seen in the response of male and female subjects. The extent of the change was similar to that reported for training studies of other muscles, thus refuting the suggestion that the elbow flexors may be especially amenable to training. Biceps and brachialis cross-sectional area (CSA) was measured from mid-upper arm X-ray computerized tomography before and after training. Muscles increased in area (5.4±3.8%) but this was smaller than, and not correlated with, the increase in strength. The main change in the first six weeks of strength training was therefore an increase in the force generated per unit cross-sectional area of muscle. The arrangement of fibres in the biceps is nearly parallel to the action of the muscle and it is argued that the increase in force per unit cross-sectional area is unlikely to be due to changes in the pennation of the muscle fibres as has been suggested for other muscles.     

Heterogeneity of mean sarcomere length in different fibres: effects on length range of active force production in rat muscle

Isometric active length-force characteristics of rat semimembranosus lateralis muscles [rats,n = 6, body mass = 292 (SD 9) g] were recorded. Muscles were photographed during the force plateaus of the tetani to determine lengths of proximal, intermediate and distal fibres. From the mean number of sarcomeres in series in those fibres, mean sarcomere length at different muscle lengths was calculated. The heterogeneity of mean sarcomere lengths for each muscle was quantified according to the coefficient of variation of sarcomere lengths at muscle optimal length. Absolute muscle length changes were equal to fibre length changes. At all muscle lengths, mean sarcomere lengths in the distal fibres were significantly greater than those in proximal and intermediate fibres. The heterogeneity of mean sarcomere lengths augmented the length range of active force production between muscle optimal length and active slack length by about 38% to 43%. We concluded that in a muscle with a low degree of pennation, the heterogeneity of mean sarcomere lengths should be considered as a substantial contributor to the length range over which active force can be produced. In our experiment, the length ranges of active force production between optimal and slack length showed considerable differences (range 18.7–24.9 mm). The coefficient of correlation between length ranges and mean number of sarcomeres in series in various muscle regions was extremely low (r = –0.14) and not significant. The coefficient of correlation between length ranges and heterogeneity was high (r = 0.88) and significant. These data would suggest that muscles with a similar number of sarcomeres in series can exhibit quite different functional characteristics.     

Habitual level of physical activity and muscle fatigue of the elbow flexor muscles in older men

Reduced muscle performance, related to the loss of muscle mass and strength, is a common and natural part of ageing. Nevertheless, it is generally believed that regular participation in activities of moderate intensity may slow down these age-related changes. This study investigated the relationship between the habitual level of physical activity (PA), assessed by the modified Baecke Questionnaire, and the mechanical and fatigue characteristics of the right elbow flexor muscles, m. biceps brachii and m. brachioradialis, in men over the age of 55 years. Muscle fatigue was quantified both by measuring the maximal voluntary contraction (MVC) torque before and after a sustained isometric contraction at 25% MVC until exhaustion, and also by the temporal changes observed in the surface electromyographic (EMG) signal recorded during the fatigue task. Results showed a decreased MVC torque at the end of the fatiguing contraction. After 20 min of recovery, the MVC force was still significantly lower than the pre-fatigue value, except for the most active subjects. Typical myoelectrical indications of fatigue were also observed: a shift in the frequency spectrum of the signal towards lower frequencies accompanied by an increase in the EMG amplitude. We concluded from this study that the level of PA was related to the absolute isometric MVC values and the measurement of neuromuscular efficiency after 20 min of recovery, but did not influence the indications of muscle fatigue during an isometric fatigue task. Electronic Publication     

Local tissue temperature effects on peak torque and muscular endurance during isometric knee extension

The aim of the study was to investigate the relationship between local tissue temperature, peak torque and time to fatigue during isometric knee extensions. Nine males performed maximum voluntary contractions (MVCs) and isometric knee extensions at 70% MVC to exhaustion after 30 min of hot [H, 47.7 (1.3)°C; mean (SD)], warm [W, 34.6 (0.4)°C], temperate [T, 24.5 (1.3)°C], and cold [C, –11.9 (1.8)°C] localized temperature applications. Isometric peak torque was not significantly affected by temperature. Time to fatigue was strongly and negatively correlated (r=–0.98) to temperature, with endurance after H [46.99 (4.98) s] and W [54.36 (9.18) s] significantly shorter than after C [73.27 (13.43) s]. We conclude that local tissue temperature does not impair peak force production but may change muscular endurance through local factors.     

EMG activation patterns during force production in precision grip

Electromyographic (EMG) activity was analyzed for the occurrence of synergistic patterns during the steady hold periods of force in the precision grip. To establish the presence of muscle synergies in the amplitude (spatial) domain, the EMG activation levels of pairs of simultaneously active muscles were linearly correlated. Cross-correlations of EMG activity were computed to quantify muscle synergies in the spatiotemporal domain (synchronization). A muscle pair was defined to be synergistically coupled or synchronously activated when the correlation (amplitude domain) or cross-correlation (time domain) was significant for at least two of the three steady state force levels. Muscle synergies in the amplitude domain were found in one-third of the 213 muscle pairs tested, distributed among 47 of the 82 tested muscle combinations. Coactivation was the predominant synergistic pattern, whereas trade-off comprised not more than 23% of the synergies. Cross-correlation peak size varied between 5 and 39% of the autocorrelation size, with delays in the range of ±8 ms and base width between 12 and 20 ms. Synchronization was found in one-fourth of the 213 muscle pairs tested and among 35 of the 82 muscle combinations, i.e., less frequently than covariation of EMG activity levels. However, the interindividual prevalence was higher for synchronization than for synergies in the amplitude domain, since, for the synergistic muscle combinations, almost twice as many muscle pairs were found to be synchronized than coupled in the amplitude domain. Synergies in the two domains occurred independently in some pairs and concurrently in other cases, and were observed between muscles moving the thumb, the index finger, or both digits. Synchronization was more frequent in pairs of muscles supplied by branches of the same peripheral nerve (46%) than in those innervated by different nerves (18%). Synergies in the amplitude domain were distributed in similar proportions across intrinsic, extrinsic, and combinations of both types of muscles, whereas synchronization mainly occurred in pairs of intrinsic muscles. When the task was repeated with slightly lower target forces, there were fewer synergies in the amplitude domain (in 52 of the 213 pairs, distributed among 35 of 82 muscle combinations) and their distribution changed, indicating a flexible, force-dependent mechanism. In conclusion, no strictly coherent interindividual pattern of synergies in the spatial domain could be established.     

Transfer of resistance training to enhance rapid coordinated force production by older adults

The purpose of this study was to examine the capacity of resistance training to enhance the rapid and coordinated production of force by older people. Thirty adults (60 years) completed a visually guided aiming task that required the generation of isometric torque in 2 df about the elbow prior to and following a 4-week training period. Groups of six participants were allocated to two progressive (40–100% maximal voluntary contraction (MVC)) resistance-training (PRT) groups, to two constant low-load (10% MVC) training groups (CLO) and to one no-training control group. Training movements required the generation of either combined flexion and supination (FLESUP), or combined extension and supination (EXTSUP). In response to training, target acquisition times in the aiming task decreased for all groups; however, both the nature of the training load and the training movement influenced the pattern and magnitude of improvements (EXTSUP_CLO: 36%, FLESUP_PRT 26%, EXTSUP_PRT 22%, FLESUP_CLO 20%, CONTROL 15%). For one group that trained with progressively increasing loads, there arose a subsequent decrease in performance in one condition of the transfer task. For each group, these adaptations were accompanied by systematic changes in the coordination of muscles about the elbow joint, particularly the biceps brachii.Abbreviations MVC maximal voluntary contraction - EMG electromyography - PRT progressive resistance training - CLO constant low-load training - FLESUP flexion plus supination - EXTSUP extension plus supination - FLEPRO flexion plus pronation - EXTPRO extension plus pronation - FLEX flexion - SUPN supination - EXTD extension - PRON pronation - RMS root mean square     

A simple electronic circuit for determining the twitch force and resting force of small heart muscle preparations

The twitch of small preparations of heart muscle is often weak and difficult to record accurately during an experiment. Measurement of small twitch forces can also be handicapped by minor disturbances in the experimental chamber. We describe here a simple electronic circuit that is useful for detecting small changes of twitch force during experimental interventions. Using this circuit, the twitch force signal can be recorded on relatively high gain and changes of twitch force can be observed that might otherwise go undetected.     

Effect of muscle length on surface EMG wave forms in isometric contractions

Summary To elucidate the influence of muscle length on surface EMG wave form, comparisons were made of surface EMGs of the biceps and triceps brachii muscles during isometric contractions at different muscle lengths. Muscle lengths were altered by setting the elbow joint angle at several intervals between the limits of extension and flexion. The intensity of the isometric contractions was 25% of maximum voluntary contraction at the individual joint angles. Slowing was obvious in the EMG wave forms of biceps as muscle length increased. The so-called Piper rhythm appeared when the muscle was more than moderately lengthened. The slowing trend with muscle lengthening, though less marked, was also seen in triceps. Zero-cross analysis revealed quasi-linear relationships between muscle length and slowing. Frequency analysis confirmed the development of Piper rhythm. An attempt was made to interpret the slowing associated with muscle lengthening in terms of the propagation of myoelectric signals in muscle fibers. Given the effect of muscle length on EMG wave forms, a careful control of joint angle may be required in assessing local muscle fatigue when using EMG spectral indices.     

EMG activation patterns during force production in precision grip. III. Synchronisation of single motor units

Motor unit (MU) synchronisation during isometric force production in the precision grip was analysed in five subjects performing a visually guided steptracking motor task with three different force levels. With this aim multi-unit electromyographic (EMG) activity of 14 intrinsic and extrinsic finger muscles from 15 experimental sessions was decomposed into the potentials of single MUs. The behaviour of 62 intrinsic and 30 extrinsic MUs in the motor task was quantified. Most MUs displayed a positive correlation between firing rate and grip force. Compared to MUs in extrinsic muscles, intrinsic MUs had steeper regression lines with negative intercepts indicating higher force sensitivity and higher recruitment thresholds. A cross-correlation analysis was performed for 69 intra- and 166 intermuscular MU pairs while steady grip force was exerted at the three force levels. Synchronisation, for at least one force level, was found in 78% of the intra- and 45% of the intermuscular pairs. The occurrence of synchronisation was not stable over the force range tested. Factors influencing the fluctuations in occurrence and strength of synchronisation were investigated. Force increase was not paralleled by increased synchronisation; in contrast, in most MU pairs, especially intermuscular pairs, synchronisation occurred preferentially at the lower force levels. The recruitment threshold appeared to play a determining role in synchronisation: the more similar the thresholds of two MUs, the greater the probability of them being synchronised at this force level. Synchronised MUs fired on average at a lower frequency than non-synchronised ones. Finally, synchronisation at the multi-unit EMG level does not indicate that all underlying MUs are synchronised, nor does the absence of temporal coupling at the multi-unit level indicate that none of the MUs is synchronised.     

Asymmetry in grasp force matching and sense of effort

While asymmetries in upper limb force matching have been observed, the mechanisms underlying asymmetry in the sense of effort have not been conceptualized. The aim of this study was to investigate asymmetries in the perception and reproduction of grasp force. Forty-two young adults, 22 right-handed (RH) and 20 left-handed (LH), were, respectively, divided into three groups according to differences between their right and left-hand strength (left stronger than right, right stronger than left and right & left equivalent). A reference force, representing 20% of the maximal voluntary contraction (MVC) produced by the right or left hand, was matched with same hand (Ipsilateral Remembered—IR) or opposite (Contralateral Remembered—CR) hand. The matching relative error was 92% (for RH) and 46% (for LH) greater in the CR than IR condition. Asymmetries in matching were significant for RH participants only in the CR condition and were dependent on right/left differences in hand strength as shown by the constant error (CE). For this RH population, right-hand overshoot of the left-hand reference and left-hand undershoot of the right-hand reference were significant when the right hand was stronger than the left. Asymmetry remained significant when CE was normalized (%MVC). Asymmetry was reduced when the strength of each hand was equivalent or when the left hand was stronger than the right. These findings suggest that effort perception is asymmetric in RH but not in LH individuals. The hand x strength interaction indicates that asymmetry in force matching is a consequence of both a difference in the respective cortical representations and motor components, which confer a different “gain” (input–output relationship) to each system. The similarity with position sense asymmetry suggests that the gain concept may be generalized to describe some functional/performance differences between the two hand/hemisphere systems. The more symmetrical performance of the LH than RH group underlines that context specific influence of handedness, hemisphere dominance and hemispheric interactions modulate performance symmetries/asymmetries.