Activation of shoulder muscles in healthy men and women under isometric conditions

Purpose: Due to the low osseous lead of the shoulder joint a large portion of the shoulder muscles, in addition to executing movements, deals with stabilising tasks. This requires a permanent readjustment of the intermuscular co-ordination of all involved muscles. The aim of the study was to verify the existence of gender dependent differences in intramuscular co-ordination patterns of shoulder muscles.

Method: Fifteen healthy men and nine healthy women, who executed 24 isometric exercises in sagittal, frontal and horizontal planes with a loading of 50% of their individual isometric maximum force, were investigated. In every plane, four angular positions were chosen and both opposite force directions were measured, respectively. SEMG was taken from 13 muscles of the shoulder and the upper arm. Due to inter-individual differences SEMG amplitudes were normalised. Results: Gender specific differences of functional intermuscular co-ordination patterns could be proven systematically. Women showed less activation of muscles acting in the main force direction. In addition, those muscles less necessary for the actual force production were more activated in women than in men.

Conclusions: Functionally comparable shoulder function showed a gender dependency in terms of functional intermuscular co-ordination.     

Forearm blood flow responses to fatiguing isometric contractions in women and men

Previous studies suggest that women experience less vascular occlusion than men when generating the same relative contractile force. This study examined forearm blood flow (FBF) in women and men during isometric handgrip exercise requiring the same relative force. Thirty-eight subjects [20 women and 18 men, 22.8 +/- 0.6 yrs old (means +/- SE)] performed low- and moderate-force handgrip exercise on two occasions. Subjects performed five maximum voluntary contractions (MVC) before exercise to determine 20% and 50% MVC target forces. Time to task failure (TTF) was determined when the subject could not maintain force within 5% of the target force. Mean blood velocity was measured in the brachial artery with the use of Doppler ultrasonography. Arterial diameter was measured at rest and used to calculate absolute FBF (FBFa; ml/min) and relative FBF (FBFr; ml.min(-1).100 ml(-1)). Women generated less (P < 0.05) absolute maximal force (208 +/- 10 N) than men (357 +/- 17 N). The TTF was longer (P < 0.05) at 20% MVC for women (349 +/- 32 s) than for men (230 +/- 23 s), but no difference between the sexes was observed at 50% MVC (women: 69 +/- 5 s; men: 71 +/- 8 s). FBFa and FBFr increased (P < 0.05) from rest to TTF in both women and men during 20% and 50% MVC trials. FBFr was greater in women than in men at > or =30% TTF during 50% MVC. At exercise durations > or =60% of TTF, FBFa was lower (P < 0.05) in women than in men during handgrip at 20% MVC. Despite the longer exercise duration for women at the lower contraction intensity, FBFr was similar between the sexes, suggesting that muscle perfusion is matched to the exercising muscle mass independent of sex.     

Differences in stretch reflex responses of elbow flexor muscles during shortening, lengthening and isometric contractions

Stretch reflexes were evoked in elbow flexor muscles undergoing three different muscle contractions, i.e. isotonic shortening (SHO) and lengthening (LEN), and isometric (ISO) contractions. The intermuscle relationships for the magnitude of the stretch reflex component in the eletromyographic (EMG) activities of two main elbow flexor muscles, i.e. the biceps brachii (BB) and the brachioradialis (BRD), were compared among the three types of contractions. The subjects were requested to move their forearms sinusoidally (0.1 Hz) against a constant pre-load between elbow joint angles of 10° (0° = full extension) and 80° during SHO and LEN, and to keep an angle of 45° during the ISO. The perturbations were applied at the elbow angle of 45° in pseudo-random order. The EMG signals were rectified and averaged over a period of 100 ms before and 400 ms after the onset of the perturbation 40–50 times. From the ensemble averaged EMG waveform, the background activity (BGA), short (20–50 ms) and long latency (M2, 50–80, M3, 80–100 ms) reflex and voluntary activity (100–150 ms) components were measured. The results showed that both BGA and reflex EMG activity of the two elbow flexor muscles were markedly decreased during the lengthening contraction compared to the SHO and ISO contractions. Furthermore, the changes of reflex EMG components in the BRD muscle were more pronounced than those in the BB muscle, i.e. the ratios of M2 and M3 magnitudes between BRD and BB (BRD:BB) were significantly reduced during the LEN contractions. These results would suggest that the gain of long latency stretch reflex EMG activities in synergistic muscles might be modulated independently according to the model of muscle contraction. Accepted: 1 September 1997     

Efficiency of light diffraction by cross-striated muscle fibers under stretch and during isometric contraction.

When light is diffracted by a single frog muscle fiber the intensities I kappa of the different orders kappa (kappa = 1,2,3) strongly depend on the angle between the axis of the incident beam and the fiber axis. Maximum intensity is not obtained with perpendicular incidence (omega = 0 degree) but at angles that can be calculated for each order number and sarcomere length using Bragg's formula. In analogy to techniques developed for x-ray structure analysis of mosaic crystals we have rotated the fiber around an axis perpendicular to the fiber axis and to the incident beam axis within an angular range delta omega = +/- 35 degrees and recorded the light intensities I kappa. Diffraction efficiencies defined as E kappa = integral of I kappa d omega were studied as a function of sarcomere length and during isometric contraction. The sarcomere length dependences of the efficiencies E kappa of the first three orders show characteristic trends. E1 increases with fiber stretch, E2 has a minimum at a sarcomere length near 2.8 micrometers, and E3 has a maximum near 2.5 micrometers. These trends as well as the observed efficiency ratios are in fairly good agreement with predictions by the intensity formula developed for x-ray structure analysis. During isometric contraction, the diffraction efficiencies of the fiber decrease, with the decreases becoming greater the higher the order number. These decreases might be caused by a longitudinal displacement of myofibrils of up to 0.4 micrometers. The efficiency of light diffraction strongly depends on the tonicity of the bathing fluid. Hypertonic (3/2 x normal) solution reduces E1 to less than half, hypotonic (2/3 x normal) solution increases E1 to almost twice the value obtained in normal Ringer's solution.     

Muscle inflammatory cells after passive stretches, isometric contractions, and lengthening contractions.

We tested the hypotheses that lengthening contractions, isometric contractions, and passive stretches increase muscle inflammatory cells (neutrophils and macrophages) and that prior conditioning with lengthening contractions, isometric contractions, or passive stretches reduces neutrophils and macrophages after subsequent lengthening contractions. Extensor digitorum longus muscles in anesthetized mice were subjected in situ to lengthening contractions, isometric contractions, or passive stretches. Six hours or 3 days after a protocol of contractions or passive stretches, neutrophils and macrophages were quantified in muscle cross sections. Three days after isometric contractions or passive stretches, neutrophils were elevated (P < 0.05) 3.7- and 5.5-fold, respectively, relative to controls. Both macrophages and neutrophils were increased 51.2- and 7.9-fold, respectively, after lengthening contractions. Prior lengthening contractions, isometric contractions, or passive stretches reduced inflammatory cells after lengthening contractions performed 2 wk later. The major finding of this study was that passive stretches and isometric contractions elevated neutrophils without causing overt signs of injury. Because both passive stretches and isometric contractions elevated neutrophils and afforded some protection from contraction-induced muscle injury, neutrophils and/or the related inflammatory events may contribute to the induction of a protective mechanism.     

Residual force enhancement exceeds the isometric force at optimal sarcomere length for optimized stretch conditions.

Residual force enhancement (FE) following stretch of an activated muscle is a well accepted property of skeletal muscle contraction. However, the mechanism underlying FE remains unknown. A crucial assumption on which some proposed mechanisms are based is the idea that forces in the enhanced state cannot exceed the steady-state isometric force at a sarcomere length associated with optimal myofilament overlap. Although there are a number of studies in which forces in the enhanced state were compared with the corresponding isometric forces on the plateau of the force-length relationship, these studies either did not show enhanced forces above the plateau or, if they did, they lacked measurements of sarcomere lengths confirming the plateau region. Here, we revisited this question by optimizing stretch conditions and measuring the average sarcomere lengths in isolated fibers, and we found that FE exceeded the maximal isometric reference force obtained at the plateau of the force-length relationship consistently (mean+/-SD: 4.8+/-2.1%) and by up to 10%. When subtracting the passive component of FE from the total FE, the enhanced forces remained greater than the isometric plateau force (mean+/-SD: 4.3+/-2.0%). Calcium-induced increases in passive forces, known to be present in single fibers and myofibrils, are too small to account for the FE observed here. We conclude that FE cannot be explained exclusively with a stretch-induced development of sarcomere length nonuniformities, that FE in single fibers may be associated with the recruitment of additional contractile force, and that isometric steady-state forces in the enhanced state are not uniquely determined by sarcomere lengths.     

Transfer characteristics between EMG activity and muscle tension under isometric conditions in man.

The relationship between motor unit activity and a voluntarily produced, sinusoidally modulated isometric tension was evaluated as a function of the modulation frequency. These date are reported in terms of the gain and phase difference of the motor activity (input) and tension (output) relationship, the gain being the logarithmic ratio of the amplitudes of the output and input sinusoids. It was found that an increase in the modulation amplitude of the motor unit activity was required to produce the same amount of modulation of the output tension as the modulation frequency was increased. For example, the modulation amplitude of the activity is about twice as much at 2 Hz as at 0.25 Hz and about 4 times as much at 5 Hz. It was also found that the maximum tension which could be produced voluntarily during brief jerks at 5 Hz was the same as the maximum sustained tension which could be attained. This latter finding emphasizes the importance of recruitment and especially synchronization of motor unit activity to the gradation of output tension.     

Analysis of various conditions in order to measure electromyography of isometric contractions in water and on air

The aim of this study was to verify if there are differences in the amplitude of signals from surface electromyography (EMG) during maximal and submaximal voluntary isometric contraction (MVC and 50% MVC, respectively) under different conditions, in our case, water and air, with and without extra protection (water-resistant tape) on the electrode. The isometric force and muscle activation of the MVC and 50% MVC of the biceps brachial muscle of nine healthy trained men were measured simultaneously, performed in water and on air, with and without protection of the EMG electrode. The multivariate analysis of variance with a post hoc Tukey test was applied to detect significant differences between the levels of muscular force. For the amplitude values of the EMG signal, the Wilcoxon signed rank test was applied to compare all experimental conditions in order to detect a significance of p < 0.05. The values of isometric force were not significantly different among conditions (MVC and 50% MVC). The results showed a significant difference among conditions in the water without extra protection compared to the conditions on air with and without extra protection and in water with extra protection. Reduced EMG amplitude was seen in water without extra protection from 37.04% to 55.81% regarding the other conditions. However, no significant difference was seen among conditions in water with extra protection in relation the conditions on air (with and without extra protection). This study suggest that it is necessary to use a water-resistant tape as an extra protection on the electrode when using EMG underwater, to avoid having a significant decrease in the EMG amplitude underwater and not to suffer interference from the water. There was no significant difference among the recordings of EMG with and without the use of protection on air; therefore, the protection does not influence the recording of EMG amplitude and isometric force on air.     

Phosphate uptake in rat skeletal muscle is reduced during isometric contractions.

During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (P(i)) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless P(i) uptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [(32)P]P(i) uptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. P(i) uptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [(32)P]P(i) uptake rates declined by approximately 25% at 0.5 Hz and 50-60% at 3 and 5 Hz. Uptake rates in white gastrocnemius decreased by 65-75% at all three stimulation frequencies. These reductions in P(i) uptake are not likely confounded by changes in precursor [(32)P]P(i) specific activity in the interstitium. In soleus and red gastrocnemius, declines in P(i) uptake rates were related to energy expenditure over the contraction duration. These data imply that P(i) uptake in skeletal muscle is acutely modulated during contractions and that decreases in P(i) uptake rates, in combination with expected increases in P(i) efflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of P(i) must subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.     

Caffeine increases endurance and attenuates force sensation during submaximal isometric contractions.

Caffeine has known ergogenic effects, some of which have been observed during submaximal isometric contractions. We used 15 subjects in a randomized, double-blind, repeated-measures experiment to determine caffeine's ergogenic effects on neuromuscular variables that would contribute to increased endurance capacity. Subjects performed repeated submaximal (50% maximal voluntary contraction) isometric contractions of the right quadriceps to the limit of endurance (T(lim)) 1 h after oral caffeine administration (6 mg/kg). Time to reach T(lim) increased by 17 +/- 5.25% (P < 0.02) after caffeine administration compared with the placebo trial. The changes in contractile properties, motor unit activation, and M-wave amplitude that occurred as the quadriceps reached T(lim) could not account for the prolonged performance after caffeine ingestion. In a separate experiment with the same subjects, we used a constant-sensation technique to determine whether caffeine influenced force sensation during 100 s of an isometric contraction of the quadriceps. The results of this experiment showed that caffeine reduced force sensation during the first 10-20 s of the contraction. The rapidity of this effect suggests that caffeine exerts its effects neurally. Based on these data, the caffeine-induced increase in T(lim) may have been caused by a willingness to maintain near-maximal activation longer because of alterations in muscle sensory processes.     

Muscle weakness following sustained and rhythmic isometric contractions in man

The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p less than 0.05) change in the time to peak tension (TPT) of the maximal twitch.     

Knee extensor torque and quadriceps femoris EMG during perceptually-guided isometric contractions.

The aim of this study was to examine superficial quadriceps femoris (QF) EMG and torque at perceived voluntary contraction efforts. Thirty subjects (15 males, 15 females) performed 9, 5 s, sub-maximal contractions at prescribed levels of perceived voluntary effort at points 1-9 on an 11-point scale (0-10), in a random order. Surface electromyograms (EMG) of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles, as well as QF peak torque (PT), average torque (AT), and torque coefficient of variation (C.V.), were sampled. The raw EMG signals were full-wave rectified and integrated over the middle three s of each contraction. The sampled EMG signals, and PT and AT at each perceived exertion level were normalized to the average of three maximal voluntary contractions. The normalized EMG and torque values at each perceived exertion level were then compared to equivalent percent values (i.e., 10% at a perceived level of 1). The results demonstrated that at all perceived exertion levels, with the exception of the RF at a level of 2 which was equivalent to 20%, and the VL and RF muscles at a level 1 in which activation was greater than 10%, activation was significantly less than the equivalent percent value at each point on the scale. VM EMG was found to be less than the VL and RF from contraction levels 3-9. PT was shown to be less than the equivalent percent values at contraction levels 6-9. The AT was found to be lower than the expected percent value at perceived effort levels 2-9. Torque C.V. was not found to be different across the range of perceived effort. The major findings of this study suggested that humans over-estimate voluntary QF muscle torque when guided by perceptual sensations. It is also suggested that the produced EMG signals revealed a reliance on the VL muscle for knee extensor torque generation at sub-maximal levels.     

Fatigue of mouse diaphragm muscle in isometric and isotonic contractions

Fatiguabilities of mouse diaphragm muscle in vitro in isometric and isotonic contractions were compared in this study. Isolated mouse diaphragm muscle was stimulated repetitively to induce fatigue during both isometric and isotonic contractions. The supramaximal electrical stimulation used was a train of 100-Hz, 0.5-ms pulses delivered to the muscle every 2 s for 0.5 s. The percentage decrease in isometric tension from beginning to end of the fatiguing process was used as the index of fatigue. The experiments were carried out at different PO2 levels in both normal and zero-glucose Ringer solutions. It was found that fatigue developed more rapidly in isotonic contractions than in isometric ones. Also, the extracellular glucose level demonstrated little effect on the muscle's short-term fatiguability, whereas reductions in the extracellular PO2 exerted a profound effect, especially in the case of isotonic fatigue.     

Mechanomyography and electromyography force relationships during concentric, isometric and eccentric contractions.

The purpose of this study was to investigate systematically if complementary knowledge could be obtained from the recordings of electromyography (EMG) and mechanomyography (MMG) signals. EMG and MMG activities were recorded from the first dorsal interosseous muscle during slow concentric, isometric, and eccentric contraction at 0, 25, 50, 75 and 100% of the maximal voluntary contraction (MVC). The combination of the EMG and MMG recordings during voluntary concentric-isometric-eccentric contraction showed significant different non-linear EMG/force and MMG/force relationships (P<0.001). The EMG root mean square (rms) values increased significantly from 0 to 50% MVC during concentric and isometric contraction and up to 75% MVC during eccentric contraction (P<0.05). The MMG rms values increased significantly from 0 to 50% MVC during concentric contraction (P<0.05). The non-linear relationships depended mainly on the type and the level of contraction together with the angular velocity. Furthermore, the type of contraction, the contraction level, and the angular velocity influenced the electromechanical efficiency evaluated as the MMG to EMG ratio (P<0.05). These results highlight that EMG and MMG provide complementary information about the electrical and mechanical activity of the muscle. Different activation strategies seem to be used during graded isometric and anisometric contraction.     

Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ.

The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.8 +/- 0.2 (SE) average fiber lengths/s; mean maximal isometric tension (Po) was 508 +/- 15 g/g. With a significant decrease of force development during isometric contractions (-27 +/- 4%, P < 0.01, n = 5), Vmax was unchanged. However, with repetitive isotonic contractions at a low load (P/Po = 0.25, n = 5), a significant decrease in Vmax was observed (-21 +/- 2%, P < 0.01), whereas Po was unchanged. Isotonic contractions at an intermediate load (P/Po = 0.5, n = 4) resulted in significant decreases in both Vmax (-26 +/- 6%, P < 0.05) and Po (-12 +/- 2%, P < 0.01). These results show that repeated contractions of canine skeletal muscle produce specific changes in the F-V relationship that are dependent on the type of contractions being performed and indicate that decreases in other contractile properties, such as velocity development and shortening, can occur independently of changes in isometric tension.     

Age-related differences in corticospinal control during functional isometric contractions in left and right hands.

The purpose of the study was to examine age-related differences in electromyographic (EMG) responses to transcranial magnetic stimulation (TMS) during functional isometric contractions in left and right hands. EMG responses were recorded from the first dorsal interosseus muscle following TMS in 10 young (26.6 +/- 1.3 yr) and 10 old (67.6 +/- 2.3 yr) right-handed subjects. Muscle evoked potentials (MEPs) and silent-period durations were obtained in the left and right hands during index finger abduction, a precision grip, a power grip, and a scissor grip, while EMG was held constant at 5% of maximum. For all tasks, MEP area was 30% (P < 0.001) lower in the left hand of old compared with young subjects, whereas there was no age difference in the right hand. The duration of the EMG silent period was 14% (P < 0.001) shorter in old (150.3 +/- 2.9 ms) compared with young (173.9 +/- 3.0 ms) subjects, and the age differences were accentuated in the left hand (19% shorter, P < 0.001). For all subjects, the largest MEP area (10-12% larger) and longest EMG silent period (8-19 ms longer) were observed for the scissor grip compared with the other three tasks, and the largest task-dependent change in these variables was observed in the right hand of older adults. These differences in corticospinal control in the left and right hands of older adults may reflect neural adaptations that occur throughout a lifetime of preferential hand use for skilled (dominant) and unskilled (nondominant) motor tasks.