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     

Impaired neuromuscular function during isometric, shortening, and lengthening contractions after exercise-induced damage to elbow flexor muscles

The purpose of this study was to examine the effect of exercise-induced damage of the elbow flexor muscles on steady motor performance during isometric, shortening, and lengthening contractions. Ten healthy individuals (age 22+/-4 yr) performed four tasks with the elbow flexor muscles: a maximum voluntary contraction, a one repetition maximum (1 RM), an isometric task at three joint angles (short, intermediate, and long muscle lengths), and a constant-load task during slow (approximately 7 degrees/s) shortening and lengthening contractions. Task performance was quantified as the fluctuations in wrist acceleration (steadiness), and electromyography was obtained from the biceps and triceps brachii muscles at loads of 10, 20, and 40% of 1 RM. Tasks were performed before, immediately after, and 24 h after eccentric exercise that resulted in indicators of muscle damage. Maximum voluntary contraction force and 1-RM load declined by approximately 45% immediately after exercise and remained lower at 24 h ( approximately 30% decrease). Eccentric exercise resulted in reduced steadiness and increased biceps and triceps brachii electromyography for all tasks. For the isometric task, steadiness was impaired at the short compared with the long muscle length immediately after exercise (P<0.01). Furthermore, despite no differences before exercise, there was reduced steadiness for the shortening compared with the lengthening contractions after exercise (P=0.01), and steadiness remained impaired for shortening contractions 24 h later (P=0.01). These findings suggest that there are profound effects for the performance of these types of fine motor tasks when recovering from a bout of eccentric exercise.     

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.     

Behavior of human muscle fascicles during shortening and lengthening contractions in vivo.

The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions at five angular velocities. Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography. During maximal isometric contraction, fascicle length was shorter and pennation angle larger compared with values at rest (P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad/s, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whereas pennation angle decreased curvilinearly from 14.8 to 9.8 degrees (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening.     

Ia-afferent input to motoneurons during shortening and lengthening muscle contractions in humans.

The central nervous system employs different strategies to execute specific motor tasks. Because afferent feedback during shortening and lengthening muscle contractions differs, the neural strategy underlying these tasks may be quite distinct. Cortical drive may be adjusted or afferent input regulated. The exact mechanisms are not clear. Here, we examine the control of synaptic transmission across the Ia synapse during shortening and lengthening muscle contractions. Subjects were instructed to maintain isolated activity in a single tibialis anterior (TA) motor unit while muscle length was varied from flexion to extension and back. At a fixed interval after a firing of the active motor unit, a single electrical stimulus was applied to the common peroneal nerve to activate Ia afferents from the TA muscle. We investigated the stimulus-induced change in firing probability of 19 individual low-threshold TA motor units during shortening and lengthening contractions. Any change in firing probability depends on both pre- and postsynaptic mechanisms. In this experiment, motoneuron firing rate was similar during both contraction types. There was no difference in the firing probability between shortening and lengthening contractions (0.23 +/- 0.03 and 0.20 +/- 0.02, respectively). We suggest that there is no contraction type-specific control of Ia input to the motoneurons during shortening and lengthening muscle contractions. Cortical adjustments may have occurred.     

Electromyography of superficial and deep neck muscles during isometric, voluntary, and reflex contractions

Increasingly complex models of the neck neuromusculature need detailed muscle and kinematic data for proper validation. The goal of this study was to measure the electromyographic activity of superficial and deep neck muscles during tasks involving isometric, voluntary, and reflexively evoked contractions of the neck muscles. Three male subjects (28-41 years) had electromyographic (EMG) fine wires inserted into the left sternocleidomastoid, levator scapulae, trapezius, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles. Surface electrodes were placed over the left sternohyoid muscle. Subjects then performed: (i) maximal voluntary contractions (MVCs) in the eight directions (45 deg intervals) from the neutral posture; (ii) 50 N isometric contractions with a slow sweep of the force direction through 720 deg; (iii) voluntary oscillatory head movements in flexion and extension; and (iv) initially relaxed reflex muscle activations to a forward acceleration while seated on a sled. Isometric contractions were performed against an overhead load cell and movement dynamics were measured using six-axis accelerometry on the head and torso. In all three subjects, the two anterior neck muscles had similar preferred activation directions and acted synergistically in both dynamic tasks. With the exception of splenius capitis, the posterior and posterolateral neck muscles also showed consistent activation directions and acted synergistically during the voluntary motions, but not during the sled perturbations. These findings suggest that the common numerical-modeling assumption that all anterior muscles act synergistically as flexors is reasonable, but that the related assumption that all posterior muscles act synergistically as extensors is not. Despite the small number of subjects, the data presented here can be used to inform and validate a neck model at three levels of increasing neuromuscular-kinematic complexity: muscles generating forces with no movement, muscles generating forces and causing movement, and muscles generating forces in response to induced movement. These increasingly complex data sets will allow researchers to incrementally tune their neck models' muscle geometry, physiology, and feedforward/feedback neuromechanics.     

Fatigue responses of human triceps surae muscles during repetitive maximal isometric contractions.

Nine healthy men (22-45 yr) completed 100 repetitive maximal isometric contractions of the ankle plantar flexor muscles in two knee positions of full extension (K0) and flexion at 90 degrees (K90), positions that varied the contribution of the gastrocnemii. Electromyographic activity was recorded from the medial and lateral gastrocnemii and soleus muscles by using surface electrodes. Plantar flexion torque in K0 was greater and decreased more rapidly than in K90. The electromyographic amplitude decreased over time, and there were no significant differences between muscles and knee joint positions. The level of voluntary effort, assessed by a supramaximal electrical stimulation during every 10th contraction, decreased from 96 to 70% (P < 0.05) with no difference between K0 and K90. It was suggested that a decrease in plantar flexion torque was attributable to both central and peripheral fatigue and that greater fatigability in K0 than in K90 would result from a greater contribution and hence more pronounced fatigue of the gastrocnemius muscle. Further support for this possibility was provided from changes in twitch torque.     

Skeletal muscle hypertrophy in response to isometric, lengthening, and shortening training bouts of equivalent duration.

Movements generated by muscle contraction generally include periods of muscle shortening and lengthening as well as force development in the absence of external length changes (isometric). However, in the specific case of resistance exercise training, exercises are often intentionally designed to emphasize one of these modes. The purpose of the present study was to objectively evaluate the relative effectiveness of each training mode for inducing compensatory hypertrophy. With the use of a rat model with electrically stimulated (sciatic nerve) contractions, groups of rats completed 10 training sessions in 20 days. Within each training session, the duration of the stimulation was equal across the three modes. Although this protocol provided equivalent durations of duty cycle, the torque integral for the individual contractions varied markedly with training mode such that lengthening > isometric > shortening. The results indicate that the hypertrophy response did not track the torque integral with mass increases of isometric by 14%, shortening by 12%, and lengthening by 11%. All three modes of training resulted in similar increases in total muscle DNA and RNA. Isometric and shortening but not lengthening mode training resulted in increased muscle insulin-like growth factor I mRNA levels. These results indicate that relatively pure movement mode exercises result in similar levels of compensatory hypertrophy that do not necessarily track with the total amount of force generated during each contraction.     

Muscle architecture and fibre characteristics of rat gastrocnemius and semimembranosus muscles during isometric contractions

Rat gastrocnemius medialis (GM) and semimembranosus (SM) muscles have a very different morphology. GM is a very pennate muscle, combining relatively short muscle fibre length with sizable fibre angles and long muscle and aponeurosis lengths. SM is a more parallel-fibred muscle, combining a relatively long fibre length with a small fibre angle and short aponeurosis length. The mechanisms of fibre shortening as well as angle increase are operational in GM as well as SM. However, as a consequence of isometric contraction, changes of fibre length and angle are greater for GM than for SM at any relative muscle length. These differences are particularly notable at short muscle lengths: at 80% of optimum muscle length, fibre length changes of approximately 30% are coupled to fibre angle changes of 15 degrees in GM, while for SM these changes are 4% and 0.6 degrees, respectively. A considerable difference was found for normalized active slack muscle length (GM approximately 80 and SM approximately 45%). This is explained by differences of degree of pennation as well as factors related to differences found for estimated fibre length-force characteristics. Estimated normalized active fibre slack length was considerably smaller for SM than for GM (approximately 40 and 60%, respectively). The most likely explanation of these findings are differences of distribution of optimum fibre lengths, possibly in combination with differences of myofilament lengths and/or fibre length distributions.     

Older adults are less steady during submaximal isometric contractions with the knee extensor muscles.

This study compared the steadiness of submaximal contractions with the knee extensor muscles in young and old adults. Twenty young and twenty old subjects underwent assessment of isometric maximum voluntary contraction (MVC), one-repetition maximum (1-RM) strength, and steadiness during isometric, concentric, and eccentric contractions with the knee extensor muscles. The old adults displayed 33% lower MVC force and a 41% lower 1-RM load. The coefficient of variation for force was significantly greater for the old adults during isometric contractions at 2, 5, and 10% of MVC but not at 50% MVC. The decline in steadiness at low forces experienced by the men was marginally greater than that experienced by the women. The steadiness of concentric and eccentric contractions was similar in young and old adults at 5, 10, and 50% of 1-RM load. Old subjects exhibited greater coactivation of an antagonist muscle compared with young subjects during the submaximal isometric and anisometric contractions. These results indicate that, whereas the ability to exert steady submaximal forces with the knee extensor muscles was reduced in old adults, fluctuations in knee joint angle during slow movements were similar for young and old adults.     

Repeatability of corticospinal and spinal measures during lengthening and shortening contractions in the human tibialis anterior muscle

Elements of the human central nervous system (CNS) constantly oscillate. In addition, there are also methodological factors and changes in muscle mechanics during dynamic muscle contractions that threaten the stability and consistency of transcranial magnetic stimulation (TMS) and perpherial nerve stimulation (PNS) measures.

Purpose

To determine the repeatability of TMS and PNS measures during lengthening and shortening muscle actions in the intact human tibialis anterior.

Methods

On three consecutive days, 20 males performed lengthening and shortening muscle actions at 15, 25, 50 and 80% of maximal voluntary contraction (MVC). The amplitude of the Motor Evoked Potentials (MEPs) produced by TMS was measured at rest and during muscle contraction at 90° of ankle joint position. MEPs were normalised to Mmax determined with PNS. The corticospinal silent period was recorded at 80% MVC. Hoffman reflex (H-reflex) at 10% isometric and 25% shortening and lengthening MVCs, and V-waves during MVCs were also evoked on each of the three days.

Results

With the exception of MEPs evoked at 80% shortening MVC, all TMS-derived measures showed good reliability (ICC = 0.81–0.94) from days 2 to 3. Confidence intervals (CI, 95%) were lower between days 2 and 3 when compared to days 1 and 2. MEPs significantly increased at rest from days 1 to 2 (P = 0.016) and days 1 to 3 (P = 0.046). The H-reflex during dynamic muscle contraction was reliable across the three days (ICC = 0.76–0.84). V-waves (shortening, ICC = 0.77, lengthening ICC = 0.54) and the H-reflex at 10% isometric MVC (ICC = 0.66) was generally less reliable over the three days.

Conclusion

Although it is well known that measures of the intact human CNS exhibit moment-to-moment fluctuations, careful experimental arrangements make it possible to obtain consistent and repeatable measurements of corticospinal and spinal excitability in the actively lengthening and shortening human TA muscle.     

Similar acute molecular responses to equivalent volumes of isometric, lengthening, or shortening mode resistance exercise.

The present study was undertaken to test the hypothesis that the contraction mode of action [static-isometric (Iso), shortening-concentric (Con), or lengthening-eccentric (Ecc)] used to stress the muscle provides a differential mechanical stimulus eliciting greater or lesser degrees of anabolic response at the initiation of a resistance training program. We performed an acute resistance training study in which different groups of rodents completed four training sessions in either the Iso, Con, or Ecc mode of contraction under conditions of activation and movement specifically designed to elicit equivalent volumes of force accumulation. The results of this experiment indicate that the three modes of contraction produced nearly identical cell signaling, indicative of an anabolic response involving factors such as increased levels of mRNA for IGF-I, procollagen III alpha1, decreased myostatin mRNA, and increased total RNA concentration. The resulting profiles collectively provide evidence that pure mode of muscle action, in and of itself, does not appear to be a primary variable in determining the efficacy of increased loading paradigms with regard to the initiation of selected muscle anabolic responses.     

Sex differences in the fatigability of arm muscles depends on absolute force during isometric contractions.

Women are capable of longer endurance times compared with men for contractions performed at low to moderate intensities. The purpose of the study was 1) to determine the relation between the absolute target force and endurance time for a submaximal isometric contraction and 2) to compare the pressor response and muscle activation patterns of men [26.3 +/- 1.1 (SE) yr] and women (27.5 +/- 2.3 yr) during a fatiguing contraction performed with the elbow flexor muscles. Maximal voluntary contraction (MVC) force was greater for men (393 +/- 23 vs. 177 +/- 7 N), which meant that the average target force (20% of MVC) was greater for men (79.7 +/- 6.5 vs. 36.7 +/- 2.0 N). The endurance time for the fatiguing contractions was 118% longer for women (1,806 +/- 239 vs. 829 +/- 94 s). The average of the rectified electromyogram (%MVC) for the elbow flexor muscles at exhaustion was similar for men (31 +/- 2%) and women (30 +/- 2%). In contrast, the heart rate and mean arterial pressure (MAP) were less at exhaustion for women (94 +/- 6 vs. 111 +/- 7 beats/min and 121 +/- 5 vs. 150 +/- 6 mmHg, respectively). The target force and change in MAP during the fatiguing contraction were exponentially related to endurance time (r(2) = 0.68 and r(2) = 0.64, respectively), whereas the change in MAP was linearly related to target force (r(2) = 0.51). The difference in fatigability of men and women when performing a submaximal contraction was related to the absolute contraction intensity and was limited by mechanisms that were distal to the activation of muscle.     

Normalization procedures using maximum voluntary isometric contractions for the serratus anterior and trapezius muscles during surface EMG analysis.

The serratus anterior and trapezius muscles are considered to be the only upward rotators of the scapula and are very important for normal shoulder function. A variety of methods have been used to produce a maximum voluntary isometric contraction (MVIC) of these muscles for normalization of EMG data. The purpose of this study was to quantify the surface EMG activity of the serratus anterior muscle and the upper, middle, and lower parts of the trapezius during 9 manual muscle tests performed with maximum effort in 30 subjects. It was found that no one muscle test produced a MVIC for all individuals. Therefore, to perform normalization within each subject, it is suggested that the 2 or 3 tests identified in this study that produce high levels of EMG activity for each muscle be performed. The scapular protraction muscle test that is often used to normalize data for the serratus anterior muscle produced relatively low levels of EMG activity and was not found to be an optimal test. Muscle tests in which an attempt was made to de-rotate the scapula from an upwardly rotated position produced much higher levels of EMG activity in the serratus anterior muscle.     

Comparison of fascicle behaviors between superficial and deeper muscles of triceps brachii during isometric contractions

PurposeWe assessed fascicle behaviors of the upper extremities during isometric contractions at different joint angles in this study.MethodsThirteen healthy men and women performed isometric elbow extension tasks at 50% and 75% of maximal voluntary contraction (MVC) at 60°, 90°, and 120° of elbow extension (full extension = 180°). Extended field-of-view B-mode ultrasonography was used to obtain sagittal plane panoramic images of the long head (TB-Long) and medial head (TB-Med) of the triceps brachii at rest and during contraction; fascicle length and pennation angle were measured.ResultsIn the TB-Long, significant fascicle shortening from rest was found during 50% and 75%MVC at 60° and during 75%MVC at 90° of extension. There was no significant fascicle shortening in the TB-Med muscle under any conditions. There was no significant pennation angle change from rest in either muscle. The pennation angle of the TB-Long was significantly greater than that of the TB-Med under all conditions.ConclusionsThese results suggest that fascicle shortening in the TB-Long muscle occurs in flexion; however, no change was found in the TB-Med. In the upper extremity muscle–tendon complex, the superficial and deeper muscles may have different force-transmission efficiency at flexed joint angles.     

Discharge properties of motor units during steady isometric contractions performed with the dorsiflexor muscles

The purpose of this study was to record the discharge characteristics of tibialis anterior motor units over a range of target forces and to import these data, along with previously reported observations, into a computational model to compare experimental and simulated measures of torque variability during isometric contractions with the dorsiflexor muscles. The discharge characteristics of 44 motor units were quantified during brief isometric contractions at torques that ranged from recruitment threshold to an average of 22 ± 14.4% maximal voluntary contraction (MVC) torque above recruitment threshold. The minimal [range: 5.8-19.8 pulses per second (pps)] and peak (range: 8.6-37.5 pps) discharge rates of motor units were positively related to the recruitment threshold torque (R(2) ≥ 0.266; P < 0.001). The coefficient of variation for interspike interval at recruitment was positively associated with recruitment threshold torque (R(2) = 0.443; P < 0.001) and either decreased exponentially or remained constant as target torque increased above recruitment threshold torque. The variability in the simulated torque did not differ from the experimental values once the recruitment range was set to ～85% MVC torque, and the association between motor twitch contraction times and peak twitch torque was defined as a weak linear association (R(2) = 0.096; P < 0.001). These results indicate that the steadiness of isometric contractions performed with the dorsiflexor muscle depended more on the distributions of mechanical properties than discharge properties across the population of motor units in the tibialis anterior.     

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.     

Reliability of EMG measurements for trunk muscles during maximal and sub-maximal voluntary isometric contractions in healthy controls and CLBP patients.

The purpose of this study was to compare the reliability of trunk muscle activity measured by means of surface electromyography (EMG) during maximal and sub-maximal voluntary isometric contractions (MVC/sub-MVC) over repeated trials within-day and between-days in healthy controls and patients with chronic low back pain (CLBP). Eleven volunteers (six controls and five CLBP patients) were assessed twice with a 1-week interval. Surface EMG signals were recorded bilaterally from six trunk muscles. Intra-class correlation coefficients (ICC) and standard error of measurement as a percentage of the grand mean (%SEM) were calculated. MVC and sub-MVC showed excellent within-day reliability in both healthy controls and CLBP patients (ICC mean 0.91; range 0.75-0.98; %SEM mean 4%; range 1-12%). Sub-MVC for both groups between-days showed excellent reliability (ICC mean 0.88; range 0.78-0.97; %SEM mean 7%; range 3-11%). The between-days MVC for both groups showed trends towards lower levels of reliability (ICC mean 0.70; range 0.19-0.99; %SEM mean 17%; range 4-36%) when compared to sub-MVC. Findings of the study provide evidence that sub-MVC are preferable for amplitude normalisation when assessing EMG signals of trunk muscles between-days.