Evidence of changes in load sharing during isometric elbow flexion with ramped torque

This study aimed to: (1) test the repeatability of Supersonic Shear Imaging measures of muscle shear elastic modulus of four elbow flexor muscles during isometric elbow flexion with ramped torque; (2) determine the relationship between muscle shear elastic modulus and elbow torque for the four elbow flexor muscles, and (3) investigate changes in load sharing between synergist elbow flexor muscles with increases in elbow flexor torque. Ten subjects performed ten isometric elbow flexions consisting of linear torque ramps of 30-s from 0 to 40% of maximal voluntary contraction. The shear elastic modulus of each elbow flexor muscle (biceps brachii long head [BB(LH)], biceps brachii short head [BB(SH)], brachialis [BA], and brachoradialis [BR]) and of triceps brachii long head [TB] was measured twice with individual muscles recorded in separate trials in random order. A good repeatability of the shape of the changes in shear elastic modulus as a function of torque was found for each elbow flexor muscle (r-values: 0.85 to 0.94). Relationships between the shear elastic modulus and torque were best explained by a second order polynomial, except BA where a higher polynomial was required. Statistical analysis showed that BB(SH) and BB(LH) had an initial slow change at low torques followed by an increasing rate of increase in modulus with higher torques. In contrast, the BA shear elastic modulus increased rapidly at low forces, but plateaued at higher forces. These results suggest that changes in load sharing between synergist elbow flexors could partly explain the non-linear EMG-torque relationship classically reported for BB during isometric efforts.     

In vivo muscular force analysis during the isometric flexion on a monkey's elbow

An experimental method has been developed to analyse muscular forces and torques in vivo during isometric flexion of the elbow in small monkeys (Macaca fascicularis). A mini-transducer was built with a view to measure forces in situ, without cutting tendons. The positions of tendon insertions were measured on anatomical parts, then integrated in a series of calculations aiming of deducing the lever arms of forces to be included in a torque equilibrium relationship. Muscle activity of the three main flexors was measured for five angles of isometric flexion between 70 degrees and 110 degrees, 0 degrees corresponding to the full extended forearm. The analysed signals were selected using physiological and biomechanical criteria. Then, results corresponding to force participation, and torque participations, were worked out; they are presented and discussed in the present paper.     

Effects of mechanical assistance on muscle activity and motor performance during isometric elbow flexion

Mechanical assistance on joint movement is generally beneficial; however, its effects on cooperative performance and muscle activity needs to be further explored. This study examined how motor performance and muscle activity are altered when mechanical assistance is provided during isometric force control of ramp-down and hold phases. Thirteen right-handed participants (age: 24.7 ± 1.8 years) performed trajectory tracking tasks. Participants were asked to maintain the reference magnitude of 47 N (REF) during isometric elbow flexion. The force was released to a step-down magnitude of either 75% REF or 50% REF and maintained, with and without mechanical assistance. The ramp-down durations of force release were set to 0.5, 2.5, or 5.0 s. Throughout the experiment, we measured the following: (1) the force output using load cells to compute force variability and overshoot ratio; (2) peak perturbation on the elbow movement using an accelerometer; (3) the surface electromyography (sEMG) from biceps brachii and triceps brachii muscles; and (4) EMG oscillation from the biceps brachii muscle in the bandwidth of 15–45 Hz. Our results indicated that mechanical assistance, which involved greater peak perturbation, demonstrated lower force variability than non-assistance (p < 0.01), while EMG oscillation in the biceps brachii muscle from 15 to 45 Hz was increased (p < 0.05). These findings imply that if assistive force is provided during isometric force control, the central nervous system actively tries to stabilize motor performance by controlling specific motor unit activity in the agonist muscle.     

Measurement of the isometric dorsiflexion and plantar flexion force in the ankle joint]

This article describes an easy to use test equipment for measuring the isometric force in the ankle joints in dorsiflexion and plantar flexion. The combination of the test equipment for measuring the voluntary maximal isometric muscle force in the ankle joint, the surface electromyograms and the motion analysis of the measured leg allow an objective comparison of the strength of the muscular force between the left and right leg. It might be also used as a control setup during rehabilitation after surgical treatment or injuries.     

Maximal voluntary isometric elbow flexion force during unilateral versus bilateral contractions in individuals with chronic stroke

The purpose of this study was to determine whether the phenomenon of bilateral deficit in muscular force production observed in healthy subjects and mildly impaired stroke patients also exists in patients with more chronic and greater levels of stroke impairment. Ten patients with chronic hemiparesis resulting from stroke performed unilateral and bilateral maximal voluntary isometric contractions of the elbow flexors. When the total force produced by both arms was compared, 12% less force was produced in the bilateral compared with unilateral condition (p=0.01). However, studying the effect of task conditions on each arm separately revealed a significant decline in nonparetic (p=0.01) but not paretic elbow flexor force in the bilateral compared with unilateral condition. Results suggest that a significant bilateral force deficit exists in the nonparetic but not the paretic arm in individuals with chronic stroke. Bilateral task conditions do not seem to benefit or impair paretic arm maximal isometric force production in individuals with moderate-severity chronic stroke.     

Muscle oxidative metabolism accelerates with mild acidosis during incremental intermittent isometric plantar flexion exercise

BACKGROUND: It has been thought that intramuscular ADP and phosphocreatine (PCr) concentrations are important regulators of mitochondorial respiration. There is a threshold work rate or metabolic rate for cellular acidosis, and the decrease in muscle PCr is accelerated with drop in pH during incremental exercise. We tested the hypothesis that increase in muscle oxygen consumption (o2mus) is accelerated with rapid decrease in PCr (concomitant increase in ADP) in muscles with drop in pH occurs during incremental plantar flexion exercise. METHODS: Five male subjects performed a repetitive intermittent isometric plantar flexion exercise (6-s contraction/4-s relaxation). Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC), starting at 10% MVC until exhaustion. The measurement site was at the medial head of the gastrocnemius muscle. Changes in muscle PCr, inorganic phosphate (Pi), ADP, and pH were measured by 31P-magnetic resonance spectroscopy. o2mus was determined from the rate of decrease in oxygenated hemoglobin and/or myoglobin using near-infrared continuous wave spectroscopy under transient arterial occlusion. Electromyogram (EMG) was also recorded. Pulmonary oxygen uptake (o2pul ) was measured by the breath-by-breath gas analysis. RESULTS: EMG amplitude increased as exercise intensity progressed. In contrast, muscle PCr, ADP, o2mus, and o2pul did not change appreciably below 40% MVC, whereas above 40% MVC muscle PCr decreased, and ADP, o2mus, and o2pul increased as exercise intensity progressed, and above 70% MVC, changes in muscle PCr, ADP, o2mus, and o2pul accelerated with the decrease in muscle pH (~6.78). The kinetics of muscle PCr, ADP, o2mus, and o2pul were similar, and there was a close correlation between each pair of parameters (r = 0.969~0.983, p < 0.001). CONCLUSION: With decrease in pH muscle oxidative metabolism accelerated and changes in intramuscular PCr and ADP accelerated during incremental intermittent isometric plantar flexion exercise. These results suggest that rapid changes in muscle PCr and/or ADP with mild acidosis stimulate accelerative muscle oxidative metabolism.     

Nonuniform shortening in the biceps brachii during elbow flexion.

This study tested the common assumption that skeletal muscle shortens uniformly in the direction of its fascicles during low-load contraction. Cine phase contrast magnetic resonance imaging was used to characterize shortening of the biceps brachii muscle in 12 subjects during repeated elbow flexion against 5 and 15% maximum voluntary contraction (MVC) loads. Mean shortening was relatively constant along the anterior boundary of the muscle and averaged 21% for both loading conditions. In contrast, mean shortening was nonuniform along the centerline of the muscle during active elbow flexion. Centerline shortening in the distal region of the biceps brachii (7.3% for 5% MVC and 3.7% for 15% MVC) was significantly less (P < 0.001) than shortening in the muscle midportion (26.3% for 5% MVC and 28.2% for 15% MVC). Nonuniform shortening along the centerline was likely due to the presence of an internal aponeurosis that spanned the distal third of the longitudinal axis of the biceps brachii. However, muscle shortening was also nonuniform proximal to the centerline aponeurosis. Because muscle fascicles follow the anterior contour and centerline of the biceps brachii, our results suggest that shortening is uniform along anterior muscle fascicles and nonuniform along centerline fascicles.     

Power produced by maximal velocity elbow flexion

An analysis of horizontal elbow flexion at maximal velocity was made to determine how different loads affected power output. Twenty male subjects operated a specially constructed dynamometer initially performing a maximal effort isometric trial with the elbow fully extended and then three dynamic trials at each of three loads equal to 75, 50, and 25 per cent of the maximal isometric strength. Angular acceleration was used to calculate forearm torque, and power was obtained by taking the product of torque and angular velocity. Power was found to be a cubic function of time and a fourth-order polynomial function of angular displacement reaching a peak early in the movement. The 50 per cent load resulted in a higher peak level of power than either the 25 or 75 per cent loads.     

Effect of hip flexion angle on stiffness of the adductor longus muscle during isometric hip flexion

This study examined the effect of hip flexion angle on the stiffness of the adductor longus (AL) muscle during isometric hip flexion. Seventeen men were recruited. Ten participants performed submaximal voluntary contraction at 0%, 25%, 50%, and 75% of maximal voluntary contraction (MVC) during isometric hip flexion after performing MVC at 0°, 40°, and 80° of hip flexion. Seven participants performed submaximal voluntary tasks during isometric hip extension in addition to hip flexion task. The shear modulus of the AL muscle was used as the index of muscle stiffness, and was measured using ultrasound shear-wave elastography during the tasks at each contraction intensity for each hip flexion angle. During hip flexion, the shear modulus of the AL muscle was higher at 0° than at 40° and 80° of hip flexion at each contraction intensity (p < 0.016). Conversely, a significant effect was not found among hip flexion angle during hip extension at 75% of MVC (p = 0.867). These results suggest that mechanical stress of the AL muscle may be higher at 0° of hip flexion during isometric hip flexion.     

The isometric functional capacity of muscles that cross the elbow

We hypothesized that muscles crossing the elbow have fundamental differences in their capacity for excursion, force generation, and moment generation due to differences in their architecture, moment arm, and the combination of their architecture and moment arm. Muscle fascicle length, sarcomere length, pennation angle, mass, and tendon displacement with elbow flexion were measured for the major elbow muscles in 10 upper extremity specimens. Optimal fascicle length, physiological cross-sectional area (PCSA), moment arm, operating range on the force-length curve, and moment-generating capacity were estimated from these data. Brachioradialis and pronator teres had the longest (17.7cm) and shortest (5.5cm) fascicles, respectively. Triceps brachii (combined heads) and brachioradialis had the greatest (14.9cm(2)) and smallest (1.2cm(2)) PCSAs, respectively. Despite a comparable fascicle length, long head of biceps brachii operates over a broader range of the force-length curve (length change=56% of optimal length, 12.8cm) than the long head of triceps brachii (length change=28% of optimal length, 12. 7cm) because of its larger moment arm (4.7cm vs. 2.3cm). Although brachioradialis has a small PCSA, it has a relatively large moment-generating capacity (6.8cm(3)) due to its large moment arm (average peak=7.7cm). These results emphasize the need to consider the interplay of architecture and moment arm when evaluating the functional capabilities of a muscle.     

Use of the latissimus dorsi muscle to restore elbow flexion in arthrogryposis

The successful use of ipsilateral pedicle latissimus dorsi muscle to restore elbow flexion in a child with arthrogryposis multiplex congenita is described. In appropriately selected patients, use of the latissimus dorsi muscle for elbow flexor reconstruction is a strong, reliable flexorplasty without significant donor-site morbidity.     

Serum creatine kinase activity following forearm flexion isometric exercise

The purpose of this study was to compare serum creatine kinase (CK) activity following two forearm flexion isometric exercise regimens differing in work to rest ratio, and examine the CK response to a repeated bout of isometric exercise. Eleven males were tested on two sessions (bouts) spaced 1 week apart. For bout 1, five subjects (group A) performed a forearm flexion isometric exercise consisting of 40 10-s maximal contractions with 20-s inter-trial rests (10:20), while six (group B) performed 40 maximal 10-s contractions with 5-s inter-trial rests (10:5). The increase in serum CK activity following the 10:20 exercise (143%) was significantly greater than that following the 10:5 exercise (52%). The 10:20 exercise was also associated with greater tension generation over trials. One week later, both groups performed a bout of 10:20 exercise. A substantial reduction in the serum CK response was found following this second bout. The data suggest that for bout 1 the isometric exercise associated with the greater overall tension levels resulted in the greater CK response. However, when the 10:20 exercise was repeated 1 week later, a substantial reduction in the CK response was found which was unrelated to the tension generated.     

Isokinetic elbow flexion and coactivation following eccentric training.

The influence of an eccentric training on torque/angular velocity relationships and coactivation level during maximal voluntary isokinetic elbow flexion was examined. Seventeen subjects divided into two groups (Eccentric Group EG, n = 9 Control Group CG, n = 8) performed on an isokinetic dynamometer, before and after training, maximal isokinetic elbow flexions at eight angular velocities (from - 120 degrees s(-1) under eccentric conditions to 240 degrees s(-1) under concentric conditions), and held maximal and submaximal isometric actions. Under all conditions, the myoelectric activities (EMG) of the biceps and the triceps brachii muscles were recorded and quantified as the RMS value. Eccentric training of the EG consisted of 5x6 eccentric muscle actions at 100 and 120% of one maximal repetition (IRM) for 21 sessions and lasted 7 weeks. In the EG after training, torque was significantly increased at all angular velocities tested (ranging from 11.4% at 30 degrees (s-1) to 45.5% at - 120 degrees s(-1)) (p < 0.05). These changes were accompanied by an increase in the RMS activities of the BB muscle under eccentric conditions (from - 120 to - 30 degrees (s-1)) and at the highest concentric angular velocities (180 and 24 degrees s(-1)) (p < 0.05). The RMS activity of the TB muscle was not affected by the angular velocity in either group for all action modes. The influence of eccentric training on the torque gains under eccentric conditions and for the highest velocities was attributed essentially to neural adaptations.     

Experimental and modelling investigation of learning a fast elbow flexion in the horizontal plane

Changes in the kinematic and electromyographic characteristics that occur while learning to move as fast as possible have been studied experimentally. Experimental investigation of what happens to the individual motor units (MUs) is more difficult. Access to each MU is impossible, and the recruitment and force developing properties of all individual MUs cannot be known. Thus, what is currently known about MU firing is based on experiments that have recorded relatively few MUs compared to what exists in the entire muscle. A recently developed muscle model (Raikova and Aladjov, 2002, J. Biomechanics, 35, 1123-1135) composed of MUs with different properties can be used for such investigation. The process of learning fast elbow flexion in the horizontal plane was simulated and the results were compared with experimentally measured data. Comparing the simulation results of the very first trial of a particular subject with those of the last trail (at the end of the learning process), it can be concluded that the speed of limb motion and muscle forces increase initially as a result of the more synchronous MUs activation and the increase of firing rate of active MUs. Further improvement necessitated an appreciable reduction in the motor task requirements (i.e. less muscle force and less MUs' activity) set in the computational algorithm by optimization criteria. This forced the next process-inclusion of additional MUs.     

Differential angle-dependent modulation of the long-latency stretch reflex responses in elbow flexion synergists.

We determined the effect of elbow joint angle on the short-(M1) and long-latency stretch reflex (M2 and M3) responses of the elbow flexion synergists, the brachioradialis (BR), and the biceps brachii (BB), during weak isometric elbow flexion tasks. The elbow joint angle was 35,75 and 115 degrees (full-extension angle was 0 degrees ), and the muscle contraction level was 0,3 and 6% of maximum voluntary contraction (MVC) of the BR. In BR, the M1, M2 and M3 responses were significantly greater at 75 and 115 degrees than at 35 degrees. On the other hand, in BB, the M2 response was significantly greater at 35 degrees than at 75 and 115 degrees, while the M1 and M3 responses were not significantly different among the elbow joint angles. These results indicated that the stretch reflex responses of BR might be dependent on the changes of muscle length in stretch stimulus, while the M2 response of BB might not be dependent on the actual stimulus intensity. Therefore, we concluded that the M2 of BB might be modulated selectively by a higher reflex center in accordance with relationships of the mechanical advantages between synergistic muscles.     

Muscle ATP turnover rate during isometric contraction in humans

ATP turnover and glycolytic rates during isometric contraction in humans have been investigated. Subjects contracted the knee extensor muscles at two-thirds maximal voluntary force to fatigue (mean +/- SE, 53 +/- 4 s). Biopsies were obtained before and after exercise and analyzed for high-energy phosphates and glycogenolytic-glycolytic intermediates. Total ATP turnover was 190 +/- 7 mmol/kg dry muscle, whereas the average turnover rate was 3.7 +/- 0.2 mmol . kg dry muscle-1 . S-1. The average ATP turnover rate was positively correlated with the percentage of fast-twitch fibers in the postexercise biopsy (r = 0.71; P less than 0.05) and negatively correlated with contraction duration to fatigue (r = -0.88; P less than 0.05). At fatigue, phosphocreatine ranged from 1 to 11 mmol/kg dry muscle (86-99% depletion of value at rest), whereas lactate ranged from 59 to 101. The mean glycolytic rate was 0.83 +/- 0.05 mmol . kg dry muscle-1 . S-1 and was positively correlated with the rate of glucose 6-phosphate accumulation (r = 0.83; P less than 0.05). It is concluded that a major determinant of the ATP turnover rate is the muscle fiber composition, which is probably explained by a higher turnover rate in fast-twitch fibers; fatigue is more closely related to a low phosphocreatine content than to a high lactate content; and the increase in prephosphofructokinase intermediates is important for stimulating glycolysis during contraction.