Measuring changes in muscle stiffness after eccentric exercise using elastography

Muscle stiffness has been reported to increase following eccentric muscle exercise, but to date only indirect methods have been used to measure it. This study aimed to use Magnetic Resonance Elastography (MRE), a noninvasive imaging technique, to assess the time‐course of passive elasticity changes in the medial gastrocnemius and soleus muscles before and after a bout of eccentric exercise. Shear storage modulus (G′) and loss modulus (G′′) measurements were made in eight healthy subjects for both muscles in vivo before, one hour after, 48 hours after and 1 week after eccentric exercise. The results show a 21% increase in medial gastrocnemius storage modulus following eccentric exercise with a peak occurring ~48 hours after exercise (before exercise 1.15 ± 0.23 kPa, 48 hours after 1.38 ± 0.27 kPa). No significant changes in soleus muscle storage modulus were measured for the exercise protocol used in this study, and no significant changes in loss modulus were observed. This study provides the first direct measurements in skeletal muscle before and after eccentric exercise damage and suggests that MRE can be used to detect the time course of changes to muscle properties. Copyright © 2012 John Wiley & Sons, Ltd.     

Magnetic resonance elastography of skeletal muscle deep tissue injury

The current state‐of‐the‐art diagnosis method for deep tissue injury in muscle, a subcategory of pressure ulcers, is palpation. It is recognized that deep tissue injury is frequently preceded by altered biomechanical properties. A quantitative understanding of the changes in biomechanical properties preceding and during deep tissue injury development is therefore highly desired. In this paper we quantified the spatial–temporal changes in mechanical properties upon damage development and recovery in a rat model of deep tissue injury. Deep tissue injury was induced in nine rats by two hours of sustained deformation of the tibialis anterior muscle. Magnetic resonance elastography (MRE), T₂‐weighted, and T₂‐mapping measurements were performed before, directly after indentation, and at several timepoints during a 14‐day follow‐up. The results revealed a local hotspot of elevated shear modulus (from 3.30 ± 0.14 kPa before to 4.22 ± 0.90 kPa after) near the center of deformation at Day 0, whereas the T₂ was elevated in a larger area. During recovery there was a clear difference in the time course of the shear modulus and T₂. Whereas T₂ showed a gradual normalization towards baseline, the shear modulus dropped below baseline from Day 3 up to Day 10 (from 3.29 ± 0.07 kPa before to 2.68 ± 0.23 kPa at Day 10, P < 0.001), followed by a normalization at Day 14. In conclusion, we found an initial increase in shear modulus directly after two hours of damage‐inducing deformation, which was followed by decreased shear modulus from Day 3 up to Day 10, and subsequent normalization. The lower shear modulus originates from the moderate to severe degeneration of the muscle. MRE stiffness values were affected in a smaller area as compared with T₂. Since T₂ elevation is related to edema, distributing along the muscle fibers proximally and distally from the injury, we suggest that MRE is more specific than T₂ for localization of the actual damaged area.     

Skeletal muscle buffering capacity is higher in the superficial vastus than in the soleus of spontaneously running rats

Skeletal muscle buffering capacity (βm_titr) was determined in soleus (type I) and superficial vastus (type II) muscles of 16 Long–Evans rats with differing levels of spontaneous activity and in 11 sedentary control rats. βm_titr was 24% higher (P<0.001) in superficial vastus muscle than in soleus muscle (268±50 vs. 216±30 μmol H⁺ g muscle dry wt^-1 pH unit^-1) (mean±SD). There was no relationship between βm_titr and mean weekly running distance amongst spontaneously running rats, nor was βm_titr any greater in these rats than in a group of sedentary control rats. Protein to wet wt ratio was 31% higher (P<0.0001) in the superficial vastus muscle when compared with soleus muscle (22.04±3.74 vs. 16.77±3.00 mg protein, 100 mg wet wt muscle^-1), but there was no relationship between protein to wet wt ratio and running distance. Initial muscle homogenate pH (pH_i) was lower in superficial vastus muscle compared with soleus muscle (6.36±0.25 vs. 6.63±0.16). Running rats had a significantly lower pH_i in both soleus and superficial vastus than sedentary controls. There was an exponential relationship between weekly running distance and pH_i in both the superficial vastus muscle (r=-0.86, P<0.001) and the soleus muscle (r=-0.73, P<0.01). Citrate synthase activity correlated with weekly running distance in superficial vastus muscle (r=0.66, P<0.01) but not in soleus muscle. The results confirm a higher βm_titr in the type II superficial vastus muscle when compared with the predominantly type I soleus muscle. We suggest that this may be partly the result of a higher protein concentration in type II muscle. Future studies measuring βm_titr in mixed muscle (e.g. human vastus lateralis) should report fibre type composition.     

Long‐term regeneration of fast and slow murine skeletal muscles after induced injury by ACL myotoxin isolated from Agkistrodon contortrix laticinctus (broad‐banded copperhead) venom

The aim of the present work was to analyze the regenerated muscle types I and II fibers of the soleus and gastrocnemius muscles of mice, 8 months after damage induced by ACL myotoxin (ACLMT). Animals received 5 mg/kg of ACLMT into the subcutaneous lateral region of the right hind limb, near the Achilles tendon; contralateral muscles received saline. Longitudinal and cross sections (10 μm) of frozen muscle tissue were evaluated. Eight months after ACLMT injection, both muscle types I and II fibers of soleus and gastrocnemius muscles still showed centralized nuclei and small regenerated fibers. Compared with the left muscle, the incidence of type I fibers increased in the right muscle (21% ± 03% versus 12% ± 06%, P = 0.009), whereas type II fibers decreased (78% ± 02% versus 88% ± 06%, P = 0.01). The incidence of type IIC fibers was normal. These results confirm that ACLMT induced muscle type fiber transformation from type II to type I, through type IIC. The area analysis of types I and II fibers of the gastrocnemius revealed that injured right muscles have a higher percentage of small fibers in both types I and II fibers (0–1,500 μm²) than left muscles, which have larger normal type I and II fibers (1,500–3,500 μm²). These results indicate that ACLMT can be used as an excellent model to study the rearrangement of motor units and the transformation of muscle fiber types during regeneration. Anat Rec 254:521–533, 1999. © 1999 Wiley‐Liss, Inc.     

Metabolite accumulation increases adenine nucleotide degradation and decreases glycogenolysis in ischaemic rat skeletal muscle

Adenine nucleotides and glycogen are degraded in skeletal muscle during no-flow ischaemia. Past investigations have ascribed these metabolic changes to the severe energetic stress which arises with the removal of exogenous substrates (principally oxygen). We tested this hypothesis by measuring the high-energy phosphagen and glycogen contents of stimulated rat hindlimb muscles (1 twitch s^?1) prior to and following 40 min of no-flow ischaemia or hypoxic perfusion without glucose (P_ao₂ = 4.6 ± 0.1 torr, plasma glucose = 0.3 ± 0.1 mmol L^?1). Both experimental protocols eliminated exogenous substrate supply; however, the maintenance of flow during hypoxic perfusion ensured the removal of metabolic by-products. A period of forty minutes of skeletal muscle ischaemia was characterized by reductions in the total adenine nucleotide pool, phosphocreatine and glycogen in the slow oxidative soleus, fast oxidative-glycolytic plantaris and the fast glycolytic white gastrocnemius. Compared to ischaemia, the total adenine nucleotide pool was higher (by 7.2–13.3 μmol g^?1 dry wt) and the glycogen content lower (by 10.0–16.6 μmol g^?1 dry wt) in skeletal muscle exposed to hypoxic perfusion without glucose. The ability of hypoxic perfusion to attenuate TAN degradation and augment glycogenolysis can be attributed to metabolic by-product removal. By limiting muscle lactate and PCO ₂ accumulation, hypoxic perfusion without glucose attenuates cellular acidification; this could in turn limit AMP deaminase activation and glycogen phosphorylase inhibition. We conclude that the ischaemia-induced alterations in adenine nucleotide and glycogen metabolism arise in response to the elimination of exogenous substrates and to the accumulation of metabolic by-products.     

Stiffness mapping of lower leg muscles during passive dorsiflexion

It is challenging to differentiate the mechanical properties of synergist muscles in vivo. Shear wave elastography can be used to quantify the shear modulus (i.e. an index of stiffness) of a specific muscle. This study assessed the passive behavior of lower leg muscles during passive dorsiflexion performed with the knee fully extended (experiment 1, n = 22) or with the knee flexed at 90° (experiment 2, n = 20). The shear modulus measurements were repeated twice during experiment 1 to assess the inter‐day reliability. During both experiments, the shear modulus of the following plantar flexors was randomly measured: gastrocnemii medialis (GM) and lateralis (GL), soleus (SOL), peroneus longus (PL), and the deep muscles flexor digitorum longus (FDL), flexor hallucis longus (FHL), tibialis posterior (TP). Two antagonist muscles tibialis anterior (TA), and extensor digitorum longus (EDL) were also recorded. Measurements were performed in different proximo‐distal regions for GM, GL and SOL. Inter‐day reliability was adequate for all muscles (coefficient of variation < 15%), except for TP. In experiment 1, GM exhibited the highest shear modulus at 80% of the maximal range of motion (128.5 ± 27.3 kPa) and was followed by GL (67.1 ± 24.1 kPa). In experiment 2, SOL exhibited the highest shear modulus (55.1 ± 18.0 kPa). The highest values of shear modulus were found for the distal locations of both the GM (80% of participants in experiment 1) and the SOL (100% of participants in experiment 2). For both experiments, deep muscles and PL exhibited low levels of stiffness during the stretch in young asymptomatic adults, which was unknown until now. These results provide a deeper understanding of passive mechanical properties and the distribution of stiffness between and within the plantar flexor muscles during stretching between them and thus could be relevant to study the effects of aging, disease progression, and rehabilitation on stiffness.     

Serotonin inhibition of 1-methylxanthine metabolism parallels its vasoconstrictor activity and inhibition of oxygen uptake in perfused rat hindlimb

The effect of serotonin (5-HT) on the metabolism of infused 1-methylxanthine (1-MX), a putative substrate of capillary endothelial xanthine oxidase (XO), and on the distribution of infused fluorescent microspheres (15 μm) by the artificially constant-flow perfused rat hindlimb preparation was investigated. 1-MX (5–100 μM ) caused a slight inhibition of oxygen uptake (V˙O ₂) but was not vasoactive, either alone or with 5-HT. 1-MX was converted to 1-methylurate (1-MU) and this conversion was inhibited by allopurinol and xanthine. 5-HT (0.35 μM ), which caused vasoconstriction and decreased V˙O ₂, also inhibited the conversion of 1-MX, indicated by a lowered venous perfusate steady-state 1-MU:1-MX ratio from 1.14 ± 0.02 to 0.71 ± 0.02 (P < 0.001), which is equivalent to the rate of conversion decreasing from 0.83 ± 0.03 to 0.63 ± 0.05 nmol min^?1 g^?1. This change closely followed the time course for changes in V˙O ₂ and perfusion pressure and all three changes reversed in parallel when 5-HT was removed. Recoveries of 1-MU plus 1-MX at all times were high (100 ± 5%). 5-HT did not act to inhibit XO. When compared with vehicle alone, 5-HT had either no effect (plantaris, gastrocnemius white, tibialis, extensor digitorum longus, vastus and thigh), or increased microsphere content (soleus and gastrocnemius red, P < 0.05) of muscles with only bone showing a significant decrease (P < 0.05). Since 5-HT did not inhibit XO or alter the net flow to individual muscles in this constant-flow model, the inhibition of conversion of 1-MX to 1-MU is concluded to be the result of a 5-HT-mediated decrease in the access of 1-MX to capillary XO within individual muscles. Possibilities include the redirection of flow to capillaries either in muscle or in connective tissue closely associated with muscle, where resistance is low and effective surface area is less. 1-MX has potential as a marker for muscle nutritive flow.     

Electrically evoked torque-velocity characteristics and isomyosin composition of the triceps surae in young and elderly men

The electrically evoked isokinetic torque-velocity relationship of the triceps surae of eight elderly and four non-trained young men was examined in relation to the isomyosin composition of the soleus and the gastrocnemius muscles, determined under non-denaturing conditions using pyrophosphate gel electrophoresis. The angle specific torque-velocity properties of the triceps surae were measured using maximal percutaneous electrical stimulation at 50 Hz and a release technique. The elderly subjects generated significantly (P < 0.05) less absolute torque at all angular velocities when compared with the young subjects. When the isokinetic data were normalized to the isometric torque, the lower normalized torques generated by the elderly subjects were not statistically different from the young. The total fast isomyosin (FM) content of the soleus and gastrocnemius in the elderly subjects was 22 ± 13 and 35 ± 18%, respectively. This compared with 29 ± 8 (n.s.) and 44 ± 8% (n.s.) in the young subjects. When the gastrocnemius and soleus muscles were given an equal weighting and considered together to represent the whole triceps surae, the normalized torque at the fixed angular velocity of 5 rad s^-1 was significantly associated with%FM (r = 0.90, P < 0.01), and the isomyosin bands%FM1 (r = 0.90, P < 0.01) and%FM2 (r = 0.93, P < 0.001) when only the elderly subjects were considered. No relationships were observed between contractile characteristics and contractile protein profile when only the young subjects were considered. This was despite the inclusion of a further two sprint and three endurance trained athletes to increase the range of contractile characteristics and differences in muscle composition.     

Differential strain patterns of the human gastrocnemius aponeurosis and free tendon,in vivo

Aim: The mechanical characteristics of the human free tendon and aponeurosis, in vivo, remains largely unknown. The present study evaluated the longitudinal displacement of the separate free Achilles tendon and distal (deep) aponeurosis of the medial gastrocnemius muscle during voluntary isometric contraction. Methods: Ultrasonography‐obtained displacement of the free tendon and tendon–aponeurosis complex, electromyography of the gastrocnemius, soleus, and dorsiflexor muscles, and joint angular rotation were recorded during isometric plantarflexion (n = 5). Tendon cross‐sectional area, moment arm and segment lengths (L_o) were measured using magnetic resonance imaging. Tendon force was calculated from joint moments and tendon moment arm, and stress was obtained by dividing force by cross‐sectional area. The difference between the free tendon and tendon–aponeurosis complex deformation yielded separate distal aponeurosis deformation. Longitudinal aponeurosis and tendon strain were obtained from the deformations normalized to segment lengths. Results: At a common tendon force of 2641 ± 306 N, the respective deformation and L_o were 5.85 ± 0.85 and 74 ± 0.8 mm for the free tendon and 2.12 ± 0.64 and 145 ± 1.3 mm for the distal aponeurosis, P < 0.05. Longitudinal strain was 8.0 ± 1.2% for the tendon and 1.4 ± 0.4% for the aponeurosis, P < 0.01. Stiffness and stored energy was 759 ± 132 N mm^?1 and 6.14 ± 1.89 J, respectively, for the free tendon. Cross‐sectional area of the Achilles tendon was 73 ± 4 mm², yielding a stress of 36.5 ± 4.6 MPa and Young's modulus of 788 ± 181 MPa. Conclusion: The free Achilles tendon demonstrates greater strain compared with that of the distal (deep) aponeurosis during voluntary isometric contraction, which suggests that separate functional roles may exist during in vivo force transmission.     

The medial gastrocnemius muscle attenuates force fluctuations during plantar flexion

Force fluctuations during steady contractions of multiple agonist muscles may be influenced by the relative contribution of force by each muscle. The purpose of the study was to compare force fluctuations during steady contractions performed with the plantar flexor muscles in different knee positions. Nine men (25.8±5.1 years) performed steady contractions of the plantar flexor muscles in the knee-flexed and knee-extended (greater involvement of the gastrocnemii muscles) positions. The maximal voluntary contraction (MVC) force was 32% greater in the knee-extended position compared with the knee-flexed position. The target forces were 2.5–10% MVC force in the respective position. The amplitude of electromyogram in the medial gastrocnemius muscle was greater in the knee-extended position (10.50±9.80%) compared with the knee-flexed position (1.26±1.15%, P<0.01). The amplitude of electromyogram in the soleus muscle was not influenced by the knee position. The amplitude of electromyogram in the lateral gastrocnemius and tibialis anterior muscles was marginal and unaltered with knee position. At the same force (in Newtons), the standard deviation of force was lower in the knee-extended position compared with the knee-flexed position. These results indicate that force fluctuations during plantar flexion are attenuated with greater involvement of the medial gastrocnemius muscle.     

Coactivation of the ankle musculature during maximal isokinetic dorsiflexion at different angular velocities

The objectives of this study were to determine the differences in the level of coactivation among the ankle agonist and antagonist muscles during maximal isokinetic dorsiflexion and to determine whether velocity alters the activation levels. Raw surface electromyograms (EMG) were recorded from six muscle sites – two over the agonist tibialis anterior (upper and lower sites) and four over the antagonist (lateral and medial gastrocnemius, plus lateral and medial soleus) muscles – in ten healthy subjects (two males and eight females) as they performed maximal dorsiflexor efforts against an isokinetic dynamometer at each of three angular velocity settings (30, 90 and 150°/s). The root-mean-square amplitude (RMS) was calculated over a 35° angular displacement for each EMG recording, then was normalized (RMS_{N ji} ) to the amplitude measured during maximal voluntary isometric contractions (MVIC) of the dorsifexors and plantarflexors (i.e. RMS_{MVIC j} ). A two-factor repeated-measures analysis of variance (ANOVA) tested the muscle site by velocity interaction and the main effects of muscle site and velocity separately. The raw EMG signals were then full-wave rectified and low-pass filtered (6 Hz) and ensemble average curves of the sample were calculated, for each muscle site, at each velocity. The ANOVA revealed a statistically significant muscle-by-velocity interaction (P < 0.05). The RMS_{N ji} values for the two tibialis anterior sites were not significantly different from each other for any of the three velocity settings, nor for the two gastrocnemius muscles at 30°/s velocity. All other between-muscle comparisons were statistically significant (P < 0.05). The RMS_{N ji} means were 28, 23, 59, 52, 98 and 98% MVIC for the lateral and medial gastrocnemius, the lateral and medial sites on the soleus, and for the upper and lower sites on the tibialis anterior, respectively. The RMS_{N ji} from the lower site on the tibialis anterior yielded the only statistically significant difference (P < 0.05) among velocities; at this site, the RMS_{N ji} at 150°/s was higher than at the other two velocities. The ensemble-average curves revealed that not all muscle sites are activated to a consistent level throughout the entire movement. The interaction between agonist and antagonist activation during isokinetic dorsiflexion has implications for interpreting the results of isokinetic dynamometry for strength assessment and for understanding the neuromuscular control strategies used in this exercise modality. Accepted: 3 March 2000     

Decreased motor unit discharge rate in the potentiated human tibialis anterior muscle

Aim The purpose of this study was to examine the influence of post‐activation potentiation (PAP), the transient increase in low‐frequency isometric force observed after muscle activity, on motor unit discharge rates measured during submaximal contractions. Methods A quadrifilar needle electrode was inserted into the tibialis anterior muscle to determine discharge rate of individual motor units while monopolar electrodes were used to monitor the root‐mean‐square (RMS) and mean power frequency (MPF) of the surface EMG signal. Control (unpotentiated) and experimental (potentiated) measures were obtained during a 5 s voluntary contraction at 50% of maximal. In between these measures, subjects performed a 10 s maximal voluntary contraction (MVC) to induce PAP. Results All subjects data are reported as means ± SEM (n = 10). Compared to baseline values measured prior to the MVC, isometric twitch force measured immediately after the MVC was increased by 260 ± 16% (day 3). Motor unit discharge rate in the potentiated tibialis anterior muscle decreased by approx. 10%, from 20.3 ± 0.8 (before) to 18.3 ± 0.99 pps (P = 0.01) (after). Moreover, the MPF was decreased by approx. 9% (from 58.1 ± 2.84 to 53.6 ± 2.85 Hz; P = 0.01) in the potentiated tibialis anterior. On the other hand, consistent with the absence of fatigue during the MVC, the RMS signal was not altered in the potentiated tibialis anterior (0.29 ± 0.03 vs. 0.33 ± 0.04 mV; P = 0.07). Conclusion Motor unit discharge rates determined during a brief, submaximal contraction were decreased in the potentiated human tibialis anterior muscle.     

Alteration of regulatory enzyme activities in fast-twitch and slow-twitch muscles and muscle fibres in low-intensity endurance-trained rats

The effect of progressive, low-intensity endurance training on regulatory enzyme activities in slow-twitch (ST) and fast-twitch (FT) muscle fibres was studied in 32 rats. Of those rats 16 were trained on a treadmill at a running speed of 10m · min^–1 5 days a week over an 8-week period. Running time was progressively increased from 15 min to 2 h · day^–1. Of the rats 4 trained and 4 sedentary rats were also subjected to acute exhausting exercise. Enzyme activities of phosphofructokinase 1 (PFKI) from glycolysis, -ketoglutarate dehydrogenase (-KGDH) from the Krebs cycle and carnitine palmitoyltransferase (CPT I and II) from fatty acid metabolism in soleus, tibialis anterior and gastrocnemius muscles were measured in trained and sedentary rats. Enzyme activities of individual ST and FT fibres were measured from the freeze-dried gastrocnemius muscle of 8 trained and 8 sedentary rats. In the sedentary rats the activity of PFK1 in tibialis anterior and soleus muscles was 141% and 41% of the activity in gastrocnemius muscle, respectively. The activity of -KGDH in tibialis anterior and soleus muscles was 164% and 278% of the activity in gastrocnemius muscle, respectively. The activity of CPT I in tibialis anterior and gastrocnemius muscles were at the same level, but in soleus muscle the activity was 127% of that in mixed muscle. Endurance training increased enzyme activities of -KGDH and CPT I significantly (P < 0.05) in gastrocnemius muscle but not in soleus or tibialis anterior muscle. After training both -KGDH and CPT II activities were elevated significantly (P < 0.05) in the ST fibres of gastrocnemius muscle, whereas in FT fibres only -KGDH was increased. For PFK1 activity no significant change was observed in ST or FT fibres. After acute exercise, activities of mitochondrial enzymes -KGDH and CPT I tended to be elevated in all muscles. Thus, low-intensity endurance training induced significant peripheral changes in regulatory enzyme activities in oxidative and fatty acid metabolism in individual ST or FT muscle fibres.     

Insulin action on rates of muscle protein synthesis following eccentric,muscle‐damaging contractions

The purpose of this study was to determine whether eccentric, muscle‐damaging contractions affect insulin action on muscle protein synthesis. Male Wistar rats (n=28) were anaesthetized either once or twice separated by 7 days’ rest, and one limb was electrically stimulated to contract eccentrically, while the contralateral limb served as a non‐stimulated control. Twenty‐four and 48 h after contractions, rates of protein synthesis were assessed in soleus and red or white gastrocnemius muscles during a hindlimb perfusion with or without insulin (20 000 μU mL^–1). Rates of protein synthesis were not different in non‐stimulated muscle, with or without insulin (P > 0.05). In red or white gastrocnemius without insulin, rates of protein synthesis were significantly reduced (P < 0.05) 24 and 48 h after a single session and 48 h after a double session of muscle contractions. However, protein synthesis was normalized with insulin 24 and 48 h after contractions in red, and 48 h after contractions in white gastrocnemius. In soleus muscle, protein synthesis was impaired only 48 h after the second session, but partially restored by insulin (P < 0.05). These results indicate that muscle becomes more sensitive to insulin action on rates of protein synthesis after muscle‐damaging contractions.     

Dynamic multivoxel‐localized 31P MRS during plantar flexion exercise with variable knee angle

Exercise studies investigating the metabolic response of calf muscles using ³¹P MRS are usually performed with a single knee angle. However, during natural movement, the distribution of workload between the main contributors to force, gastrocnemius and soleus is influenced by the knee angle. Hence, it is of interest to measure the respective metabolic response of these muscles to exercise as a function of knee angle using localized spectroscopy. Time‐resolved multivoxel ³¹P MRS at 7 T was performed simultaneously in gastrocnemius medialis and soleus during rest, plantar flexion exercise and recovery in 12 healthy volunteers. This experiment was conducted with four different knee angles. PCr depletions correlated negatively with knee angle in gastrocnemius medialis, decreasing from 79±14 % (extended leg) to 35±23 %(～40°), and positively in soleus, increasing from 20±21 % to 36±25 %; differences were significant. Linear correlations were found between knee angle and end‐exercise PCr depletions in gastrocnemius medialis (R²=0.8) and soleus (R²=0.53). PCr recovery times and end‐exercise pH changes that correlated with PCr depletion were consistent with the literature in gastrocnemius medialis and differences between knee angles were significant. These effects were less pronounced in soleus and not significant for comparable PCr depletions. Maximum oxidative capacity calculated for all knee angles was in excellent agreement with the literature and showed no significant changes between different knee angles. In conclusion, these findings confirm that plantar flexion exercise with a straight leg is a suitable paradigm, when data are acquired from gastrocnemius only (using either localized MRS or small surface coils), and that activation of soleus requires the knee to be flexed. The present study comprises a systematic investigation of the effects of the knee angle on metabolic parameters, measured with dynamic multivoxel ³¹P MRS during muscle exercise and recovery, and the findings should be used in future study design.     

Die abfallende Phase des aktiven Zustandes des Tibialis anterior,Gastrocnemius und Soleus der Katze und ihre Beeinflussung durch Dehnung und Sympathicomimetica

Summary The time course of the active state curve has been studied in the tibialis anterior, gastrocnemius, and soleus muscles of the cat. The technique employed was the method of quick release described by Hill (1953), Ritchie (1954), and Ritchie and Wilkie (1955).The rate of relaxation of the contractile elements was higher in the fast-contracting tibialis anterior and gastrocnemius muscles than in the slow-contracting soleus muscle. The active state of the tibialis anterior, gastrocnemius, and soleus muscle ended at about 90 msec, 100 msec, and 210 msec respectively after a single stimulus. The duration of the active state was prolonged by a stretch applied to the muscles prior to stimulation.Adrenaline reduced the rate of relaxation of the contractile elements in the tibialis anterior muscle and increased that of the soleus muscle. This effect was diminished by stretch of the muscles. Isoprenaline exerted a similar effect on the duration of the active state as did adrenaline, whereas noradrenaline proved to be much less potent in influencing the rate of relaxation.It is therefore concluded that the difference in fusion frequency of fast- and slow-contracting muscles of the cat is due to a different time course of relaxation of the contractile elements. The changes in tension development in the soleus and tibialis anterior muscles following administration of adrenaline, noradrenaline, and isoprenaline result from a reduced (soleus) or an increased (tibialis anterior) duration of the active state.

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Two‐dimensional spectroscopic imaging with combined free induction decay and long‐TE acquisition (FID echo spectroscopic imaging,FIDESI) for the detection of intramyocellular lipids in calf muscle at 7 T

The aim of this study was to introduce a two‐dimensional chemical shift imaging (2D CSI) sequence, with simultaneous acquisition of free induction decay (FID) and long TEs, for the detection and quantification of intramyocellular lipids (IMCLs) in the calf at 7 T. The feasibility of the new 2D CSI sequence, which acquires FID (acquisition delay, 1.3 ms) and an echo (long TE) in one measurement, was evaluated in phantoms and volunteers (n = 5): TR/TE*/TE = 800/1.3/156 ms; 48 × 48 matrix; field of view, 200 × 200 × 20 mm³; Hamming filter; no water suppression; measurement time, 22 min 2 s. The IMCL concentration and subcutaneous lipid contamination were assessed. Spectra in the tibialis anterior (TA), gastrocnemius (GM) and soleus (SOL) muscles were analyzed. The water signal from the FID acquisition was used as an internal concentration reference. In the spectra from subcutaneous adipose tissue (SUB) and bone marrow (BM), an unsaturation index (UI) of the vinyl‐H (5.3 ppm) to methyl‐CH₃ ratio, and a polyunsaturation index (pUI) of the diallylic‐H (2.77 ppm) to ‐CH₃ ratio, were calculated. Long‐TE spectra from muscles showed a simplified spectral pattern with well‐separated IMCL for several muscle groups in the same scan. The IMCL to water ratio was largest in SOL (0.66% ± 0.23%), and lower in GM (0.37% ± 0.14%) and TA (0.36% ± 0.12%). UI and pUI for SUB were 0.65 ± 0.06 and 0.18 ± 0.04, respectively, and for BM were 0.60 ± 0.16 and 0.18 ± 0.08, respectively. The new sequence, with the proposed name ‘free induction decay echo spectroscopic imaging’ (FIDESI), provides information on both specific lipid resonances and water signal from different tissues in the calf, with high spectral and spatial resolution, as well as minimal voxel bleeding and subcutaneous lipid contamination, in clinically acceptable measurement times. Copyright © 2014 John Wiley & Sons, Ltd.     

Myosin heavy chains in skeletal muscles of the common shrew (Sorex araneus): absence of a slow isoform and transitions of fast isoforms with ageing

Myosin heavy chain (MHC) composition in seven skeletal muscles of the common shrew (Sorex araneus) was analysed by gradient SDS-PAGE and immunoblotting with monoclonal antibodies. Characteristic for the studied muscles (diaphragm, lateral gastrocnemius, medial gastrocnemius, masseter, plantaris, soleus and tibialis anterior) was a total absence of the slow isoform, MHCI; all muscles were exclusively composed of two fast isoforms MHCIId and MHCIIb. In young adults the amount of MHCIId varied between 34% (tibialis anterior) and 97% (masseter). In over-wintered senescent individuals MHCIId was clearly the dominant isoform in all muscles studied; the lowest MHCIId content was measured for tibialis anterior (69%), while in diaphragm, masseter and soleus it was practically the sole isoform (over 96%). Ageing associated isoform transition from MHCIIb to MHCIId occurred in all seven muscles studied (P< 0.05).     

Endotoxin‐induced shock in the pig – limited effects of low and high concentrations of inhaled nitric oxide

Aim: This study was carried out to study the prophylactic effects of inhalation of nitric oxide (NO) before and during the induction of endotoxic shock. Methods: Eighteen anaesthetized pigs received an infusion of 10–20 μg kg^?1 endotoxin during 2 h after pre‐treatment with the cortisol‐synthesis inhibitor metyrapone. Three groups were tested (n = 6 each) and received 0, 0.2 or 20 ppm inhaled NO from 30 min before start of endotoxin infusion until 4 h after start of endotoxin. Both 0.2 and 20 ppm NO were able to improve blood gas values. Results: Area above curve values of arterial P₂/FiO₂ from 0 to 4 h were 0.83 ± 0.09 kPa h (control), 0.78 ± 0.22 (0.2 ppm NO, non‐significant) and 0.31 ± 0.06 (20 ppm NO, P < 0.01, Mann–Whitney U‐test, compared to control). Area under curve values of PCO₂ from 0 to 4 h were 3.96 ± 0.66 kPa h (control), 1.20 ± 0.46 (0.2 ppm NO, P < 0.05, Mann–Whitney U‐test, compared to control) and 2.78 ± 1.06 (20 ppm NO group, non‐significant). The increase in pulmonary arterial pressure (PAP) was partly prevented by 20 ppm NO, but not by 0.2 ppm NO at 4 h. Inhaled NO did not affect the levels of BAL fluid total protein, tumour necrosis factor‐α, interleukin‐8 and neutrophil counts. Conclusions: The addition of a high (20 ppm), but not a low (0.2 ppm), concentration of NO to the inhaled air during endotoxin shock improves arterial oxygen tension and reduces pulmonary artery pressure. Neither dose affects lung mechanics or inflammatory indices, in spite of being given prophylactically.     

Feasibility and repeatability of localized 31P‐MRS four‐angle saturation transfer (FAST) of the human gastrocnemius muscle using a surface coil at 7 T

Phosphorus (³¹P) MRS, combined with saturation transfer (ST), provides non‐invasive insight into muscle energy metabolism. However, even at 7 T, the standard ST method with T₁^app measured by inversion recovery takes about 10 min, making it impractical for dynamic examinations. An alternative method, i.e. four‐angle saturation transfer (FAST), can shorten the examination time. The aim of this study was to test the feasibility, repeatability, and possible time resolution of the localized FAST technique measurement on an ultra‐high‐field MR system, to accelerate the measurement of both P_i‐to‐ATP and PCr‐to‐ATP reaction rates in the human gastrocnemius muscle and to test the feasibility of using the FAST method for dynamic measurements. We measured the exchange rates and metabolic fluxes in the gastrocnemius muscle of eight healthy subjects at 7 T with the depth‐resolved surface coil MRS (DRESS)‐localized FAST method. For comparison, a standard ST localized method was also used. The measurement time for the localized FAST experiment was 3.5 min compared with the 10 min for the standard localized ST experiment. In addition, in five healthy volunteers, P_i‐to‐ATP and PCr‐to‐ATP metabolic fluxes were measured in the gastrocnemius muscle at rest and during plantar flexion by the DRESS‐localized FAST method. The repeatability of PCr‐to‐ATP and P_i‐to‐ATP exchange rate constants, determined by the slab‐selective localized FAST method at 7 T, is high, as the coefficients of variation remained below 20%, and the results of the exchange rates measured with the FAST method are comparable to those measured with standard ST. During physical activity, the PCr‐to‐ATP metabolic flux decreased (from F_CK = 8.21 ± 1.15 mM s^?1 to F_CK = 3.86 ± 1.38 mM s^?1) and the P_i‐to‐ATP flux increased (from F_ATP = 0.43 ± 0.14 mM s^?1 to F_ATP = 0.74 ± 0.13 mM s^?1). In conclusion, we could demonstrate that measurements in the gastrocnemius muscle are feasible at rest and are short enough to be used during exercise with the DRESS‐localized FAST method at 7 T. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.