Sex-based differences in skeletal muscle function and morphology with short-term limb immobilization.

The purpose of this study was to determine the effects of short-term (14-day) unilateral leg immobilization using a simple knee brace (60 degree flexion)- or crutch-mediated model on muscle function and morphology in men (M, n = 13) and women (W, n = 14). Isometric and isokinetic (concentric-slow, 0.52 rad/s and fast, 5.24 rad/s) knee extensor peak torque was determined at three time points (Pre, Day-2, and Day-14). At the same time points, magnetic resonance imaging was used to measure the cross-sectional area of the quadriceps femoris and dual-energy X-ray absorptiometry scanning was used to calculate leg lean mass. Muscle biopsies were taken from vastus lateralis at Pre and Day-14 for myosin ATPase and myosin heavy chain analysis. Women showed greater decreases (Pre vs. Day-14) compared with men in specific strength (N/cm2) for isometric [M = 3.1 +/- 13.3, W = 17.1 +/- 15.9%; P = 0.055 (mean +/- SD)] and concentric-slow (M = 4.7 +/- 11.3, W = 16.6 +/- 18.4%; P < 0.05) contractions. There were no immobilization-induced sex-specific differences in the decrease in quadriceps femoris cross-sectional area (M = 5.7 +/- 5.0, W = 5.9 +/- 5.2%) or leg lean mass (M = 3.7 +/- 4.2, W = 2.7 +/- 2.8%). There were no fiber-type transformations, and the decreases in type I (M = 4.8 +/- 5.0, W = 5.9 +/- 3.4%), IIa (M = 7.9 +/- 9.9, W = 8.8 +/- 8.0%), and IIx (M = 10.7 +/- 10.8, W = 10.8 +/- 12.1%) fiber areas were similar between sexes. These findings indicate that immobilization-induced loss of knee extensor muscle strength is greater in women compared with men despite a similar extent of atrophy at the myofiber and whole muscle levels after 14 days of unilateral leg immobilization. Furthermore, we have described an effective and safe knee immobilization method that results in reductions in quadriceps muscle strength and size.     

Changes in inorganic phosphate and force production in human skeletal muscle after cast immobilization.

Cast immobilization is associated with decreases in muscle contractile area, specific force, and functional ability. The pathophysiological processes underlying the loss of specific force production as well as the role of metabolic alterations are not well understood. The aim of this study was to quantify changes in the resting energy-rich phosphate content and specific force production after immobilization. (31)P-magnetic resonance spectroscopy, three-dimensional magnetic resonance imaging, and isometric strength testing were performed in healthy subjects and patients with an ankle fracture after 7 wk of immobilization and during rehabilitation. Muscle biopsies were obtained in a subset of patients. After immobilization, there was a significant decrease in the specific plantar flexor torque and a significant increase in the inorganic phosphate (P(i)) concentration (P < 0.001) and the P(i)-to-phosphocreatine (PCr) ratio (P < 0.001). No significant change in the PCr content or basal pH was noted. During rehabilitation, both the P(i) content and the P(i)-to-PCr ratio decreased and specific torque increased, approaching control values after 10 wk of rehabilitation. Regression analysis showed an inverse relationship between the in vivo P(i) concentration and specific torque (r = 0.65, P < 0.01). In vitro force mechanics performed on skinned human muscle fibers demonstrated that varying the P(i) levels within the ranges observed across individuals in vivo (4-10 mM) changed force production by approximately 16%. In summary, our findings clearly depict a change in the resting energy-rich phosphate content of skeletal muscle with immobilization, which may negatively impact its force generation.     

Recovery in skeletal muscle contractile function after prolonged hindlimb immobilization

Contractile properties of slow-twitch soleus (SOL), fast-twitch extensor digitorum longus (EDL), and fast-twitch superficial region of the vastus lateralis were determined in vitro (22 degrees C) in rats remobilized after prolonged (3 mo) hindlimb immobilization (IM). For all muscles the muscle-to-body weight ratio was significantly depressed by IM, and the ratios failed to completely recover even after 90 days. The contractile properties of the fast-twitch muscles were less affected by IM than the slow-twitch SOL. The IM shortened the SOL isometric twitch duration due to a reduced contraction and half-relaxation time. These parameters returned to control levels by the 14th day of recovery. Peak tetanic tension (Po, g/cm2) declined with IM by 46% in the SOL but showed no significant change in the fast-twitch muscles. After IM the SOL Po (g/cm2) recovered to control values by 28 days. The recovery of Po in absolute units (g) was considerably slower and did not return to control levels until 60 (SOL) to 90 (EDL) days. The maximum shortening velocity was not altered by IM in any of the muscles studied. These results demonstrate that both fast- and slow-twitch skeletal muscles possess the ability to completely recover normal contractile function following prolonged periods of hindlimb IM.     

Near infrared monitoring of human skeletal muscle oxygenation during forearm ischemia

Changes in tissue oxygenation of forearm muscles were measured by near infrared (NIR) spectrophotometry in 10 healthy adults during tourniquet ischemia and venous outflow restriction. Muscle O2 stores were depleted rapidly by forearm ischemia manifest by a progressive decrease in tissue oxyhemoglobin and oxymyoglobin over 4-5 min. Muscle ischemia significantly decreased the oxidation level of cytochrome aa3, to below resting base line after only 1.5 min, and the enzyme became fully reduced after 6.5 min. After 8 min of ischemia, tourniquet release was accompanied by a transient increase in muscle blood volume due to influx of oxyhemoglobin. The cytochrome aa3 oxidation level increased above resting base line within 1 min after tourniquet release. Transcutaneous PO2 measurements recorded simultaneously from the same forearm correlated poorly with the kinetics of O2 availability and cytochrome oxidation in the underlying muscle tissue; this was not unexpected because overlying skin did not contribute significantly to NIR muscle signals. Venous outflow restriction without inflow obstruction increased muscle deoxyhemoglobin and tissue blood volume but did not change muscle O2 stores or cytochrome aa3 oxidation level. The ability of the NIR technique to detect dynamic trends in tissue oxygenation reveals that muscle O2 is rapidly consumed during tourniquet ischemia and rapidly restored by hyperemic responses after brief ischemia.     

Neuromuscular plasticity during and following 3 wk of human forearm cast immobilization

Prolonged reductions in muscle activity results in alterations in neuromuscular properties; however, the time course of adaptations is not fully understood, and many of the specific adaptations have not been identified. This study evaluated the temporal evolution of adaptations in neuromuscular properties during and following 3 wk of immobilization. We utilized a combination of techniques involving nerve stimulation and transcranial magnetic stimulation to assess changes in central activation of muscle, along with spinal (H reflex) and corticospinal excitability [i.e., motor-evoked potential (MEP) amplitude, silent period (SP)] and contractile properties in 10 healthy humans undergoing 3 wk of forearm immobilization and 9 control subjects. Immobilization induced deficits in central activation (85 +/- 3 to 67 +/- 7% ) that returned to baseline levels 1 wk after cast removal. The flexor carpii radialis MEP amplitude increased greater than twofold after the first week of immobilization and remained elevated throughout immobilization and 1 wk after cast removal. Additionally, we observed a prolongation of the SP 1 wk after cast removal compared with baseline (78.5 +/- 7.1 to 98.2 +/- 8.7 ms). The contractile properties were also altered, since the rate of evoked force relaxation was slower following immobilization (-14.5 +/- 1.4 to -11.3 +/- 1.0% peak force/ms), and remained depressed 1 wk after cast removal (-10.5 +/- 0.8% peak force/ms). These observations detail the time course of adaptations in corticospinal and contractile properties associated with disuse and illustrate the profound effect of immobilization on the human neuromuscular system as evidenced by the alterations in corticospinal excitability persisting 1 wk following cast removal.     

Low-volume muscular endurance and strength training during 3-week forearm immobilization was effective in preventing functional deterioration

Purpose

The purpose of this study was to determine whether endurance and strength hand grip exercises during 3-week upper limb immobilization preserve muscle oxidative capacity, endurance performance and strength.

Methods

Ten healthy adult men underwent non-dominant forearm immobilization by plaster cast for 21 days. Five healthy adult subjects were designated as the immobilization (IMM) group and five were designated as the immobilization + training (IMM+TRN) group. Grip strength, forearm circumference, dynamic handgrip endurance and muscle oxygenation response were measured before and after the 21 day immobilization period. Using near-infrared spectroscopy (NIRS), muscle oxygen consumption recovery (VO₂mus) was recorded after a submaximal exercise and the recovery time constant (TcVO₂mus) was calculated. Reactive hyperemic oxygenation recovery was evaluated after 5 minutes ischemia. Two training programs were performed by the IMM+TRN group twice a week. One exercise involved a handgrip exercise at 30% maximum voluntary contraction (MVC) at a rate of 1 repetition per 1 second until exhaustion (about 60 seconds). The other involved a handgrip exercise at 70% MVC for 2 seconds with a 2 second rest interval, repeated 10 times (40 seconds).

Results

There was a significant group-by-time interaction between the IMM and IMM+TRN groups in the TcVO₂mus (p = 0.032, F = 6.711). A significant group-by-time interaction was observed between the IMM and IMM+TRN groups in the MVC (p = 0.001, F = 30.415) and in grip endurance (p = 0.014, F = 9.791). No significant group-by-time interaction was seen in forearm circumference and reactive hyperemic oxygenation response either in IMM or IMM+TRN group.

Conclusion

The training programs during immobilization period used in this experiment were effective in preventing a decline in muscle oxidative function, endurance and strength.

     

Noninvasive quantitative analysis of blood oxygenation in rat skeletal muscle

Using the isolated perfused rat hindlimb and the fluorocarbon-transfused rat, we have examined the optical characteristics of the rat skeletal muscle in the near-infrared region. The total contribution of myoglobin and cytochromes to the overall absorbance change was less than 10%. Analyzing transmitted light at 700, 730, and 805 nm, we found linear relationships between the absorbance and the hemoglobin concentrations at hematocrit values from 15 to 50% in the inflowing perfusate. Based on the relationship, we determined the ratio of absorption coefficients at 700, 730, and 805 nm of oxy- and deoxy-hemoglobins of blood in the thigh muscle. The values in thigh muscle were significantly smaller than those in hemoglobin solutions for deoxygenated blood. On the other hand, the values in thigh muscle were larger than those in hemoglobin solutions for oxygenated blood. Solving simultaneous equations by the use of these absorption coefficients, we calculated the changes in the contents of oxy-, deoxy-, and total hemoglobins in the anesthetized rat hindlimb under various conditions. The oxygen saturation of blood determined by our optical method in the thigh muscle was very close to that in the vena cava measured directly with a gas analyzer.     

Populations of rat skeletal muscle mitochondria after exercise and immobilization

We slightly modified an existing procedure (Palmer et al., J. Biol. Chem. 252: 8731-8739, 1977) to isolate two distinct populations of mitochondria from rat skeletal muscle; initial brief Polytron homogenization released the subsarcolemmal mitochondria, and brief exposure of the resultant intact myofibrils to the proteolytic enzyme, Nagarse, extracted the intermyofibrillar mitochondria. The intermyofibrillar mitochondria differed from the subsarcolemmal mitochondr. ia by higher state III respiration measurements and enzymatic activities. These two populations of mitochondria were then isolated from the gastrocnemius muscle that had been induced to perform different amounts of contractile activity. The endurance training program of daily running significantly increased state III respiration and respiratory control index in the subsarcolemmal mitochondria, but the program did not increase these measurements in the intermyofibrillar mitochondria. In addition, 2 days of hindlimb immobilization resulted in a significant decrease in state II respiration and the respiratory control index of the subsarcolemmal mitochondria; however, immobilization did not affect the intermyofibrillar mitochondria. These measurements suggest that the subsarcolemmal mitochondria adapt in response to chronic changes in the level of contractile activity.     

Glucose uptake after immobilization in fast and slow skeletal muscle in rats

1. 3-O-methylglucose uptake was studied after immobilization in rat extensor digitorum longus (EDL) and soleus (Sol) muscles. 2. The immobilization of the ankle was done in one of extreme positions by plaster casts. 3. In both positions, 3-O-methylglucose uptake in EDL increased and that in Sol decreased after immobilization. 4. When immobilization was released uptake returned to control level. 5. The change in uptake after immobilization and after release of immobilization was earlier in Sol.     

Laminin-211 in skeletal muscle function

A chain is no stronger than its weakest link is an old idiom that holds true for muscle biology. As the name implies, skeletal muscle’s main function is to move the bones. However, for a muscle to transmit force and withstand the stress that contractions give rise to, it relies on a chain of proteins attaching the cytoskeleton of the muscle fiber to the surrounding extracellular matrix. The importance of this attachment is illustrated by a large number of muscular dystrophies caused by interruption of the cytoskeletal-extracellular matrix interaction. One of the major components of the extracellular matrix is laminin, a heterotrimeric glycoprotein and a major constituent of the basement membrane. It has become increasingly apparent that laminins are involved in a multitude of biological functions, including cell adhesion, differentiation, proliferation, migration and survival. This review will focus on the importance of laminin-211 for normal skeletal muscle function.