Persisting side-to-side differences in bone mineral content, but not in muscle strength and tendon stiffness after anterior cruciate ligament reconstruction

Tendon stiffness may be involved in limiting peak musculoskeletal forces and thus may constitute an upper limit for bone strength. The patellar tendon bone (PTB) graft, which is harvested from the patellar tendon during surgical reconstruction of the anterior cruciate ligament (ACL), is an ideal scenario to test this hypothesis. Eleven participants were recruited who had undergone surgical reconstruction of the ACL with a PTB graft 1-10 years prior to study inclusion. As previously reported, there was no side-to-side difference in thigh muscle cross-sectional area, in maximum voluntary knee extension torque, or in patellar tendon stiffness, suggesting full recovery of musculature and tendon. However, in the present study bone mineral content (BMC), assessed by peripheral quantitative computed tomography, was lower on the operated side than on the control side in four regions studied (P = 0·0019). Differences were less pronounced in the two sites directly affected by the operation (patella and tibia epiphysis) when compared to the more remote sites. Moreover, significant side-to-side differences were found in BMC in the trabecular compartment in the femoral and tibial epiphysis (P = 0·004 and P = 0·047, respectively) with reductions on the operated side, but increased in the patella (P = 0·00016). Cortical BMC, by contrast, was lower on the operated side at all sites except the tibia epiphysis (P = 0·09). These findings suggest that impaired recovery of BMC following ACL reconstruction is not because of lack of recovery of knee extensor strength or patellar tendon stiffness. The responsible mechanisms still remain to be determined.

Changes in lower extremity muscle function after 56 days of bed rest

Preservation of muscle function, known to decline in microgravity and simulation (bed rest), is important for successful spaceflight missions. Hence, there is great interest in developing interventions to prevent muscle-function loss. In this study, 20 males underwent 56 days of bed rest. Ten volunteers were randomized to do resistive vibration exercise (RVE). The other 10 served as controls. RVE consisted of muscle contractions against resistance and concurrent whole-body vibration. Main outcome parameters were maximal isometric plantar-flexion force (IPFF), electromyography (EMG)/force ratio, as well as jumping power and height. Measurements were obtained before and after bed rest, including a morning and evening assessment on the first day of recovery from bed rest. IPFF (-17.1%), jumping peak power (-24.1%), and height (-28.5%) declined (P < 0.05) in the control group. There was a trend to EMG/force ratio decrease (-20%; P = 0.051). RVE preserved IPFF and mitigated the decline of countermovement jump performance (peak power -12.2%; height -14.2%). In both groups, IPFF was reduced between the two measurements of the first day of reambulation. This study indicates that bed rest and countermeasure exercises differentially affect the various functions of skeletal muscle. Moreover, the time course during recovery needs to be considered more thoroughly in future studies, as IPFF declined not only with bed rest but also within the first day of reambulation. RVE was effective in maintaining IPFF but only mitigated the decline in jumping performance. More research is needed to develop countermeasures that maintain muscle strength as well as other muscle functions including power.

Comparison of interpolation and central activation ratios as measures of muscle inactivation

The objective of this study was to investigate different methods of estimating muscle inactivation, derived from single and multiple voluntary contractions. Ten subjects performed maximal and submaximal leg extensor contractions to determine an interpolation (IT) or central activation ratio (CAR). A superimposed evoked force was compared with the force output of either a voluntary (CAR) or resting evoked contraction (IT ratio), or the ratios were inserted into regression equations (linear, polynomial, exponential). Linear-regression estimates of CAR using doublets and tetanus provided physiologically inaccurate values. Whereas IT ratios using doublets (IT-doublet) and tetanus (IT-tetanus) had a significant difference in only one interaction, IT-tetanus and CAR using a tetanus (CAR-tetanus) estimates provided the most extensive correlation within and between measures. Thus, tetanic stimulation superimposed upon single maximal or multiple contractions seems to provide the most valid measure of muscle inactivation when using the interpolated-twitch technique.

Adrenaline increases skeletal muscle glycogenolysis, pyruvate dehydrogenase activation and carbohydrate oxidation during moderate exercise in humans

# 1.
To evaluate the role of adrenaline in regulating carbohydrate metabolism during moderate exercise, 10 moderately trained men completed two 20 min exercise bouts at 58 ± 2 % peak pulmonary oxygen uptake (̇V_o2,peak). On one occasion saline was infused (CON), and on the other adrenaline was infused intravenously for 5 min prior to and throughout exercise (ADR). Glucose kinetics were measured by a primed, continuous infusion of 6,6-[2H]glucose and muscle samples were obtained prior to and at 1 and 20 min of exercise.

# 2.
The infusion of adrenaline elevated (P < 0.01) plasma adrenaline concentrations at rest (pre-infusion, 0.28 ± 0.09; post-infusion, 1.70 ± 0.45 nmol l−1; means ±s.e.m.) and this effect was maintained throughout exercise. Total carbohydrate oxidation increased by 18 % and this effect was due to greater skeletal muscle glycogenolysis (P < 0.05) and pyruvate dehydrogenase (PDH) activation (P < 0.05, treatment effect). Glucose rate of appearance was not different between trials, but the infusion of adrenaline decreased (P < 0.05, treatment effect) skeletal muscle glucose uptake in ADR.

# 3.
During exercise muscle glucose 6-phosphate (G-6-P) (P = 0.055, treatment effect) and lactate (P < 0.05) were elevated in ADR compared with CON and no changes were observed for pyruvate, creatine, phosphocreatine, ATP and the calculated free concentrations of ADP and AMP.

# 4.
The data demonstrate that elevated plasma adrenaline levels during moderate exercise in untrained men increase skeletal muscle glycogen breakdown and PDH activation, which results in greater carbohydrate oxidation. The greater muscle glycogenolysis appears to be due to increased glycogen phosphorylase transformation whilst the increased PDH activity cannot be readily explained. Finally, the decreased glucose uptake observed during exercise in ADR is likely to be due to the increased intracellular G-6-P and a subsequent decrease in glucose phosphorylation.

Differential effects of exercise on insulin-signaling gene expression in human skeletal muscle.

Skeletal muscle insulin sensitivity is enhanced after acute exercise and short-term endurance training. We investigated the impact of exercise on the gene expression of key insulin-signaling proteins in humans. Seven untrained subjects (4 women and 3 men) completed 9 days of cycling at 63 ± 2% of peak O₂ uptake for 60 min/day. Muscle biopsies were taken before, immediately after, and 3 h after the exercise bouts (on days 1 and 9). The gene expression of insulin receptor substrate-2 and the p85α subunit of phosphatidylinositol 3-kinase was significantly higher 3 h after a single exercise bout, although short-term training ameliorated this effect. Gene expression of insulin receptor and insulin receptor substrate-1 was not significantly altered at any time point. These results suggest that exercise may have a transitory impact on the expression of insulin receptor substrate-2 and phosphatidylinositol 3-kinase; however, the predominant actions of exercise on insulin sensitivity appear not to reside in the transcriptional activation of the genes encoding major insulin-signaling proteins.

Creatine transporter protein content, localization, and gene expression in rat skeletal muscle

The present study examined the gene expression and cellular localization of the creatine transporter (CreaT) protein in rat skeletal muscle. Soleus (SOL) and red (RG) and white gastrocnemius (WG) muscles were analyzed for CreaT mRNA, CreaT protein, and total creatine (TCr) content. Cellular location of the CreaT protein was visualized with immunohistochemical analysis of muscle cross sections. TCr was higher (P <= 0.05) in WG than in both RG and SOL, and was higher in RG than in SOL. Total CreaT protein content was greater (P <= 0.05) in SOL and RG than in WG. Two bands (55 and 70 kDa) of the CreaT protein were found in all muscle types. Both the 55-kDa (CreaT-55) and the 70-kDa (CreaT-70) bands were present in greater (P <= 0.05) amounts in SOL and RG than in WG. SOL and RG had a greater amount (P <= 0.05) of CreaT-55 than CreaT-70. Immunohistochemical analysis revealed that the CreaT was mainly associated with the sarcolemmal membrane in all muscle types. CreaT mRNA expression per microgram of total RNA was similar across the three muscle types. These data indicate that rat SOL and RG have an enhanced potential to transport Cr compared with WG, despite a higher TCr in the latter.

Differences in the neuromuscular capacity and lean muscle tissue in old and older community-dwelling adults

This study investigated whether there was a worsening of the neuromuscular capacity of older adults after the seventh decade of life. The results suggest that the age-related deterioration in maximal strength measures and rapid force production characteristics in older adults could be related to a reduction in the mass and neural activation of the thigh muscles.

Calpain 3 gene expression in skeletal muscle is associated with body fat content and measures of insulin resistance

OBJECTIVE: To investigate whether skeletal muscle gene expression of calpain 3 is related to obesity and insulin resistance.

DESIGN: Cross-sectional studies in 27 non-diabetic human subjects and in Psammomys obesus, a polygenic animal model of obesity and type 2 diabetes.

MEASUREMENTS: Expression of CAPN3 in skeletal muscle was measured using Taqman fluorogenic PCR. In the human subjects, body composition was assessed by DEXA and insulin sensitivity was measured by euglycemic-hyperinsulinemic clamp. In Psammomys obesus, body composition was determined by carcass analysis, and substrate oxidation rates, physical activity and energy expenditure were measured by whole-body indirect calorimetry.

RESULTS: In human subjects, calpain 3 gene expression was negatively correlated with total (P=0.022) and central abdominal fat mass (P=0.034), and with blood glucose concentration in non-obese subjects (P=0.017). In Psammomys obesus, calpain 3 gene expression was negatively correlated with circulating glucose (P=0.013) and insulin (P=0.034), and with body fat mass (P=0.049). Indirect calorimetry revealed associations between calpain 3 gene expression and carbohydrate oxidation (P=0.009) and energy expenditure (P=0.013).

CONCLUSION/INTERPRETATION: Lower levels of expression of calpain 3 in skeletal muscle were associated with reduced carbohydrate oxidation and elevated circulating glucose and insulin concentrations, and also with increased body fat and in particular abdominal fat. Therefore, reduced expression of calpain 3 in both humans and Psammomys obesus was associated with phenotypes related to obesity and insulin resistance.

Fasting activates the gene expression of UCP3 independent of genes necessary for lipid transport and oxidation in skeletal muscle

Fasting triggers a complex array of adaptive metabolic and hormonal responses including an augmentation in the capacity for mitochondrial fatty acid (FA) oxidation in skeletal muscle. This study hypothesized that this adaptive response is mediated by increased mRNA of key genes central to the regulation of fat oxidation in human skeletal muscle. Fasting dramatically increased UCP3 gene expression, by 5-fold at 15 h and 10-fold at 40 h. However the expression of key genes responsible for the uptake, transport, oxidation, and re-esterification of FA remained unchanged following 15 and 40 h of fasting. Likewise there was no change in the mRNA abundance of transcription factors. This suggests a unique role for UCP3 in the regulation of FA homeostasis during fasting as adaptation to 40 h of fasting does not require alterations in the expression of other genes necessary for lipid metabolism.

Exercise, diet and skeletal muscle gene expression

Skeletal muscle, as a consequence of its mass and great capacity for altered metabolism, has a major impact on whole-body metabolic homeostasis and is capable of remarkable adaptation in response to various physiological stimuli, including exercise and dietary intervention. Exercise-induced increases in skeletal muscle mRNA levels of a number of genes have been reported, due to transcriptional activation and/or increased mRNA stability. The cellular adaptations to exercise training appear to be due to the cumulative effects of transient increases in gene transcription after repeated exercise bouts. The relative importance of transcriptional (mRNA synthesis) and translational (mRNA stability or translational efficiency) mechanisms for the training-induced increases in skeletal muscle protein abundance remains to be fully elucidated. Dietary manipulation, and the associated alterations in nutrient availability and hormone levels, can also modify skeletal muscle gene expression, although fewer studies have been reported. A major challenge is to understand how exercise and diet exert their effects on gene and protein expression in skeletal muscle. In relation to exercise, potential stimuli include stretch and muscle tension, the pattern of motor nerve activity and the resultant calcium transients, the energy charge of the cell and substrate availability, oxygen tension and circulating hormones. These are detected by various cellular signaling mechanisms, acting on a range of downstream targets and a wide range of putative transcription factors. A key goal in the years ahead is to identify how alterations at the level of gene expression are coupled to the changes in skeletal muscle phenotype. It is clear that gene expression, although representing a specific site of regulation, is only one step in a complex cascade from the initial stimulus to the final phenotypic adaptation and integrated physiological response.

Effect of epinephrine on glucose disposal during exercise in humans: role of muscle glycogen

This study examined the effect of epinephrine on glucose disposal during moderate exercise when glycogenolytic flux was limited by low preexercise skeletal muscle glycogen availability. Six male subjects cycled for 40 min at 59 ± 1% peak pulmonary O₂ uptake on two occasions, either without (CON) or with (EPI) epinephrine infusion starting after 20 min of exercise. On the day before each experimental trial, subjects completed fatiguing exercise and then maintained a low carbohydrate diet to lower muscle glycogen. Muscle samples were obtained after 20 and 40 min of exercise, and glucose kinetics were measured using [6,6-²H]glucose. Exercise increased plasma epinephrine above resting concentrations in both trials, and plasma epinephrine was higher (P < 0.05) during the final 20 min in EPI compared with CON. Muscle glycogen levels were low after 20 min of exercise (CON, 117 ± 25; EPI, 122 ± 20 mmol/kg dry matter), and net muscle glycogen breakdown and muscle glucose 6-phosphate levels during the subsequent 20 min of exercise were unaffected by epinephrine infusion. Plasma glucose increased with epinephrine infusion (i.e., 20-40 min), and this was due to a decrease in glucose disposal (R_d) (40 min: CON, 33.8 ± 3; EPI, 20.9 ± 4.9 µmol · kg^-1 · min^-1, P < 0.05), because the exercise-induced rise in glucose rate of appearance was similar in the trials. These results show that glucose R_d during exercise is reduced by elevated plasma epinephrine, even when muscle glycogen availability and utilization are low. This suggests that the effect of epinephrine does not appear to be mediated by increased glucose 6-phosphate, secondary to enhanced muscle glycogenolysis, but may be linked to a direct effect of epinephrine on sarcolemmal glucose transport.

Exercise training increases lipid metabolism gene expression in human skeletal muscle

The effects of a single bout of exercise and exercise training on the expression of genes necessary for the transport and beta -oxidation of fatty acids (FA), together with the gene expression of transcription factors implicated in the regulation of FA homeostasis were investigated. Seven human subjects (3 male, 4 female, 28.9 ± 3.1 yr of age, range 20-42 yr, body mass index 22.6 kg/m², range 17-26 kg/m²) underwent a 9-day exercise training program of 60 min cycling per day at 63% peak oxygen uptake (V_{O2 peak}; 104 ± 14 W). On days 1 and 9 of the program, muscle biopsies were sampled from the vastus lateralis muscle at rest, at the completion of exercise, and again 3 h postexercise. Gene expression of key components of FA transport [FA translocase (FAT/CD36), plasma membrane-associated FA-binding protein], beta -oxidation [carntine palmitoyltransferase(CPT) I, beta -hydroxyacyl-CoA dehydrogenase] and transcriptional control [peroxisome proliferator-activated receptor (PPAR)alpha , PPARgamma , PPARgamma coactivator 1, sterol regulatory element-binding protein-1c] were unaltered by exercise when measured at the completion and at 3 h postexercise. Training increased total lipid oxidation by 24% (P < 0.05) for the 1-h cycling bout. This increased capacity for lipid oxidation was accompanied by an increased expression of FAT/CD36 and CPT I mRNA. Similarly, FAT/CD36 protein abundance was also upregulated by exercise training. We conclude that enhanced fat oxidation after exercise training is most closely associated with the genes involved in regulating FA uptake across the plasma membrane (FAT/CD36) and across the mitochondrial membrane (CPT I).

The role of puberty, media and popularity with peers on strategies to increase weight, decrease weight and increase muscle tone among adolescent boys and girls

The present study was concerned with the impact of pubertal development, relationships with peers and perceived pressure from the media on body dissatisfaction and body change behaviors among adolescent boys and girls. In particular, the study investigated the underresearched area of strategies to increase weight and muscle. The exploration of body change strategies among adolescent boys has been a neglected area of research. Methods: Respondents were 1185 adolescents (527 males, 598 females) who were enrolled in Grades 7 and 9. Participants completed measures of pubertal development, media and peer influence, body dissatisfaction and strategies to lose weight, increase weight and to increase muscle. Results: The findings demonstrated that girls were more likely than boys to adopt strategies to lose weight, whereas boys were more likely to adopt strategies to increase muscle tone (but not weight). For boys in both Years 7 and 9, the main predictors of body change strategies were puberty and, to a lesser extent, perceived popularity with peers. The major influences for Years 7 and 9 girls were puberty and the media, but these mainly focused on weight loss. For Year 9 girls, perceived popularity with opposite-sex peers also predicted body dissatisfaction and strategies to increase muscle tone. Conclusion: The implications of these findings for understanding factors related to a range of body change strategies for adolescent boys and girls are discussed.

Social comparisons and negative affect as indicators of problem eating and muscle preoccupation among children

The present study examined the utilization of social comparison practices and the role of negative affect in predicting body dissatisfaction, problem eating, and muscle preoccupation among young children. Participants were 236 children aged between 8 and 10 years. Children's eating, exercising, and muscle concerns were examined using a modified version of the Children's Eating Attitudes Test (ChEAT), which included additional items pertaining to muscle bulk and exercising. Consistent with past findings, body mass index (BMI) was found to be the sole unique indicator of body dissatisfaction for both boys and girls. Utilization of social comparison practices with adults was the main unique indicator of the modified ChEAT factors for boys, while BMI was the main unique indicator of the modified ChEAT factors for girls. In addition, negative affect was associated with binging, food preoccupation, and social pressure to eat for boys and dieting and muscle preoccupation for girls. Findings are discussed in relation to previous studies with adolescents and adults.