Proteome profiles of intramuscular connective tissue: influence of aging and physical training.

Both aging and physical activity can influence the amount of intramuscular connective tissue in skeletal muscle, but the impact of these upon specific extracellular matrix (ECM) proteins in skeletal muscle is unknown. We investigated the proteome profile of intramuscular connective tissue by label-free proteomic analysis of cellular protein-depleted extracts from lateral gastrocnemius muscle of old (22-23 mo old) and middle-aged (11 mo old) male mice subjected to three different levels of regular physical activity for 10 wk (high-resistance wheel running, low-resistance wheel running, or sedentary controls). We hypothesized that aging is correlated with an increased amount of connective tissue proteins in skeletal muscle and that regular physical activity can counteract these age-related changes. We found that dominating cellular proteins were diminished in the urea/thiourea extract, which was therefore used for proteomics. Proteomic analysis identified 482 proteins and showed enrichment for ECM proteins. Statistical analysis revealed that the abundances of 86 proteins changed with age. Twenty-three of these differentially abundant proteins were identified as structural ECM proteins (e.g., collagens and laminins) and all of these were significantly more abundant with aging. No significant effect of training or interaction between training and advance in age was found for any proteins. Finally, we found a lower protein concentration in the urea/thiourea extracts from the old mice compared with the middle-aged mice. Our findings indicate that the intramuscular ECM solubility is affected by increased age but is not altered by physical training.NEW & NOTEWORTHY We investigated the impact of aging and exercise on extracellular matrix components of intramuscular connective tissue using proteomics. Middle-aged and old mice were subjected to three different levels of regular physical activity for 10 wk (high-resistance wheel running, low-resistance wheel running, or sedentary controls). We prepared extracts of extracellular matrix proteins depleted of cellular proteins. Our findings indicate that intramuscular connective tissue alters its soluble protein content with age but is unaffected by training.

Intramuscular connective tissue content and mechanical properties: Influence of aging and physical activity in mice.

Aging is accompanied by morphological and mechanical changes to the intramuscular connective tissue (IMCT) of skeletal muscles, but whether physical exercise can influence these changes is debated. We investigated the effects of aging and exercise with high or low resistance on composition and mechanical properties of the IMCT, including direct measurements on isolated IMCT which has rarely been reported. Middle-aged (11 months, n = 24) and old (22 months, n = 18) C57BL/6 mice completed either high (HR) or low (LR) resistance voluntary wheel running or were sedentary (SED) for 10 weeks. Passive mechanical properties of the intact soleus and plantaris muscles and the isolated IMCT of the plantaris muscle were measured in vitro. IMCT thickness was measured on picrosirius red stained cross sections of the gastrocnemius and soleus muscle and for the gastrocnemius hydroxyproline content was quantified biochemically and advanced glycation end-products (AGEs) estimated by fluorometry. Mechanical stiffness, IMCT content and total AGEs were all elevated with aging in agreement with previous findings but were largely unaffected by training. Conclusion: IMCT accumulated with aging with a proportional increase in mechanical stiffness, but even the relatively high exercise volume achieved with voluntary wheel-running with or without resistance did not significantly influence these changes.

The influence of fibrillin-1 and physical activity upon tendon tissue morphology and mechanical properties in mice.

Fibrillin-1 mutations cause pathological changes in connective tissue that constitute the complex phenotype of Marfan syndrome. In this study, we used fibrillin-1 hypomorphic and haploinsufficient mice (Fbn1mgr/mgR and Fbn1+/- mice, respectively) to investigate the impact of fibrillin-1 deficiency alone or in combination with regular physical activity on tendon tissue morphology and mechanical properties. Morphological and biomechanical analyses revealed that Fbn1mgr/mgR but not Fbn1+/- mice displayed smaller tendons with physical properties that were unremarkable when normalized to tendon size. Fbn1mgR/mgR mice (n = 43) Fbn1+/- mice (n = 27) and wild-type mice (WT, n = 25) were randomly assigned to either control cage conditions (n = 54) or to a running on a running wheel for 4 weeks (n = 41). Both fibrillin-1-deficient mice ran voluntarily on the running wheel in a manner similar to WT mice (3-4 km/24 h). Regular exercise did not mitigate aneurysm progression in Fbn1mgR/mgR mice (P < 0.05) as evidenced by unmodified median survival. In spite of the smaller size, tendons of fibrillin-1-deficient mice subjected to regular exercise showed no evidence of overt histopathological changes or tissue overload. We therefore concluded that lack of optimal fibrillin-1 synthesis leads to a down regulation of integrated tendon formation, rather than to a loss of tendon quality, which also implies that fibrillin-1 deficiency in combination with exercise is not a suitable animal model for studying the development of tendon overuse (tendinopathy).

Muscle-specific changes in length-force characteristics of the calf muscles in the spastic Han-Wistar rat.

The purpose of the present study was to investigate muscle mechanical properties and mechanical interaction between muscles in the lower hindlimb of the spastic mutant rat. Length-force characteristics of gastrocnemius (GA), soleus (SO), and plantaris (PL) were assessed in anesthetized spastic and normally developed Han-Wistar rats. In addition, the extent of epimuscular myofascial force transmission between synergistic GA, SO, and PL, as well as between the calf muscles and antagonistic tibialis anterior (TA), was investigated. Active length-force curves of spastic GA and PL were narrower with a reduced maximal active force. In contrast, active length-force characteristics of spastic SO were similar to those of controls. In reference position (90° ankle and knee angle), higher resistance to ankle dorsiflexion and increased passive stiffness was found for the spastic calf muscle group. At optimum length, passive stiffness and passive force of spastic GA were decreased, whereas those of spastic SO were increased. No mechanical interaction between the calf muscles and TA was found. As GA was lengthened, force from SO and PL declined despite a constant muscle-tendon unit length of SO and PL. However, the extent of this interaction was not different in spastic rats. In conclusion, the effects of spasticity on length-force characteristics were muscle specific. The changes observed for GA and PL muscles are consistent with the changes in limb mechanics reported for human patients. Our results indicate that altered mechanics in spastic rats cannot be attributed to differences in mechanical interaction, but originate from individual muscular structures.

Effect of generalized joint hypermobility on knee function and muscle activation in children and adults.

INTRODUCTION: We investigated muscle activation strategy and performance of knee extensor and flexor muscles in children and adults with generalized joint hypermobility (GJH) and compared them with controls. METHODS: Muscle activation, torque steadiness, electromechanical delay, and muscle strength were evaluated in 39 children and 36 adults during isometric knee extension and flexion. Subjects performed isometric maximum contractions, submaximal contractions at 25% maximum voluntary contraction (MVC), and explosive contractions. RESULTS: Agonist activation was reduced, and coactivation ratio was greater in GJH during knee flexion compared with controls. Torque steadiness was impaired in adults with GJH during knee flexion. No effect of GJH was found on muscle strength or electromechanical delay. Correlation analysis revealed an association between GJH severity and function in adults. CONCLUSIONS: The results indicate that muscle activation strategy and quality of force control were significantly affected in adults with GJH during knee flexion, whereas only muscle activation strategy was affected in children with GJH.