Changes in morphological and functional characteristics of male rat EDL muscle during growth

The influence of longitudinal muscle fascicle growth on mechanical function

Skeletal muscle has the remarkable ability to remodel and adapt, such as the increase in serial sarcomere number (SSN) or fascicle length (FL) observed after overstretching a muscle. This type of remodelling is termed longitudinal muscle fascicle growth, and its impact on biomechanical function has been of interest since the 1960s due to its clinical applications in muscle strain injury, muscle spasticity, and sarcopenia. Despite simplified hypotheses on how longitudinal muscle fascicle growth might influence mechanical function, existing literature presents conflicting results partly due to a breadth of methodologies. The purpose of this review is to outline what is currently known about the influence of longitudinal muscle fascicle growth on mechanical function and suggest future directions to address current knowledge gaps and methodological limitations. Various interventions indicate longitudinal muscle fascicle growth can increase the optimal muscle length for active force, but whether the whole force-length relationship widens has been less investigated. Future research should also explore the ability for longitudinal fascicle growth to broaden the torque-angle relationship's plateau region, and the relation to increased force during shortening. Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth also reduces passive tension at long muscle lengths; further research is required to understand whether this translates to increased joint range of motion. Lastly, some evidence suggests longitudinal fascicle growth can increase maximum shortening velocity and peak isotonic power, however, there has yet to be direct assessment of these measures in a neurologically intact model of longitudinal muscle fascicle growth.

Aging related changes in determinants of muscle force generating capacity: a comparison of muscle aging in men and male rodents

Human aging is associated with a progressive decline in skeletal muscle mass and force generating capacity, however the exact mechanisms underlying these changes are not fully understood. Rodents models have often been used to enhance our understanding of mechanisms of age-related changes in human skeletal muscle. However, to what extent age-related alterations in determinants of muscle force generating capacity observed in rodents resemble those in humans has not been considered thoroughly. This review compares the effect of aging on muscle force generating determinants (muscle mass, fiber size, fiber number, fiber type distribution and muscle specific tension), in men and male rodents at similar relative age. It appears that muscle aging in male F344*BN rat resembles that in men most; 32-35-month-old rats exhibit similar signs of muscle weakness to those of 70-80-yr-old men, and the decline in 36-38-month-old rats is similar to that in men aged over 80 yrs. For male C57BL/6 mice, age-related decline in muscle force generating capacity seems to occur only at higher relative age than in men. We conclude that the effects on determinants of muscle force differ between species as well as within species, but qualitatively show the same pattern as that observed in men.

Muscle-fiber characteristics in adult mouse-tongue muscles

To clarify functions of the mouse-tongue muscles, proteins such as myocin heavy chain (MHC) 2a and MHC-2b, which are isoforms of the fast-twitch fiber type myosin heavy chain, in the lateral margin of the tongue were observed by reverse transcription polymerase chain reaction and immunohistochemical analyses. The main MHC isoform in the superior longitudinal muscle of the tongue was MHC-2b, with the fastest function and the main MHC isoform in the transverse muscle of the tongue was MHC-2a. These findings suggested that the fastest function is necessary for the superior longitudinal muscle of the tongue, which is useful for moving the tongue in and out of the mouth in the sagittal direction, showing different cellular biological properties of the myofibers from those of the transverse muscle of the tongue.

Characteristics of myofibres in the masseter muscle of mice during postnatal growth period

Functional maturation of muscles is related to the constitutional proportion of muscle protein isoforms during development and growth. Although the mouse masseter muscle (MS) is classified as a fast limb muscle, its functions are different from those of a limb muscle. This study investigated the differentiation of myosin heavy chain (MHC) isoforms during the postnatal development periods in mouse MS and mouse tibialis anterior (TA), which is a fast limb muscle. Many anti-MHC slow-type-positive fibres were observed in neonatal MS and TA; these fibres decreased during development. Adult MS was composed of anti-MHC fast-type-positive fibres. MHC isoforms in MS were composed of MHC-2a and MHC-2d soon after birth. MHC-2b was expressed, but MHC-2a was not seen after 21 days. Expression of MHC-2b agreed with the weaning period, that is 2-3 weeks after birth. This fact suggested that the transformation from suckling to mastication changed the MHC isoforms during this period. In this study, the expressions of MHC-2b agree with the weaning period.

Specific force deficit in skeletal muscles of old rats is partially explained by the existence of denervated muscle fibers

We tested the hypothesis that denervated muscle fibers account for part of the specific force (sF(o)) deficit observed in muscles from old adult (OA) mammals. Whole muscle force (F(o)) was quantified for extensor digitorum longus (EDL) muscles of OA and young adult (YA) rats. EDL muscle sF(o) was calculated by dividing F(o) by either total muscle fiber cross-sectional area (CSA) or by innervated fiber CSA. Innervated fiber CSA was estimated from EDL muscle cross sections labeled for neural cell adhesion molecules, whose presence is a marker for muscle fiber denervation. EDL muscles from OA rats contained significantly more denervated fibers than muscles from YA rats (5.6% vs 1.1% of total CSA). When compared with YA muscle, OA muscle demonstrated deficits of 34.1% for F(o), 28.3% for sF(o), and 24.9% for sF(o) calculated by using innervated CSA as the denominator. Denervated muscle fibers accounted for 11.3% of the specific force difference between normal YA and OA skeletal muscle. Other mechanisms in addition to denervation account for the majority of the sF(o) deficit with aging.

Stapedius muscle fibre composition in the rat

The stapedius muscle (SM) is supposed to prevent cochlear damage by noise. Consequently functional demands are the ability of fast contraction with long endurance. This implies the presence of a large fraction of myosin type II fibres with an appreciable oxidative capacity. We determined the myosin composition of SM fibres using consecutive complete SM cross-sections (6 week old rats) which were processed by enzyme histochemistry (EHC) to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) or by immunohistochemistry (IHC) using myosin heavy chain (MyHC) antibodies. Method accuracy was determined in co-processed extensor digitorum longus (EDL). Four hundred SM and 200 EDL fibres were assigned to mATPase type I, IIA, IIB, IIX or 'miscellaneous' ('Misc') categories. Per mATPase category the fibres were attributed to groups with specific MyHC composition. In the EDL, mATPase type I and IIB fibres expressed only MyHC I and IIB respectively, whereas about 10% of the type IIA and 40% of the type IIX fibres expressed more than one MyHC. Thus IHC detects amounts of myosin isoforms which are not detected by EHC. The mATPase IIX category criterion leaves the possibility that this category contains fibres with myosin type IIA and/or IIB in larger amounts. The criteria of the mATPase categories type I, IIA or IIB preclude assignment to these categories of fibres which also contain other myosin isoforms in larger amounts. Such fibres were classified in one of the mATPase 'Misc' categories. Thus in the EDL the capability of the EHC criteria to select 'pure' fibres in terms of myosin differs per mATPase category. None of the SM fibres were assigned to the mATPase type I or IIB categories, about 25% to the type IIA, 60% to type IIX and 15% (including most fibres which expressed MyHC I) to a 'Misc' category. All SM fibres expressed two or more MyHC isoforms, MyHC IIB occurring in all fibres and substantial amounts of MyHC IIA and/or IIX in most. These findings confirm the hypothesis that such fibres have the capacity to contract fast and have the better fatigue resistance.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Effect of growth on efficiency and fatigue in extensor digitorum longus muscle of the rat

The effect of growth on work output, energy consumption and efficiency during repetitive dynamic contractions was determined using extensor digitorum longus muscles of 40-, 60-, 120- and 700-day-old male Wistar rats. When work output of each contraction was normalized to the work output of the first contraction it was found that work output initially increased over the first 10-20 contractions by approximately 8% in each age group. Thereafter a faster decrease in work output was found in the youngest group (approximately 2% each contraction) compared to the older groups (approximately 0.7% each contraction). After 40 contractions the reduction in work output was significantly different only between the youngest group and the two oldest groups (-30% vs -5%). These differences in fatigue were not associated with differences in adenosine 5'-triphosphate and phosphocreatine concentrations or in lactate production. Total work output and high-energy phosphate consumption increased by approximately 555% and 380% from age 40 to 120 days, respectively. Consequently, efficiency was significantly higher (approximately 32%) in the older groups compared to 40-day-old animals. Normalized for muscle mass, mean rate of high-energy phosphate consumption was similar in all groups whereas mean power output was significantly lower in the youngest group (approximately 46%). Thus, the difference in efficiency between the young and the other groups may be attributed to a lower external power production in the youngest group rather than changes in energy turnover.