No abstract

Most of what we know about whole muscle behaviour comes from experiments on single fibres or small muscles that are scaled up in size without considering the effects of the additional muscle mass. Previous modelling studies have shown that tissue inertia acts to slow the rate of force development and maximum velocity of muscle during shortening contractions and decreases the work and power per cycle during cyclic contractions; however, these results have not yet been confirmed by experiments on living tissue. Therefore, in this study we conducted in situ work-loop experiments on rat plantaris muscle to determine the effects of increasing the mass of muscle on mechanical work during cyclic contractions. We additionally simulated these experimental contractions using a mass-enhanced Hill-type model to validate our previous modelling work. We found that greater added mass resulted in lower mechanical work per cycle relative to the unloaded trials in which no mass was added to the muscle (P=0.041 for both 85 and 123% increases in muscle mass). We additionally found that greater strain resulted in lower work per cycle relative to unloaded trials at the same strain to control for length change and velocity effects on the work output, possibly due to greater accelerations of the muscle mass at higher strains. These results confirm that tissue mass reduces muscle mechanical work at larger muscle sizes, and that this effect is likely amplified for lower activations.

Muscle is highly hierarchically organized, with functions shaped by genetically controlled expression of protein ensembles with different isoform profiles at the sarcomere scale. However, it remains unclear how isoform profiles shape whole muscle performance. We compared two mouse hind limb muscles, the slow, relatively parallel-fibered soleus (SOL) and the faster, more pennate-fibered tibialis anterior (TA), across scales: from gene regulation, isoform expression and translation speed, to force-length-velocity-power for intact muscles. Expression of myosin heavy-chain (MHC) isoforms directly corresponded with contraction velocity. The fast-twitch TA with fast MHC isoforms had faster unloaded velocities (actin sliding velocity, VACTIN; peak fiber velocity, VMAX) than slow-twitch SOL. For SOL, VACTIN was biased towards VACTIN for purely slow MHC I, despite this muscle's even fast and slow MHC isoform composition. Our multi-scale results clearly identified a consistent and significant dampening in fiber shortening velocities for both muscles, underscoring an indirect correlation between VACTIN and fiber VMAX that may be influenced by differences in fiber architecture, along with internal loading due to both passive and active effects. These influences correlate with the increased peak force and power in the slightly more pennate TA, leading to a broader length range of near-optimal force production. Conversely, a greater force-velocity curvature in the near-parallel fibered SOL highlights the fine-tuning by molecular-scale influences including myosin heavy and light chain expression along with whole muscle characteristics. Our results demonstrate that the individual gene, protein, and whole fiber characteristics do not directly reflect overall muscle performance but that intricate fine-tuning across scales shapes specialized muscle function.

Objectives To analyze the impact of different sources of protein (pea, whey or casein) on functional muscle performance in C57BL/6 mice. Methods A total of 21 mice were randomized to protein intervention groups. Mice were individually caged in a temperature controlled and 12-h light-dark cycle room. Subjects were randomly assigned to casein, whey, and pea protein sourced diets, matched by sex. Total energy (3.77 ± 0.04 kcal/g) and macronutrient composition (% of total energy: carbohydrate 66%, protein 18% and fat 16%) were matched across diets. Body weight and amount of food consumed was measured weekly. Functional muscle performance was measured by forelimb grip strength test using an Accuforce Cadet Force Gage and hanging test using Kondziela's inverted screen test capped at 600 seconds (at intervention week 9). The recorded strength and hang time measurements were corrected for body-mass. Data processing and analyses were performed in IBM SPSS Statistics 25. Results Excluding 2 mice due to outliers, the total number of mice per group were: casein (n = 9), whey (n = 6), and pea (n = 5). No baseline differences in body weight or average amount of food consumed per week were observed between groups. However, mice on the pea protein diet gained significantly more weight (8 ± 2g) compared to whey (4 ± 2g) and casein (2 ± 2g) diet groups (P < 0.007). Mice fed with whey protein sourced diets showed significantly stronger maximum fore limb grip strength (237 ± 21g) compared to pea (200 ± 7g) and casein (219 ± 26g) fed mice (P < 0.005). Body-mass corrected average and maximum grip strength tests showed that mice consuming pea protein sourced were significantly weaker compared to the casein protein sourced diet group, but not to whey protein sourced diet group (Table 1). Conclusions No observable differences were found between whey and casein in functional muscle performance of C57BL/6 mice when corrected for body weight. However, the pea sourced protein diet resulted in higher weight gain and weaker functional muscle performance measurements. Funding Sources University of Massachusetts Lowell Seed Grant (NK, KM, MG). Supporting Tables, Images and/or Graphs