Repetition Performance, Rating of Perceived Discomfort, and Blood Lactate Responses to Different Rest Interval Lengths in Single-Joint and Multijoint Lower-Body Exercise.

ABSTRACT: Rosa, A, Coleman, M, Haun, C, Grgic, J, and Schoenfeld, BJ. Repetition performance, rating of perceived discomfort, and blood lactate responses to different rest interval lengths in single-joint and multijoint lower-body exercise. J Strength Cond Res 37(7): 1350-1357, 2023-The purpose of this study was to examine the effects of different rest interval lengths (RILs) on repetition performance, rating of discomfort, and blood lactate responses during lower-body single-joint and multijoint exercises. This study used a counterbalanced design where each subject performed the Smith machine back squat (BS) and leg extension (LE) using 3 different RIL configurations (1, 2, and 3 minutes) in a randomized fashion. Data collection occurred over the span of 3 separate days. Volunteers were randomly allocated to perform the independent variables (RILs and exercises) in 1 of 12 potential configurations. The initial session was allotted for familiarization with the rating of discomfort scale and 10 repetition maximum testing. The other 2 sessions involved training with the different configurations of RIL length using both the BS and LE. Randomization ensured that the BS was performed first in one of the training sessions and the LE was performed first in the other session. Results indicated that longer RILs had a small positive effect on repetition performance, with longer rest durations allowing for more repetitions compared with shorter durations. The largest difference in repetition performance between RILs was observed between 1 minute and 2-3 minutes rest; there were trivial differences in repetition performance between 2 and 3 minutes rest for both the BS and LE. Blood lactate levels were slightly higher with longer RILs. Overall, BS showed greater increases in blood lactate compared with LE, and these differences were magnified over time. Exercise selection and RIL both influenced rating of discomfort, with LE producing less discomfort than BS and longer RILs reducing perceived discomfort. Our findings suggest that RIL influences the repetition performance, blood lactate, and rating of discomfort responses between single-joint and multijoint exercises.

Beyond Mechanical Tension: A Review of Resistance Exercise-Induced Lactate Responses & Muscle Hypertrophy.

The present review aims to explore and discuss recent research relating to the lactate response to resistance training and the potential mechanisms by which lactate may contribute to skeletal muscle hypertrophy or help to prevent muscle atrophy. First, we will discuss foundational information pertaining to lactate including metabolism, measurement, shuttling, and potential (although seemingly elusive) mechanisms for hypertrophy. We will then provide a brief analysis of resistance training protocols and the associated lactate response. Lastly, we will discuss potential shortcomings, resistance training considerations, and future research directions regarding lactate's role as a potential anabolic agent for skeletal muscle hypertrophy.

Effects of High-Volume Versus High-Load Resistance Training on Skeletal Muscle Growth and Molecular Adaptations.

We evaluated the effects of higher-load (HL) versus (lower-load) higher-volume (HV) resistance training on skeletal muscle hypertrophy, strength, and muscle-level molecular adaptations. Trained men (n = 15, age: 23 ± 3 years; training experience: 7 ± 3 years) performed unilateral lower-body training for 6 weeks (3× weekly), where single legs were randomly assigned to HV and HL paradigms. Vastus lateralis (VL) biopsies were obtained prior to study initiation (PRE) as well as 3 days (POST) and 10 days following the last training bout (POSTPR). Body composition and strength tests were performed at each testing session, and biochemical assays were performed on muscle tissue after study completion. Two-way within-subject repeated measures ANOVAs were performed on most dependent variables, and tracer data were compared using dependent samples t-tests. A significant interaction existed for VL muscle cross-sectional area (assessed via magnetic resonance imaging; interaction p = 0.046), where HV increased this metric from PRE to POST (+3.2%, p = 0.018) whereas HL training did not (-0.1%, p = 0.475). Additionally, HL increased leg extensor strength more so than HV training (interaction p = 0.032; HV < HL at POST and POSTPR, p < 0.025 for each). Six-week integrated non-myofibrillar protein synthesis (iNon-MyoPS) rates were also higher in the HV versus HL condition, while no difference between conditions existed for iMyoPS rates. No interactions existed for other strength, VL morphology variables, or the relative abundances of major muscle proteins. Compared to HL training, 6 weeks of HV training in previously trained men optimizes VL hypertrophy in lieu of enhanced iNon-MyoPS rates, and this warrants future research.

Whey Protein Supplementation Effects on Body Composition, Performance, and Blood Biomarkers During Army Initial Entry Training.

This study assesses if a lower dose of whey protein can provide similar benefits to those shown in previous work supplementing Army Initial Entry Training (IET) Soldiers with two servings of whey protein (WP) per day. Eighty-one soldiers consumed one WP or a calorie matched carbohydrate (CHO) serving/day during IET (WP: n = 39, height = 173 ± 8 cm, body mass = 76.8 ± 12.8 kg, age = 21 ± 3 years; CHO: n = 42, 175 ± 8 cm, 77.8 ± 15.3 kg, 23 ± 4 years). Physical performance (push-ups, sit-ups, and a two-mile run) was assessed during weeks two and eight. All other measures (dietary intake, body composition, blood biomarkers) at weeks one and nine. There was a significant group difference for fat mass (p = 0.044) as WP lost 2.1 ± 2.9 kg and had a moderate effect size (Cohen's d: -0.24), whereas the CHO group lost 0.9 ± 2.5 kg and had only a small effect size (d: -0.1). There was no significant group-by-time interaction on fat-free mass (p = 0.069). WP gained 1.2 ± 2.4 (d: 0.1) and CHO gained 0.1 ± 3 (d: 0) kg of FFM on average. There was a significant group by week 1-fat free mass interaction (p = 0.003) indicating individuals with higher initial fat-free mass benefitted more from WP. There were no group differences for push-up (p = 0.514), sit-up (p = 0.429) or run (p = 0.313) performance. For all biomarkers there was a significant effect of time as testosterone (p < 0.01), testosterone to cortisol ratio (p = 0.39), and IGF-1 (p < 0.01) increased across training and cortisol (p = 0.04) and IL-6 (p < 0.01) decreased. There were no differences in groups across IET for any of the biomarkers. We conclude one WP serving is beneficial for FM and for FFM in soldiers with high baseline FFM but may not significantly alter biomarker response or physical performance of IET soldiers who have high relative dietary protein intakes.

Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives.

Human muscle fibers are generally classified by myosin heavy chain (MHC) isoforms characterized by slow to fast contractile speeds. Type I, or slow-twitch fibers, are seen in high abundance in elite endurance athletes, such as long-distance runners and cyclists. Alternatively, fast-twitch IIa and IIx fibers are abundant in elite power athletes, such as weightlifters and sprinters. While cross-sectional comparisons have shown marked differences between athletes, longitudinal data have not clearly converged on patterns in fiber type shifts over time, particularly between slow and fast fibers. However, not all fiber type identification techniques are created equal and, thus, may limit interpretation. Hybrid fibers, which express more than one MHC type (I/IIa, IIa/IIx, I/IIa/IIx), may make up a significant proportion of fibers. The measurement of the distribution of fibers would necessitate the ability to identify hybrid fibers, which is best done through single fiber analysis. Current evidence using the most appropriate techniques suggests a clear ability of fibers to shift between hybrid and pure fibers as well as between slow and fast fiber types. The context and extent to which this occurs, along with the limitations of current evidence, are discussed herein.

Molecular Differences in Skeletal Muscle After 1 Week of Active vs. Passive Recovery From High-Volume Resistance Training.

ABSTRACT: Vann, CG, Haun, CT, Osburn, SC, Romero, MA, Roberson, PA, Mumford, PW, Mobley, CB, Holmes, HM, Fox, CD, Young, KC, and Roberts, MD. Molecular differences in skeletal muscle after 1 week of active vs. passive recovery from high-volume resistance training. J Strength Cond Res 35(8): 2102-2113, 2021-Numerous studies have evaluated how deloading after resistance training (RT) affects strength and power outcomes. However, the molecular adaptations that occur after deload periods remain understudied. Trained, college-aged men (n = 30) performed 6 weeks of whole-body RT starting at 10 sets of 10 repetitions per exercise per week and finishing at 32 sets of 10 repetitions per exercise per week. After this period, subjects performed either active (AR; n = 16) or passive recovery (PR; n = 14) for 1 week where AR completed ∼15% of the week 6 training volume and PR ceased training. Variables related to body composition and recovery examined before RT (PRE), after 6 weeks of RT (POST), and after the 1-week recovery period (DL). Vastus lateralis (VL) muscle biopsies and blood samples were collected at each timepoint, and various biochemical and histological assays were performed. Group × time interactions (p < 0.05) existed for skeletal muscle myosin heavy chain (MHC)-IIa mRNA (AR > PR at POST and DL) and 20S proteasome activity (post-hoc tests revealed no significance in groups over time). Time effects (P < 0.05) existed for total mood disturbance and serum creatine kinase and mechano growth factor mRNA (POST > PRE &D L), VL pressure to pain threshold and MHC-IIx mRNA (PRE&DL > POST), Atrogin-1 and MuRF-1 mRNA (PRE < POST < DL), MHC-I mRNA (PRE < POST & DL), myostatin mRNA (PRE & POST < DL), and mechanistic target of rapamycin (PRE > POST & DL). No interactions or time effects were observed for barbell squat velocity, various hormones, histological metrics, polyubiquitinated proteins, or phosphorylated/pan protein levels of 4E-BP1, p70S6k, and AMPK. One week of AR after a high-volume training block instigates marginal molecular differences in skeletal muscle relative to PR. From a practical standpoint, however, both paradigms elicited largely similar responses.

An intron variant of the GLI family zinc finger 3 (GLI3) gene differentiates resistance training-induced muscle fiber hypertrophy in younger men.

We examined the association between genotype and resistance training-induced changes (12 wk) in dual x-ray energy absorptiometry (DXA)-derived lean soft tissue mass (LSTM) as well as muscle fiber cross-sectional area (fCSA; vastus lateralis; n = 109; age = 22 ± 2 y, BMI = 24.7 ± 3.1 kg/m2 ). Over 315 000 genetic polymorphisms were interrogated from muscle using DNA microarrays. First, a targeted investigation was performed where single nucleotide polymorphisms (SNP) identified from a systematic literature review were related to changes in LSTM and fCSA. Next, genome-wide association (GWA) studies were performed to reveal associations between novel SNP targets with pre- to post-training change scores in mean fCSA and LSTM. Our targeted investigation revealed no genotype-by-time interactions for 12 common polymorphisms regarding the change in mean fCSA or change in LSTM. Our first GWA study indicated no SNP were associated with the change in LSTM. However, the second GWA study indicated two SNP exceeded the significance level with the change in mean fCSA (P = 6.9 × 10-7 for rs4675569, 1.7 × 10-6 for rs10263647). While the former target is not annotated (chr2:205936846 (GRCh38.p12)), the latter target (chr7:41971865 (GRCh38.p12)) is an intron variant of the GLI Family Zinc Finger 3 (GLI3) gene. Follow-up analyses indicated fCSA increases were greater in the T/C and C/C GLI3 genotypes than the T/T GLI3 genotype (P < .05). Data from the Auburn cohort also revealed participants with the T/C and C/C genotypes exhibited increases in satellite cell number with training (P < .05), whereas T/T participants did not. Additionally, those with the T/C and C/C genotypes achieved myonuclear addition in response to training (P < .05), whereas the T/T participants did not. In summary, this is the first GWA study to examine how polymorphisms associate with the change in hypertrophy measures following resistance training. Future studies are needed to determine if the GLI3 variant differentiates hypertrophic responses to resistance training given the potential link between this gene and satellite cell physiology.

Alterations in Body Composition, Resting Metabolic Rate, Muscular Strength, and Eating Behavior in Response to Natural Bodybuilding Competition Preparation: A Case Study.

Schoenfeld, BJ, Alto, A, Grgic, J, Tinsley, G, Haun, CT, Campbell, BI, Escalante, G, Sonmez, GT, Cote, G, Francis, A, and Trexler, ET. Alterations in body composition, resting metabolic rate, muscular strength, and eating behavior in response to natural bodybuilding competition preparation: A case study. J Strength Cond Res 34(11): 3124-3138, 2020-We carried out a prospective case study in a high-level amateur natural male bodybuilder throughout preparation for 4 competitions and during the ensuing postcontest recovery period. Laboratory testing was conducted monthly over a 1-year period, which included the following assessments: B-mode ultrasound evaluation of muscle thickness (MT), multifrequency bioelectrical impedance analysis, blood pressure and heart rate assessment, resting metabolic rate via indirect calorimetry, skinfold testing, vertical jump height, isometric lower-body strength testing, and a 3-factor eating questionnaire. Blood work (including testosterone, thyroid hormone, sex hormone binding globulin, glomerular filtration rate, blood urea nitrogen, aspartate aminotransferase, alanine aminotransferase, white blood count, albumin to globulin ratio, and lipoprotein A) was obtained separately from an outside laboratory at 4 time points. We also assessed the effectiveness of a carbohydrate (carb) deplete and carb load peaking strategy employed immediately before competition. The subject employed a high-volume, high-frequency, whole-body training program throughout the study period. Average daily nutritional intakes ranged from 1,953 to 3,415 kcal: 104-386 g carb; 253-263 g protein, and; 57-95 g lipid. Body fat was reduced to very low levels (∼5%) immediately before competition, but this corresponded with a loss of lean mass. Alterations in metabolism, hormonal status, explosive strength, and psychological aspects of eating were observed during precontest preparation; however, all of these variables recovered quickly postcompetition. The implementation of a carb depleteand carb load peaking strategy acutely increased MT and thus may be a viable precontest approach to maximize muscular aesthetics.

Sarcoplasmic Hypertrophy in Skeletal Muscle: A Scientific "Unicorn" or Resistance Training Adaptation?

Skeletal muscle fibers are multinucleated cells that contain mostly myofibrils suspended in an aqueous media termed the sarcoplasm. Select evidence suggests sarcoplasmic hypertrophy, or a disproportionate expansion of the sarcoplasm relative to myofibril protein accretion, coincides with muscle fiber or tissue growth during resistance training. There is also evidence to support other modes of hypertrophy occur during periods of resistance training including a proportional accretion of myofibril protein with fiber or tissue growth (i.e., conventional hypertrophy), or myofibril protein accretion preceding fiber or tissue growth (i.e., myofibril packing). In this review, we discuss methods that have been used to investigate these modes of hypertrophy. Particular attention is given to sarcoplasmic hypertrophy throughout. Thus, descriptions depicting this process as well as the broader implications of this phenomenon will be posited. Finally, we propose future human and rodent research that can further our understanding in this area of muscle physiology.

Markers of Bone Health and Impact of Whey Protein Supplementation in Army Initial Entry Training Soldiers: A Double-Blind Placebo-Controlled Study.

Training civilians to be soldiers is a challenging task often resulting in musculoskeletal injuries, especially bone stress injuries. This study evaluated bone health biomarkers (P1NP/CTX) and whey protein or carbohydrate supplementations before and after Army initial entry training (IET). Ninety male IET soldiers participated in this placebo-controlled, double-blind study assessing carbohydrate and whey protein supplementations. Age and fat mass predicted bone formation when controlling for ethnicity, explaining 44% (p < 0.01) of bone formation variations. Age was the only significant predictor of bone resorption (p = 0.02) when controlling for run, fat, and ethnicity, and these factors together explained 32% of the variance in bone resorption during week one (p < 0.01). Vitamin D increased across training (p < 0.01). There was no group by time interaction for supplementation and bone formation (p = 0.75), resorption (p = 0.73), Vitamin D (p = 0.36), or calcium (p = 0.64), indicating no influence of a supplementation on bone biomarkers across training. Age, fitness, fat mass, and ethnicity were important predictors of bone metabolism. The bone resorption/formation ratio suggests IET soldiers are at risk of stress injuries. Male IET soldiers are mildly to moderately deficient in vitamin D and slightly deficient in calcium throughout training. Whey protein or carbohydrate supplementations did not affect the markers of bone metabolism.

Do the anatomical and physiological properties of a muscle determine its adaptive response to different loading protocols?

It has been proposed that superior muscle hypertrophy may be obtained by training muscles predominant in type I fibers with lighter loads and those predominant in type II fibers with heavier loads. PURPOSE: To evaluate longitudinal changes in muscle strength and hypertrophy of the soleus (a predominantly slow-twitch muscle) and gastrocnemius (muscle with a similar composition of slow and fast-twitch fibers) when subjected to light (20-30 repetition maximum) and heavy (6-10 repetition maximum) load plantarflexion exercise. METHODS: The study employed a within-subject design whereby 26 untrained young men had their lower limbs randomized to perform plantarflexion with a low-load (LIGHT) and a high-load (HEAVY) for 8 weeks. Muscle thickness was estimated via B-mode ultrasound and maximal strength was determined by isometric dynamometry. RESULTS: Results showed that changes in muscle thickness were similar for the soleus and the gastrocnemius regardless of the magnitude of load used in training. Furthermore, each of the calf muscles demonstrated robust hypertrophy, with the lateral gastrocnemius showing greater gains compared to the medial gastrocnemius and soleus. Both HEAVY and LIGHT training programs elicited similar hypertrophic increases in the triceps surae. Finally, isometric strength increases were similar between loading conditions. CONCLUSIONS: The triceps surae muscles respond robustly to regimented exercise and measures of muscle hypertrophy and isometric strength appear independent of muscle fiber type composition. Moreover, the study provides further evidence that low-load training is a viable strategy to increase hypertrophy in different human muscles, with hypertrophic increases similar to that observed using heavy loads.

An optimized procedure for isolation of rodent and human skeletal muscle sarcoplasmic and myofibrillar proteins.

Several published protocols exist for isolating contractile or myofibrillar (MF) proteins from skeletal muscle, however, achieving complete resuspension of the myofibril pellet can be technically challenging. We performed several previously published MF isolation methods with the intent of determining which method was most suitable for MF protein isolation and solubilization. Here, we provide an optimized protocol to isolate sarcoplasmic and solubilized MF protein fractions from mammalian skeletal muscle suitable for several downstream assays.

Skeletal Muscle Myofibrillar Protein Abundance Is Higher in Resistance-Trained Men, and Aging in the Absence of Training May Have an Opposite Effect.

Resistance training generally increases skeletal muscle hypertrophy, whereas aging is associated with a loss in muscle mass. Interestingly, select studies suggest that aging, as well as resistance training, may lead to a reduction in the abundance of skeletal muscle myofibrillar (or contractile) protein (per mg tissue). Proteomic interrogations have also demonstrated that aging, as well as weeks to months of resistance training, lead to appreciable alterations in the muscle proteome. Given this evidence, the purpose of this small pilot study was to examine total myofibrillar as well as total sarcoplasmic protein concentrations (per mg wet muscle) from the vastus lateralis muscle of males who were younger and resistance-trained (denoted as YT, n = 6, 25 ± 4 years old, 10 ± 3 self-reported years of training), younger and untrained (denoted as YU, n = 6, 21 ± 1 years old), and older and untrained (denoted as OU, n = 6, 62 ± 8 years old). The relative abundances of actin and myosin heavy chain (per mg tissue) were also examined using SDS-PAGE and Coomassie staining, and shotgun proteomics was used to interrogate the abundances of individual sarcoplasmic and myofibrillar proteins between cohorts. Whole-body fat-free mass (YT > YU = OU), VL thickness (YT > YU = OU), and leg extensor peak torque (YT > YU = OU) differed between groups (p < 0.05). Total myofibrillar protein concentrations were greater in YT versus OU (p = 0.005), but were not different between YT versus YU (p = 0.325). The abundances of actin and myosin heavy chain were greater in YT versus YU (p < 0.05) and OU (p < 0.001). Total sarcoplasmic protein concentrations were not different between groups. While proteomics indicated that marginal differences existed for individual myofibrillar and sarcoplasmic proteins between YT versus other groups, age-related differences were more prominent for myofibrillar proteins (YT = YU > OU, p < 0.05: 7 proteins; OU > YT = YU, p < 0.05: 11 proteins) and sarcoplasmic proteins (YT = YU > OU, p < 0.05: 8 proteins; OU > YT&YU, p < 0.05: 29 proteins). In summary, our data suggest that modest (~9%) myofibrillar protein packing (on a per mg muscle basis) was evident in the YT group. This study also provides further evidence to suggest that notable skeletal muscle proteome differences exist between younger and older humans. However, given that our n-sizes are low, these results only provide a preliminary phenotyping of the reported protein and proteomic variables.

LAT1 Protein Content Increases Following 12 Weeks of Resistance Exercise Training in Human Skeletal Muscle.

Introduction: Amino acid transporters are essential for cellular amino acid transport and promoting protein synthesis. While previous literature has demonstrated the association of amino acid transporters and protein synthesis following acute resistance exercise and amino acid supplementation, the chronic effect of resistance exercise and supplementation on amino acid transporters is unknown. The purpose herein was to determine if amino acid transporters and amino acid metabolic enzymes were related to skeletal muscle hypertrophy following resistance exercise training with different nutritional supplementation strategies. Methods: 43 college-aged males were separated into a maltodextrin placebo (PLA, n = 12), leucine (LEU, n = 14), or whey protein concentrate (WPC, n = 17) group and underwent 12 weeks of total-body resistance exercise training. Each group's supplement was standardized for total energy and fat, and LEU and WPC supplements were standardized for total leucine (6 g/d). Skeletal muscle biopsies were obtained prior to training and ~72 h following each subject's last training session. Results: All groups increased type I and II fiber cross-sectional area (fCSA) following training (p < 0.050). LAT1 protein increased following training (p < 0.001) and increased more in PLA than LEU and WPC (p < 0.050). BCKDHα protein increased and ATF4 protein decreased following training (p < 0.001). Immunohistochemistry indicated total LAT1/fiber, but not membrane LAT1/fiber, increased with training (p = 0.003). Utilizing all groups, the change in ATF4 protein, but no other marker, trended to correlate with the change in fCSA (r = 0.314; p = 0.055); however, when regression analysis was used to delineate groups, the change in ATF4 protein best predicted the change in fCSA only in LEU (r 2 = 0.322; p = 0.043). In C2C12 myoblasts, LAT1 protein overexpression caused a paradoxical decrease in protein synthesis levels (p = 0.002) and decrease in BCKDHα protein (p = 0.001). Conclusions: Amino acid transporters and metabolic enzymes are affected by resistance exercise training, but do not appear to dictate muscle fiber hypertrophy. In fact, overexpression of LAT1 in vitro decreased protein synthesis.

Synergist ablation-induced hypertrophy occurs more rapidly in the plantaris than soleus muscle in rats due to different molecular mechanisms.

We examined molecular mechanisms that were altered during rapid soleus (type I fiber-dominant) and plantaris (type II fiber-dominant) hypertrophy in rats. Twelve Wistar rats (3.5 mo old; 6 female, 6 male) were subjected to surgical right-leg soleus and plantaris dual overload [synergist ablation (SA)], and sham surgeries were performed on left legs (CTL). At 14 days after surgery, the muscles were dissected. Plantaris mass was 27% greater in the SA than CTL leg (P < 0.001), soleus mass was 13% greater in the SA than CTL leg (P < 0.001), and plantaris mass was higher than soleus mass in the SA leg (P = 0.001). Plantaris total RNA concentrations and estimated total RNA levels (suggestive of ribosome density) were 19% and 47% greater in the SA than CTL leg (P < 0.05), protein synthesis levels were 64% greater in the SA than CTL leg (P = 0.038), and satellite cell number per fiber was 60% greater in the SA than CTL leg (P = 0.003); no differences in these metrics were observed between soleus SA and CTL legs. Plantaris, as well as soleus, 20S proteasome activity was lower in the SA than CTL leg (P < 0.05), although the degree of downregulation was greater in the plantaris than soleus muscle (-63% vs. -20%, P = 0.001). These data suggest that early-phase plantaris hypertrophy occurs more rapidly than soleus hypertrophy, which coincided with greater increases in ribosome biogenesis, protein synthesis, and satellite cell density, as well as greater decrements in 20S proteasome activity, in the plantaris muscle.

Calculating Set-Volume for the Limb Muscles with the Performance of Multi-Joint Exercises: Implications for Resistance Training Prescription.

Resistance training volume, determined by the number of sets performed (set-volume) is considered one of the key variables in promoting muscle hypertrophy. To better guide resistance exercise prescription for weekly per-muscle training volume, the purpose of this paper is to provide evidence-based considerations for set-volume ratios between multi-joint (MJ) and single-joint (SJ) exercises so that practitioners can better manage prescription of training volume in program design. We analyzed this topic from three primary areas of focus: (1) biomechanical and physiological factors; (2) acute research; and (3) longitudinal research. From a biomechanical and physiological standpoint, when considering force production of different muscle groups, the moment arm of a given muscle, "motor abundance", the link between biomechanics and exercise-induced fatigue, as well as the amount of time in voluntary muscle activation, a logical rationale can be made for SJ exercises producing greater hypertrophy of the limb muscles than MJ exercises (at least from specific exercises and under certain conditions). This would mean that sets for a MJ exercise should be counted fractionally for select muscles compared to an SJ exercise (i.e., less than a 1:1 ratio) when prescribing set-volumes for given muscles. When considering results from acute studies that measured muscle activation during the performance of SJ and MJ exercises, it seems that MJ exercises are not sufficient to maximize muscle activation of specific muscles. For example, during performance of the leg press and squat, muscle activation of the hamstrings is markedly lower than that of the quadriceps. These results suggest that a 1:1 ratio cannot be assumed. Current longitudinal research comparing the effects of training with MJ vs. SJ or MJ + SJ exercises is limited to the elbow flexors and the evidence is somewhat conflicting. Until more research is conducted to derive stronger conclusions on the topic, we propose the best advice would be to view set-volume prescription on a 1:1 basis, and then use logical rationale and personal expertise to make determinations on program design. Future research should focus on investigating longitudinal hypertrophic changes between MJ and SJ in a variety of populations, particularly resistance-trained individuals, while using site-specific measures of muscle growth to more systematically and precisely compute effective individualized set-volumes.

Muscle fiber hypertrophy in response to 6 weeks of high-volume resistance training in trained young men is largely attributed to sarcoplasmic hypertrophy.

Cellular adaptations that occur during skeletal muscle hypertrophy in response to high-volume resistance training are not well-characterized. Therefore, we sought to explore how actin, myosin, sarcoplasmic protein, mitochondrial, and glycogen concentrations were altered in individuals that exhibited mean skeletal muscle fiber cross-sectional area (fCSA) hypertrophy following 6 weeks of high-volume resistance training. Thirty previously resistance-trained, college-aged males (mean ± standard deviation: 21±2 years, 5±3 training years) had vastus lateralis (VL) muscle biopsies obtained prior to training (PRE), at week 3 (W3), and at week 6 (W6). Muscle tissue from 15 subjects exhibiting PRE to W6 VL mean fCSA increases ranging from 320-1600 µm2 was further interrogated using various biochemical and histological assays as well as proteomic analysis. Seven of these individuals donated a VL biopsy after refraining from training 8 days following the last training session (W7) to determine how deloading affected biomarkers. The 15 fCSA hypertrophic responders experienced a +23% increase in mean fCSA from PRE to W6 (p<0.001) and, while muscle glycogen concentrations remained unaltered, citrate synthase activity levels decreased by 24% (p<0.001) suggesting mitochondrial volume decreased. Interestingly, repeated measures ANOVAs indicated that p-values approached statistical significance for both myosin and actin (p = 0.052 and p = 0.055, respectively), and forced post hoc tests indicated concentrations for both proteins decreased ~30% from PRE to W6 (p<0.05 for each target). Phalloidin-actin staining similarly revealed actin concentrations per fiber decreased from PRE to W6. Proteomic analysis of the sarcoplasmic fraction from PRE to W6 indicated 40 proteins were up-regulated (p<0.05), KEGG analysis indicated that the glycolysis/gluconeogenesis pathway was upregulated (FDR sig. <0.001), and DAVID indicated that the following functionally-annotated pathways were upregulated (FDR value <0.05): a) glycolysis (8 proteins), b) acetylation (23 proteins), c) gluconeogenesis (5 proteins) and d) cytoplasm (20 proteins). At W7, sarcoplasmic protein concentrations remained higher than PRE (+66%, p<0.05), and both actin and myosin concentrations remained lower than PRE (~-50%, p<0.05). These data suggest that short-term high-volume resistance training may: a) reduce muscle fiber actin and myosin protein concentrations in spite of increasing fCSA, and b) promote sarcoplasmic expansion coincident with a coordinated up-regulation of sarcoplasmic proteins involved in glycolysis and other metabolic processes related to ATP generation. Interestingly, these effects seem to persist up to 8 days following training.

Bovine Milk Extracellular Vesicles (EVs) Modification Elicits Skeletal Muscle Growth in Rats.

The current study investigated how bovine milk extracellular vesicles (EVs) affected rotarod performance and biomarkers of skeletal muscle physiology in young, growing rats. Twenty-eight-day Fisher 344 rats were provided an AIN-93G-based diet for 4 weeks that either remained unadulterated [EVs and RNA-sufficient (ERS; n = 12)] or was sonicated [EVs and RNA-depleted (ERD; n = 12)]. Prior to (PRE) and on the last day of the intervention (POST), animals were tested for maximal rotarod performance. Following the feeding period, the gastrocnemius muscle was analyzed at the histological, biochemical, and molecular levels and was also used to measure mitochondrial function and reactive oxygen species (ROS) emission. A main effect of time was observed for rotarod time (PRE > POST, p = 0.001). Terminal gastrocnemius mass was unaffected by diet, although gastrocnemius muscle fiber cross sectional area was 11% greater (p = 0.018) and total RNA (a surrogate of ribosome density) was 24% greater (p = 0.001) in ERD. Transcriptomic analysis of the gastrocnemius indicated that 22 mRNAs were significantly greater in ERS versus ERD (p < 0.01), whereas 55 mRNAs were greater in ERD versus ERS (p < 0.01). There were no differences in gastrocnemius citrate synthase activity or mitochondrial coupling (respiratory control ratio), although mitochondrial ROS production was lower in ERD gastrocnemius (p = 0.016), which may be explained by an increase in glutathione peroxidase protein levels (p = 0.020) in ERD gastrocnemius. Dietary EVs profiling confirmed that sonication in the ERD diet reduced EVs content by ∼60%. Our findings demonstrate that bovine milk EVs depletion through sonication elicits anabolic and transcriptomic effects in the gastrocnemius muscle of rapidly maturing rats. While this did not translate into a functional outcome between diets (i.e., rotarod performance), longer feeding periods may be needed to observe such functional effects.

Pre-training Skeletal Muscle Fiber Size and Predominant Fiber Type Best Predict Hypertrophic Responses to 6 Weeks of Resistance Training in Previously Trained Young Men.

Limited evidence exists regarding differentially expressed biomarkers between previously-trained low versus high hypertrophic responders in response to resistance training. Herein, 30 college-aged males (training age 5 ± 3 years; mean ± SD) partook in 6 weeks of high-volume resistance training. Body composition, right leg vastus lateralis (VL) biopsies, and blood were obtained prior to training (PRE) and at the 3-week (W3) and 6-week time points (W6). The 10 lowest (LOW) and 10 highest (HIGH) hypertrophic responders were clustered based upon a composite hypertrophy score of PRE-to-W6 changes in right leg VL mean muscle fiber cross-sectional area (fCSA), VL thickness assessed via ultrasound, upper right leg lean soft tissue mass assessed via dual x-ray absorptiometry (DXA), and mid-thigh circumference. Two-way ANOVAs were used to compare biomarker differences between the LOW and HIGH clusters over time, and stepwise linear regression was performed to elucidate biomarkers that explained significant variation in the composite hypertrophy score from PRE to W3, W3 to W6, and PRE to W6 in all 30 participants. PRE-to-W6 HIGH and LOW responders exhibited a composite hypertrophy change of +10.7 ± 3.2 and -2.1 ± 1.6%, respectively (p < 0.001). Compared to HIGH responders, LOW responders exhibited greater PRE type II fCSA (+18%, p = 0.022). Time effects (p < 0.05) existed for total RNA/mg muscle (W6 > W3 > PRE), phospho (p)-4EBP1 (PRE > W3&W6), pan-mTOR (PRE > W3 < W6), p-mTOR (PRE > W3 < W6), pan-AMPKα (PRE > W3 < W6), pan-p70s6k (PRE > W3), muscle ubiquitin-labeled proteins (PRE > W6), mechano growth factor mRNA (W6 > W3&PRE), 45S rRNA (PRE > W6), and muscle citrate synthase activity (PRE > W3&W6). No interactions existed for the aforementioned biomarkers and/or other assayed targets (muscle 20S proteasome activity, serum total testosterone, muscle androgen receptor protein levels, muscle glycogen, or serum creatine kinase). Regression analysis indicated PRE type II fiber percentage (R 2 = 0.152, ß = 0.390, p = 0.033) and PRE type II fCSA (R 2 = 0.207, ß = -0.455, p = 0.019) best predicted the PRE-to-W6 change in the composite hypertrophy score. While our sample size is limited, these data suggest: (a) HIGH responders may exhibit more growth potential given that they possessed lower PRE type II fCSA values and (b) possessing a greater type II fiber percentage as a trained individual may be advantageous for hypertrophy in response to resistance training.