Efficacy of myonuclear addition may explain differential myofiber growth among resistance-trained young and older men and women

2006 ◽  
Vol 291 (5) ◽  
pp. E937-E946 ◽  
Author(s):  
John K. Petrella ◽  
Jeong-su Kim ◽  
James M. Cross ◽  
David J. Kosek ◽  
Marcas M. Bamman
Keyword(s):  
Resistance Training ◽  
Cell Activation ◽  
Vastus Lateralis ◽  
Knee Extensor ◽  
Free Testosterone ◽  
Sex Hormone Binding Globulin ◽  
Muscle Stem Cells ◽  
Igf I ◽  
Paracrine Growth Factors ◽  
Igf Binding

Skeletal muscle stem (satellite) cells supporting growth/regeneration are thought to be activated and incorporated into growing myofibers by both endocrine and locally expressed autocrine/paracrine growth factors, the latter being load sensitive. We recently found that myofiber hypertrophy with resistance training is superior in young men (YM) vs. young women and older adults (Kosek DJ, Kim JS, Petrella JK, Cross JM, and Bamman MM. J Appl Physiol 101: 531–544, 2006). We hypothesized that the advanced myofiber hypertrophy in YM is facilitated by myonuclear addition in response to a milieu promoting stem cell activation. Twenty-six young (27.0 ± 1 yr, 50% women) and 26 older (63.7 ± 1 yr, 50% women) adults completed 16 wk of knee extensor resistance training. Vastus lateralis biopsies were obtained at baseline, 24 h after one bout, and after 16 wk. Muscle stem cells were identified immunohistochemically with anti-neural cell adhesion molecule (NCAM+). Muscle transcript levels of IGF-I and mechanogrowth factor (MGF) were determined by RT-PCR. Serum IGF-I, IGF-binding protein (IGFBP)-3, IGFBP-1, total and free testosterone, sex hormone-binding globulin (SHBG), and androstenedione were assessed by radioimmunoassay. Myofiber hypertrophy was twofold greater in YM vs. others, and only YM increased NCAM+ cells per 100 myofibers (49%) and myonuclei per fiber (19%) ( P < 0.05). IGF-IEa mRNA was higher in young and increased acutely (29%) with summation by 16 wk (96%) ( P < 0.05). MGF mRNA increased only in young after one bout (81%) and by 16 wk (85%) ( P < 0.001). Circulating IGF-I was twofold higher in young, whereas IGFBP-1 was lowest in YM ( P < 0.05). Among men, free testosterone was 59% higher in YM ( P < 0.01). Myonuclear addition was most effectively accomplished in YM, which likely drove the superior growth.

Effects of heavy-resistance training on hormonal response patterns in younger vs. older men

1999 ◽  
Vol 87 (3) ◽  
pp. 982-992 ◽  
Author(s):  
William J. Kraemer ◽  
Keijo Häkkinen ◽  
Robert U. Newton ◽  
Bradley C. Nindl ◽  
Jeff S. Volek ◽  
...  
Keyword(s):  
Resistance Training ◽  
Training Program ◽  
Free Testosterone ◽  
Older Men ◽  
Total Testosterone ◽  
Igf I ◽  
Before And After ◽  
Igf Binding ◽  
Heavy Resistance Training ◽  
Igf Binding Protein

To examine the adaptations of the endocrine system to heavy-resistance training in younger vs. older men, two groups of men (30 and 62 yr old) participated in a 10-wk periodized strength-power training program. Blood was obtained before, immediately after, and 5, 15, and 30 min after exercise at rest before and after training and at rest at −3, 0, 6, and 10 wk for analysis of total testosterone, free testosterone, cortisol, growth hormone, lactate, and ACTH analysis. Resting values for insulin-like growth factor (IGF)-I and IGF-binding protein-3 were determined before and after training. A heavy-resistance exercise test was used to evaluate the exercise-induced responses (4 sets of 10-repetition maximum squats with 90 s of rest between sets). Squat strength and thigh muscle cross-sectional area increased for both groups. The younger group demonstrated higher total and free testosterone and IGF-I than the older men, training-induced increases in free testosterone at rest and with exercise, and increases in resting IGF-binding protein-3. With training the older group demonstrated a significant increase in total testosterone in response to exercise stress along with significant decreases in resting cortisol. These data indicate that older men do respond with an enhanced hormonal profile in the early phase of a resistance training program, but the response is different from that of younger men.

scholarly journals Discontinuation of estrogen replacement therapy in GH-treated hypopituitary women alters androgen status and IGF-I

2005 ◽  
Vol 152 (5) ◽  
pp. 719-726 ◽  
Author(s):  
Jens Juel Christiansen ◽  
Sanne Fisker ◽  
Claus Højbjerg Gravholt ◽  
Paul Bennett ◽  
Birgit Svenstrup ◽  
...  
Keyword(s):  
Menopausal Status ◽  
Free Testosterone ◽  
Sex Hormone Binding Globulin ◽  
Control Group ◽  
Estrogen Replacement ◽  
Igf I ◽  
Igf Binding ◽  
Igf Binding Protein ◽  
Healthy Females ◽  
Igfbp 3

Objective and design: Compared with their male counterparts, healthy females secrete more growth hormone (GH) and those with GH-deficiency have lower insulin-like growth factor I (IGF-I) levels and are less responsive to GH substitution. To test whether this gender difference is related to sex hormones we measured androgen status and IGF-I related parameters in 38 hypopituitary women (mean (range) age 41.5 (20–58) years) during continued GH substitution as compared with a control group of 38 healthy women matched for age and menopausal status. Twenty six patients were studied twice: with estrogen replacement and after 28 days of estrogen discontinuation in a randomised design. Results: The patients were androgen deficient compared with controls (median, range), dehydroepiandrosterone sulphate (DHEAS): 185 (99–7800) nmol/l vs 4400 (820–13 000) nmol/l, P = < 0.001; androstenedione: 0.5 (0.1–7.1) nmol/l vs 4.3 (1.6–8.8) nmol/l, P = < 0.001; dihydrotestosterone (DHT): 0.13 (0.09–0.54) nmol/l vs 0.55 (0.09–0.89) nmol/l, P = < 0.001; testosterone: 0.28 (0.09–1.56) nmol/l vs 1.1 (0.71–2.24) nmol/l, (P = < 0.001); free testosterone: 0.004 (0.001–0.030) nmol/l vs 0.016 (0.001–0.030) nmol/l, P = < 0.001. The circulating levels of IGF-I, IGF-II, IGF-binding protein 1 (IGFBP-1), and IGFBP-3 did not differ between patients and controls. The subgroup of patients receiving hydrocortisone (HC) replacement (n = 24) had significantly lower levels of androgens (suppressed by 80–100%) as well as IGF-I and IGFBP-3 as compared with the patients not receiving HC. IGF-I was correlated to free testosterone in patients (r = 0.57, P = 0.0005) as well as controls (r = 0.43, P = 0.008), and free testosterone was a significant positive predictor of IGF-I. Estrogen discontinuation induced an increase in IGF-I (167 ± 15 vs 206 ± 14 μg/l, P = 0.005 and IGFBP-3 (3887 ± 139 vs 4309 ± 138 μg/l, P = 0.0005). Estrogen discontinuation was associated with a significant increase in median (range) free testosterone (0.004 (0–0.02) vs 0.0065 (0–0.03) nmol/l, P = 0.001) and a significant decrease in median (range) sex-hormone binding globulin (SHBG; 93 (11–278) vs 55.5 (20–142) nmol/l, P = 0.001). ΔIGF-I correlated with ΔSHBG (r = −0.45 P = 0.033) and ΔIGFBP-3 (r = 0.67 P = < 0.001). In a regression model ΔE2, Δtestosterone, ΔSHBG and ΔIGFBP-3 explained 93% of the variation in ΔIGF-I. Conclusions: Androgen levels are low in hypopituitary women and free testosterone correlates with IGF-I. Discontinuation of estrogen replacement in these patients induces elevations in IGF-I as well as free testosterone, and ΔIGF-I correlated positively with Δfree testosterone. These effects may contribute to the gender differences observed in the GH–IGF axis in healthy adults as well as in the responsiveness of hypopituitary patients to GH substitution.

Effects of Heavy Resistance/Power Training on Maximal Strength, Muscle Morphology, and Hormonal Response Patterns in 60-75-Year-Old Men and Women

2002 ◽  
Vol 27 (3) ◽  
pp. 213-231 ◽  
Author(s):  
Keijo Häkkinen ◽  
William J. Kraemer ◽  
Arto Pakarinen ◽  
Travis Tripleltt-Mcbride ◽  
Jeffrey M. Mcbride ◽  
...  
Keyword(s):  
Growth Hormone ◽  
Rise Time ◽  
Vastus Lateralis ◽  
Free Testosterone ◽  
Dehydroepiandrosterone Sulfate ◽  
Sex Hormone Binding Globulin ◽  
Type I ◽  
Key Words Aging ◽  
Power Training ◽  
Chair Rise

Eleven women (TRW; 64 ± 4 yrs) and ten men (TRM; 65 ± 5 yrs) participated in the strength/power training twice a week for 24 weeks. Basal concentrations of serum total and free testosterone, growth hormone (GH), dehydroepiandrosterone sulfate (DHEAS), cortisol and sex hormone binding globulin (SHBG) as well as acute responses of serum total and free testosterone, growth hormone (GH) were measured. Maximal 1RM strength in the squat, chair rise time and muscle fibre distribution and areas of type I and IIa and IIb of the vastus lateralis were also examined. 1RM squat increased in TRW by 26 (SDIO) % (p < .001), and in TRM by 35(7) % (p < .001) and chair rise time improved in both groups (p < .001). Fibre areas increased in type I, (p < .01), IIa (p < .01) and IIb (p < .01) in TRM and type I (p < .05) and IIa (p < .05) in TRW. The proportion of type IIa increased from 31% to 43% (p < .05) in TRW and that of type IIb decreased from 27% to 17% (p < .05) in TRW and from 25% to 17% (p < .05) in TRM. Individual concentrations of testosterone/cortisol ratios correlated (r = 0.63; p < .05) with the individual increases in 1RM strength in TRW. The exercise sessions resulted in acute increases in serum GH in both groups (p < .05) with a further increase (p < .01) up to 10 minutes post-loading in TRM at post-training. Key words: aging, strength training, power training, muscle hypertrophy, serum hormones

scholarly journals Sexual dimorphism in the maturation of the pituitary-gonadal axis, assessed by GnRH agonist challenge

1999 ◽  
pp. 27-34 ◽  
Author(s):  
N Potau ◽  
L Ibanez ◽  
M Sentis ◽  
A Carrascosa
Keyword(s):  
Gnrh Agonist ◽  
Cross Sectional Study ◽  
Leuprolide Acetate ◽  
Sex Hormone Binding Globulin ◽  
Healthy Population ◽  
Aromatase Activity ◽  
Cross Sectional ◽  
Igf I ◽  
Igf Binding ◽  
Gender Specific

OBJECTIVE: To assess whether the maturational changes of the pituitary--gonadal axis in a healthy population show gender-specific changes and to establish normative data for the different Tanner stages. DESIGN: Prospective, cross-sectional study. METHODS: The GnRH agonist leuprolide acetate (500 microgram) was administered s.c. to 60 boys and 81 girls (age range, 5--17 years). Serum steroids and gonadotropins were determined at 0 and 24 h and at 0, 3 and 24 h after GnRH agonist challenge respectively, whereas IGF-I, IGF-binding protein-1 (IGFBP-1), IGFBP-3 and sex hormone-binding globulin were measured at baseline. RESULTS: Baseline and peak LH responses to the agonist in late puberty, and basal and peak FSH levels at all Tanner stages, were higher in girls than in boys. Girls showed higher IGF-I levels than boys throughout puberty, sharper decreases in IGFBP-1 and earlier and greater increases in 17-hydroxypregnenolone, dehydroepiandrosterone (DHEA) and DHEA-sulfate. Testosterone responses to the agonist increased during puberty in males, and showed no changes in females. Conversely, estradiol responses rose throughout puberty in females and remained unchanged until late puberty in males. CONCLUSION: Leuprolide acetate stimulates gonadotropin and gonadal steroid secretion during puberty in both sexes and increases FSH levels in prepubertal girls. Pubertal maturation of gonadotrope function is gender specific, as it appears to involve increases in both the releasable and reserve pools of LH in males, and of LH and FSH in females. The earlier increase in Delta(5)-steroids in girls may suggest a sharper rise in ovarian cytochrome P450c17 activity along the Delta(5)-steroid pathway, while the failure of estradiol to increase in response to leuprolide acetate in early pubertal males suggests a late maturation of aromatase activity.

Weight loss and wrestling training: effects on growth-related hormones

1997 ◽  
Vol 82 (6) ◽  
pp. 1760-1764 ◽  
Author(s):  
James N. Roemmich ◽  
Wayne E. Sinning
Keyword(s):  
Weight Loss ◽  
Free Testosterone ◽  
Sex Hormone Binding Globulin ◽  
Training Effects ◽  
Dietary Practices ◽  
Late Season ◽  
Igf I ◽  
Receptor Number ◽  
Serial Samples ◽  
Gh Resistance

Roemmich, James N., and Wayne E. Sinning. Weight loss and wrestling training: effects on growth-related hormones. J. Appl. Physiol. 82(6): 1760–1764, 1997.—Adolescent wrestlers ( n = 9, 15.4 yr) and recreationally active control males ( n = 7, 15.7 yr) were measured before, at the end of, and 3.5–4 mo after a competitive wrestling season to assess the influence of dietary restriction on growth-related hormones. Wrestlers had significant elevations preseason to late season for morning serum concentrations (mean of 8 serial samples) of growth hormone (GH; 2.9 ± 0.7 vs. 6.5 ± 1.4 ng/ml) and sex hormone-binding globulin (SHBG; 16.1 ± 2.3 vs. 27.9 ± 6.9 nmol/l) and significant reductions in GH-binding protein (GHBP; 178 ± 19 vs. 109 ± 17 pmol/l), insulin-like growth factor I (IGF-I; 332 ± 30 vs. 267 ± 34 ng/ml), testosterone (T; 4.9 ± 0.4 vs. 3.6 ± 0.4 ng/ml), and free testosterone (Free-T; 22.4 ± 3.6 vs. 15.7 ± 2.8 pg/ml). Wrestlers had significant postseason reductions in GH (3.44 ± 1.30 ng/ml) and SHBG (10.43 ± 4.13 nmol/l) but elevations in GHBP (66.7 ± 23.8 pmol/l), IGF-I (72.9 ± 25.1 ng/ml), T (2.10 ± 0.46 ng/ml), and Free-T (9.76 ± 3.01 pg/ml). Concentrations of luteinizing hormone (LH), estradiol, prolactin, cortisol, insulin, and thyroid hormones did not differ because of exercise-dietary practices of wrestlers. In-season elevations in GH, with concomitant reductions in GHBP and IGF-I, that were reversed during the postseason suggest a reduction in GH receptor number and partial GH resistance during the season. Nonelevated LH with reduced T levels suggests a central hypothalamic-pituitary-gonadal (H-P-G) axis impairment. In conclusion, undernutrition may lead to altered H-P-G and GH-IGF-I axes function in adolescent wrestlers. However, only the wrestlers’ late-season Free-T concentrations were outside the normal range, and the hormone axis impairments were quickly reversed. The present data do not address hormonal axis responses to several years of wrestling and weight loss.

Lunges activate the gluteus maximus muscles more than back squats when both exercises are standardized

2021 ◽  
pp. 1-7
Author(s):  
Rodrigo Ramalho Aniceto ◽  
André Luiz Torres Pirauá ◽  
Leonardo da Silva Leandro ◽  
Hélen Cristina Ferreira da Silva ◽  
Diego Mesquita Silva ◽  
...  
Keyword(s):  
Resistance Training ◽  
Vastus Lateralis ◽  
Knee Extensor ◽  
Gluteus Maximus ◽  
Biceps Femoris ◽  
Lower Limbs ◽  
Emg Activity ◽  
Flexor Muscle ◽  
Back Squat ◽  
Trunk Inclination

BACKGROUND: Squats are considered one of the main exercises for the lower limbs and are used in resistance training under different contexts, including rehabilitation and sports performance. OBJECTIVE: To compare the EMG activity of different muscles in back squat and lunge exercises in trained women. METHODS: Ten healthy women experienced in resistance training performed back squat and lunge exercises on a Smith machine (total work: 70% of 1RM, 1 set, 10 repetitions and 2-s/2-s of execution speed) with an interval of 20-min between exercises. Both exercises were standardized in relation to the trunk inclination and were performed with an erect trunk parallel to the cursor of the guided bar. RESULTS: The EMG activity of the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF), and gluteus maximus (GM) were analyzed. There were no significant differences in the EMG activity of the VM, VL, and BF muscles between the back squat and lunge exercises (P> 0.05); however, GM activation was greater during the lunge exercise (effect size = 1.15; P= 0.001). CONCLUSIONS: Lunges were more effective in recruiting the GM when compared to back squats. However, both exercises can be recommended when the goal is knee extensor and flexor muscle activity.

scholarly journals An 8-week resistance training protocol is effective in adapting quadriceps but not patellar tendon shear modulus measured by Supersonic Shearwave Imaging

2018 ◽  
Author(s):  
P. Mannarino ◽  
T. T. Matta ◽  
F. O. Oliveira
Keyword(s):  
Mechanical Properties ◽  
Resistance Training ◽  
Shear Modulus ◽  
Patellar Tendon ◽  
Vastus Lateralis ◽  
Knee Extensor ◽  
Muscle Thickness ◽  
Knee Extension ◽  
Post Intervention ◽  
Training Protocol

ABSTRACTHabitual loading and resistance training (RT) can determine changes in muscle and tendon morphology but also in its mechanical properties. Conventional ultrasound (US) evaluation of these mechanical properties present limitations that can now be overcome with the advent of Supersonic Shearwave Imaging (SSI). The objective of this study was to analyze the Vastus Lateralis (VL) and patellar tendon (PT) mechanical properties adaptations to an 8-week RT protocol using SSI. We submitted 15 untrained health young men to an 8-week RT directed knee extensor mechanism. VL and PT shear modulus (μ) was assessed pre and post intervention with SSI. VL muscle thickness (VL MT) and knee extension torque (KT) was also measure pre and post intervention to ensure the RT efficiency. Significant increases were observed in VL MT and KT (pre= 2.40 ± 0.40 cm and post= 2.63 ± 0.35 cm, p = 0.0111, and pre= 294.66 ± 73.98 Nm and post= 338.93 ± 76.39 Nm, p = 0.005, respectively). The 8-week RT was also effective in promoting VL μ adaptations (pre= 4.87 ± 1.38 kPa and post= 9.08.12 ± 1.86 kPa, p = 0.0105), but not in significantly affecting PT μ (pre= 78.85 ± 7.37 kPa and post= 66.41 ± 7.25 kPa, p = 0.1287). The present study showed that an 8-week resistance training protocol was effective in adapting VL μ but not PT μ. Further investigation should be conducted with special attention to longer interventions, to possible PT differential individual responsiviness and to the muscle-tendon resting state tension environment.

Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles

2007 ◽  
Vol 103 (5) ◽  
pp. 1565-1575 ◽  
Author(s):  
Anthony J. Blazevich ◽  
Dale Cannavan ◽  
David R. Coleman ◽  
Sara Horne
Keyword(s):  
Resistance Training ◽  
Time Course ◽  
Close Association ◽  
Vastus Lateralis ◽  
Knee Extensor ◽  
Mechanical Stimuli ◽  
Fascicle Length ◽  
And Shifts ◽  
Fascicle Angle ◽  
Training And Detraining

Studies using animal models have been unable to determine the mechanical stimuli that most influence muscle architectural adaptation. We examined the influence of contraction mode on muscle architectural change in humans, while also describing the time course of its adaptation through training and detraining. Twenty-one men and women performed slow-speed (30°/s) concentric-only (Con) or eccentric-only (Ecc) isokinetic knee extensor training for 10 wk before completing a 3-mo detraining period. Fascicle length of the vastus lateralis (VL), measured by ultrasonography, increased similarly in both groups after 5 wk (ΔCon = +6.3 ± 3.0%, ΔEcc = +3.1 ± 1.6%, mean = +4.7 ± 1.7%; P < 0.05). No further increase was found at 10 wk, although a small increase (mean ∼2.5%; not significant) was evident after detraining. Fascicle angle increased in both groups at 5 wk (ΔCon = +11.1 ± 4.0%, ΔEcc = +11.9 ± 5.4%, mean = 11.5 ± 3.2%; P < 0.05) and 10 wk (ΔCon = +13.3 ± 3.0%, ΔEcc = +21.4 ± 6.9%, mean = 17.9 ± 3.7%; P < 0.01) in VL only and remained above baseline after detraining (mean = 13.2%); smaller changes in vastus medialis did not reach significance. The similar increase in fascicle length observed between the training groups mitigates against contraction mode being the predominant stimulus. Our data are also strongly indicative of 1) a close association between VL fascicle length and shifts in the torque-angle relationship through training and detraining and 2) changes in fascicle angle being driven by space constraints in the hypertrophying muscle. Thus muscle architectural adaptations occur rapidly in response to resistance training but are strongly influenced by factors other than contraction mode.

Population-based reference values for IGF-I and IGF-binding protein-1: relations with metabolic and anthropometric variables

1997 ◽  
Vol 136 (2) ◽  
pp. 165-172 ◽  
Author(s):  
Fredrik H Nyström ◽  
Peter K Öhman ◽  
Bertil Å Ekman ◽  
Maria K Österlund ◽  
Bengt E Karlberg ◽  
...  
Keyword(s):  
Reference Values ◽  
Plasma Lipids ◽  
Binding Protein ◽  
Venous Blood ◽  
Population Based ◽  
Sex Hormone Binding Globulin ◽  
Elderly Subjects ◽  
Igf I ◽  
Igf Binding ◽  
Igf Binding Protein

Abstract Population-based reference values for IGF-I and IGF-binding protein-1 (IGFBP-1) have been established. One hundred and one women and the same number of men, 20–70 years old, were randomly selected from the population registry in the community of Linköping. Participation rate was 67%. Venous blood was drawn in the fasting state. Serum IGF-I was measured by RIA after acid-ethanol extraction and IGFBP-1 was determined by ELISA. IGF-I levels did not differ between genders and the decline with age was similar in men and women (men: Y=366–3·28×age (years), r =−0·61, P<0·0001; women: Y=386–3·49×age, r =−0·57, P<0·0001, P=0·4 for difference in slope). There were negative correlations between IGF-I and plasma lipids and blood pressure in both genders, but none was independent of age. Serum angiotensin-converting enzyme activity correlated positively with IGF-I in men independently from age (r =0·21, P=0·01). The distribution of IGFBP-1 was positively skewed and it was higher in women than in men (5·9±4·8 μg/l and 4·0±3·3 μg/l respectively; Mann–Whitney, P=0·002). In men and in the women not taking oestrogen, IGFBP-1 correlated positively with age (Spearman rank correlation (Spearman): men: r=0·32, P=0·002; women: r=0·24, P=0·03). C-peptide correlated negatively (Spearman: men: r =−0·38, P=0·002; women: r =−0·49, P<0·000) and sex hormone binding globulin positively with IGFBP-1 (Spearman: men: r=0·50, P<0·0001; women: r =0·55, P<0·0001). IGF-I declined with age while IGFBP-1, which is considered to modulate the free bioactive fraction of IGF-I, increased. This suggests that IGF-I activity might be even lower in elderly subjects than is accounted for by the low total IGF-I. European Journal of Endocrinology 136 165–172

Potent myofiber hypertrophy during resistance training in humans is associated with satellite cell-mediated myonuclear addition: a cluster analysis

2008 ◽  
Vol 104 (6) ◽  
pp. 1736-1742 ◽  
Author(s):  
John K. Petrella ◽  
Jeong-su Kim ◽  
David L. Mayhew ◽  
James M. Cross ◽  
Marcas M. Bamman
Keyword(s):  
Cluster Analysis ◽  
Resistance Training ◽  
Satellite Cell ◽  
Vastus Lateralis ◽  
Knee Extensor ◽  
Surface Expression ◽  
Skeletal Muscle Hypertrophy ◽  
Neural Cell Adhesion ◽  
Myonuclear Domain ◽  
Muscle Biology

A present debate in muscle biology is whether myonuclear addition is required during skeletal muscle hypertrophy. We utilized K-means cluster analysis to classify 66 humans after 16 wk of knee extensor resistance training as extreme (Xtr, n = 17), modest (Mod, n = 32), or nonresponders (Non, n = 17) based on myofiber hypertrophy, which averaged 58, 28, and 0%, respectively (Bamman MM, Petrella JK, Kim JS, Mayhew DL, Cross JM. J Appl Physiol 102: 2232–2239, 2007). We hypothesized that robust hypertrophy seen in Xtr was driven by superior satellite cell (SC) activation and myonuclear addition. Vastus lateralis biopsies were obtained at baseline and week 16. SCs were identified immunohistochemically by surface expression of neural cell adhesion molecule. At baseline, myofiber size did not differ among clusters; however, the SC population was greater in Xtr ( P < 0.01) than both Mod and Non, suggesting superior basal myogenic potential. SC number increased robustly during training in Xtr only (117%; P < 0.001). Myonuclear addition occurred in Mod (9%; P < 0.05) and was most effectively accomplished in Xtr (26%; P < 0.001). After training, Xtr had more myonuclei per fiber than Non (23%; P < 0.05) and tended to have more than Mod (19%; P = 0.056). Both Xtr and Mod expanded the myonuclear domain to meet (Mod) or exceed (Xtr) 2,000 μm2 per nucleus, possibly driving demand for myonuclear addition to support myofiber expansion. These findings strongly suggest myonuclear addition via SC recruitment may be required to achieve substantial myofiber hypertrophy in humans. Individuals with a greater basal presence of SCs demonstrated, with training, a remarkable ability to expand the SC pool, incorporate new nuclei, and achieve robust growth.

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