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Myosin Heavy Chain Composition, Creatine Analogues, and the Relationship of Muscle Creatine Content and Fast-Twitch Proportion to Wilks Coefficient in Powerlifters

Machek, Steven B.1; Hwang, Paul S.1; Cardaci, Thomas D.1; Wilburn, Dylan T.1; Bagley, James R.2; Blake, Daniel T.3; Galpin, Andrew J.3; Willoughby, Darryn S.4

Author Information
Journal of Strength and Conditioning Research: November 2020 - Volume 34 - Issue 11 - p 3022-3030
doi: 10.1519/JSC.0000000000003804
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Abstract

Introduction

Powerlifting is a barbell sport comprised of 3 competition lifts in sequential order: back squat, bench press, and deadlift. In a sanctioned competition, the highest loads lifted of 3 attempts in each event are summed and a total is established; this number is then often normalized to body weight to produce a validated figure denoted by the Wilks Coefficient (46). This figure uses an equation which accounts for the total competition weight lifted (kg) and 6 sex-specific constants multiplied against the athlete's body weight (kg) (12). Ultimately, the Wilks Coefficient is used to determine the best male and female lifters in a meet, respective of category (age, weight class, etc.). Muscle fiber type characteristics and proportion heavily affect muscle performance, and thus may play an important role in powerlifting skill. Furthermore, fiber type shifts are commensurate to biochemical adaptations that accommodate intensity-dependent and exercise-specific stimuli (5). Skeletal muscle fiber types are classified on a continuum based on myosin heavy chain (MHC) isoform, whereby humans express slow-twitch (MHC I), fast-twitch (MHC IIa), and super-fast-twitch (MHC IIx) phenotypes (27). Myosin occupies ∼25% of all muscle proteins and thus myosin-associated adaptations impart large effects on exercise performance (48). Similar to classical resistance training, previous research has demonstrated that powerlifters primarily exhibit higher fast-twitch/MHC IIa isoform composition and slow-twitch/MHC I content to a lesser extent, commensurate with decreased super-fast-twitch/MHC IIx content (14,22,28,43). This suggests a relationship between resistance/powerlifting training-associated greater MHC IIa isoform content and subsequent improved performance. Unfortunately, research on fiber type composition of powerlifters is almost entirely limited to 6 investigations across 1976–2005 (Table 1). In this brief timeframe, less than 40 total athletes were examined with fewer than 30 subject samples taken from the vastus lateralis. Therefore, the existing literature on competitive powerlifter fiber type profiles is not only scarce with inconsistent methodology, but also ultimately nonexistent in women; nearly all previous research has used predominantly men.

Table 1 - List of previous research detailing fiber type profiles in powerlifting subjects.*,†
References Subject # & demographics Muscle Typing method % FT/ST
ST FT
SO/I FOG/IIa FG/IIx
Prince et al. (28) 5 CON VL Histochemical staining 35.5 38.1 26.2
4 PL 45 10.5 33.3
3 runners 44.3 39.7 4.5
MacDougall et al. (21) 7 elite (2 PL/5 BB) Triceps brachii Histochemical staining 29 71
5 post-RT “normals” 34 66
Tesch & Karlsson (41) 12 students VL & Deltoid Histochemical staining 43 57
8 wrestlers 47 53
9 kayakers 41 59
9 runners 67 33
7 WL & PL 54 46
Kadi et al. (18) 10 elite PL Trapezius Histochemical staining 47 44 9
6 CON 54 27 19
Fry et al. (13) 5 PL VL Histochemical staining & SDS-PAGE 28.5 59.9 11.7
5 CON 33.1 46.5 20.2
Eriksson et al. (11) 10 PL VL Histochemical staining 46 35 7
9 PL using AAS 40 40 0.7
*AAS = anabolic androgenic steroids; BB = bodybuilders; CON = control; F = female; FG = fast-glycolytic; FOG = fast oxidative-glycolytic; FT = fast-twitch; M = male; PL = powerlifters; VL = vastus lateralis; RT = resistance training SDS-PAGE = sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SO = slow-oxidative; ST = slow-twitch; WL = Olympic weightlifters.
†The above table is not an exhaustive list of the literature investigating fiber type in powerlifting subjects.

Powerlifting training and competition are highly dependent on the phosphagen system, where adenosine triphosphate (ATP) is made available by the ATP-phosphocreatine (ATP-PCr) system (7). In this reaction, creatine kinase catalyzes the transfer of a high-energy phosphate from phosphocreatine (PCr) to the hydrolysis product of ATP, adenosine diphosphate (ADP), resynthesizing ATP for extended energy provision (PCr + ADP + H+ ↔ ATP + Creatine) (47). The ATP-PCr system uses substrate-level phosphorylation to primarily support brief exercise durations (<30 seconds), highly characteristic of typical powerlifting activity. Powerlifting competition demands (total of 9 maximal lifts over several hours) do not reasonably deplete skeletal muscle PCr, especially considering phosphocreatine is not substantially depleted after a single exhaustive set and that athletes are not commonly in a position of hampered resynthesis rates (metabolic acidosis or hypoxia) (15,39). Nonetheless, typical powerlifter training regimens commonly use combined low and high volumes between a range of submaximal-to-maximal loads that likely recruit chronic and consistent immediate pathway utilization (2,45). Previous data have illustrated a higher PCr depletion in lower repetition, equivalent load (∼83% one-repetition maximum) resistance training compared with an exhaustive condition over 5 sets (15). This evidence is especially relevant for powerlifters, whereby frequently high relative loads used with the primary compound lifts largely discourage exhaustive sets for fear of subsequent performance decrement/injury (2,23). Therefore, it can be surmised that adaptations in total creatine and various creatine analogues potentially contribute to powerlifting performance via energy provision in competition and amidst training (10,50).

Total creatine is absorbed from the blood by the sodium-dependent transporter (SLC6A8) within the sarcolemma (35). The SLC6A8 transport protein has demonstrated an inverse fiber type-dependent relationship with total creatine content, where fast-twitch fibers have lower SLC6A8 levels and typically higher total creatine (35). Conversely, slow-twitch fibers demonstrate greater SLC6A8 concentrations and typically lower total creatine. Upon skeletal muscle uptake, creatine becomes phosphorylated to PCr (35). Total creatine (PCr + free creatine) is ultimately related to the enzymatic resynthesis of ATP from ADP (35). This process is vital in stabilizing cellular energy levels, and thus increased total creatine content is implicated in a cell's ability to meet increased energy demands (34). Creatinine is formed via random cyclization of the total creatine pool at a rate of ∼2% and can vary by muscle mass (4,29,40). As a result, serum creatinine content may be used as an indirect measure of total creatine content, but can also provide insight into habitual dietary protein intake and the intensity of physical activity (4). Altogether, establishing comparative fiber type and creatine analogue profiles provides novel understanding into competitive powerlifter-specific skeletal muscle adaptations.

To date, the potential relationship between powerlifter-specific MHC isoform content and creatine-associated markers on powerlifting performance remains unexplored and has yet to be compared between sexes. Therefore, we aimed to (a) compare profiles of MHC composition and creatine analogues between competitive powerlifters and controls of both sexes, and (b) determine whether specific targeted markers (MHC IIa and muscle total creatine content) are predictors of powerlifting performance via Wilks coefficient.

Methods

Experimental Approach to the Problem

This investigation strived to examine the potential relationships between powerlifting performance, multiple markers of creatine content, and skeletal muscle fiber type. In doing so, a cross-sectional design was used to facilitate the recruitment of experienced powerlifter subjects. Sedentary controls were additionally recruited to compare against the targeted athletic demographic and both sexes were enlisted during the study to comprehensively assess potential sex-specific interactions. In both groups, skeletal muscle MHC (I, IIa, and IIx) isoform content, and several indicators of serum and skeletal muscle creatine content were examined. Myosin heavy chain IIa and muscle total creatine were compared for predictive ability against all powerlifters' most recent Wilks coefficient as the surrogate powerlifting performance indicator.

Subjects

Twenty-four apparently healthy and physically active men and women between the ages of 18–35 participated in the study (Table 2). Groups consisted of actively competing powerlifters (PL; n = 12; 6 men & 6 women) and sedentary controls (CON; n = 10; 5 men & 5 women). Before the study commencing, eligible powerlifter subjects must have competed in a sanctioned meet within the previous 12 months, actively engaged in “powerlifting-style” training (≥3x·wk−1), and reported no anabolic androgenic steroids (AAS) use within the past year. Furthermore, 8 of the 12 subjects are active competitors in the drug-tested USA Powerlifting (USAPL) federation. The USAPL membership agreement stipulates a written agreement that signees have not used any strength inducing drugs (AAS, natural hormone or synthetic growth hormone) in the past 36 months. All powerlifter subject meet dates, years competed, and highest Wilks coefficients (≤12 months) were verified using data from the OpenPowerlifting project database (OpenPowerlifting.org). Powerlifters subjects varied in their competitive history, whereby some could be characterized as recreational competitors, whereas 7 had qualified and participated in national meets. Although there are no established standards for powerlifting performance, all PL were required to have an arbitrary minimum Wilks coefficient of 300 to prevent extremely unexperienced subjects from entering the investigation. Finally, although recent training history was not collected (phase of training, number of sessions per week, etc.) none of the powerlifter subjects were in preparation for a competition and were considered in their “off season”. Sedentary control subjects were inactive as defined by the American College of Sports Medicine (ACSM) guidelines (≤30 minutes exercise, ≤ 3 d·wk, over the last 3 months) (31). Also, subjects were ineligible if they identified as vegetarian or vegan or had been on a purposeful creatine supplement regimen within the last 30 days (based on previously demonstrated creatine supplementation washout timeframe) (44). The use of blood thinning (e.g., warfarin, Jantoven, etc.), heart, pulmonary, thyroid, antihypertensive, antihyperlipidemic, hypoglycemic, endocrinologic (e.g., thyroid, insulin, etc.) emotional/psychotropic (e.g., Prednisone, Ritalin, Adderall, etc.), or neuromuscular/neurological medications were further prohibited for eligibility. All subjects were required to refrain from exercise for 48 hours before laboratory visit. The investigators explained both the purpose and procedures to be used before allowing prospective subjects to enter. All eligible subjects signed university-approved informed consent documents and approval was granted by the Institutional Review Board for Human Subjects at Baylor University. In addition, all experimental procedures involved in the study conformed to ethical considerations of the Helsinki Code.

Table 2 - Basic subject descriptive and powerlifter-specific experience data.*
Mean ± SD PL (n = 12) 6M/6F CON (n = 10) 5M/5F p (<0.05)
Basic subject descriptives
 Age (y) 0.016
  Males 21.3 ± 1.0 19.4 ± 2.0
  Females 24.3 ± 4.9 19.6 ± 1.5
  M + F 22.8 ± 3.7 19.5 ± 1.7
 Height (cm) 0.997
  Males 171.3 ± 5.4 174.7 ± 7.6
  Females 156.4 ± 5.2 160.1 ± 2.7
  M + F 163.9 ± 9.3 168.1 ± 9.4
 Body mass (kg) 0.388
  Males 86.0 ± 12.9 94.0 ± 37.1
  Females 63.7 ± 9.3 60.7 ± 12.7
  M + F 74.8 ± 15.9 77.8 ± 31.9
 BF% 0.009
  Males 13.22 ± 5.05 26.78 ± 8.33
  Females 24.55 ± 5.07 32.42 ± 5.89
  M + F 18.88 ± 7.64 29.12 ± 7.48
Powerlifter-specific experience
 Wilks coefficient SQ/CBW ratio
  Males 430.3 ± 52.1 Males 2.86 ± 0.40
  Females 380.8 ± 44.8 Females 2.05 ± 0.28
  M + F 405.5 ± 53.1 M + F 2.45 ± 0.53
 Years competing BP/CBW ratio
  Males 3.8 ± 1.7 Males 1.90 ± 0.23
  Females 2.2 ± 1.5 Females 1.18 ± 0.21
  M + F 3.0 ± 1.8 M + F 1.54 ± 0.43
 Meets attended DL/CBW ratio
  Males 8.2 ± 6.2 Males 3.06 ± 0.69
  Females 6.3 ± 7.3 Females 2.36 ± 0.39
  M + F 7.3 ± 6.6 M + F 2.71 ± 0.65
*BF = body fat; BP = bench press; CBW = competition body weight (kg); CON = control; DL = deadlift; F = female; PL = powerlifters; M = male; SQ = squat.
All data are presented as means ± SD.
Significant differences in basic subjects descriptive data between sex-collapsed groups (p < 0.05). Powerlifters (PL) were significantly both older and displayed lower body fat percentages compared with sedentary controls (CON).

Procedures

Dietary Records

Subject diets were not standardized and they were asked to not change their dietary habits before visiting the laboratory. In addition, subjects were required to keep dietary records for 48 hours before visit, using the MyFitnessPal mobile or desktop application (MyFitnessPal; San Francisco, CA). If the subject was unfamiliar with either application, they were provided with instructions on how to use relevant features. MyFitnessPal data was analyzed for macronutrient (protein, carbohydrate, and fat) content, and fiber intake by the same investigator across all subjects. All dietary data were normalized to mass (kg) for further statistical analysis.

Body Composition Testing

Total body mass (kg) and height (cm) were determined on a standard dual beam balance scale (Detecto Bridgeview, IL). Percent body fat, fat mass, and fat free mass were determined using dual-energy-x-ray-absorptiometry (DEXA) (Hologic Discovery Series W, Waltham, MA). Quality control calibration procedures were performed on a spine phantom (Hologic X-CAIBER Model DPA/QDR-1 anthropometric spine phantom) and a density step calibration phantom before each testing session.

Venipuncture

Venous blood samples were obtained from the antecubital vein into a 10-ml collection tube via a standard Vacutainer apparatus. Afterwards, blood samples were centrifuged at room temperature at 2,500 rpm for 15 minutes. The serum was then isolated and frozen at −80° C for future analysis.

Muscle Biopsy and Muscle Tissue Processing

All subjects received 1 ml of anesthetic (1% lidocaine without epinephrine) via subcutaneous administration at the mid-muscle belly (half-way between the greater trochanter and patella) of the vastus lateralis. Subjects then underwent a resting biopsy at the pilot hole site using the 14-gauge fine needle aspiration method using a TRU-CORE 1 Automatic Disposable Biopsy Instrument (Angiotech, Medical Device Technologies, Inc., Gainsville, FL) previously described by our laboratory (6). The biopsy needle was inserted into the pilot hole at a depth of ∼5–10 mm for 2–3 passes. As a result, muscle samples totaling ∼30 mg (2–3 ∼10–15 mg samples collected per subject) were separated from connective tissue and adipose tissues, and immediately frozen and stored at −80° C for later analysis.

Homogenate Myosin Heavy Chain Fiber Type Analysis

Each sample was placed into 80 μL of sodium dodecyl sulfate (SDS) buffer (1% SDS, 23 mM EDTA, 0.008% bromophenol blue, 15% glycerol, and 715 mM b-mercanptoethanol [pH 6.8]). Homogenate samples (∼5 mg) were hand homogenized and then diluted between 1:10 to 1:15 depending on sample amount and protein quantity. As described in our previous laboratory methods (3,25,36), 1–2µL aliquots were loaded into individual wells in a 3.5% loading and 5% separating gel (SDS-PAGE), run at 5° C for 15.5 hours (SE 600 Series; Hoefer, San Francisco, CA), and silver-stained for MHC identification. Subsequently, densitometry (ImageJ, National Institutes of Health, Bethesda, MD) was used to quantify the relative MHC protein composition of individual isoforms (i.e., percent area occupied by each pure isoform; MHC I, IIa, and IIx) of each sample.

Serum and Muscle Analysis of Creatine Analogues

Total muscle (MTC) and serum creatine (STC) were determined using a commercially available colorimetric assay kit, detected at a wavelength of 570 nm (BioVision, Milpitas, CA). Muscle SLC6A8 and serum creatinine (CRT) were determined by commercially available enzyme-linked immunosorbent assay kits (MyBioSource, San Diego, CA) detected at a wavelength of 450 nm. All samples were compared against control peptides of known concentrations in which a standard curve is generated and sample concentrations determined. Assays involved the use of a microplate reader (X-Mark, Bio-Rad, Hercules, CA) and data reduction software (Microplate Manager, Bio-Rad, Hercules, CA). Intra-assay coefficients of variations (CV) and standard curve coefficients of determination (r2) were 2.838% (r2 = 0.998), 4.501% (r2 = 0.966), 5.346% (r2 = 0.998), and 4.309% (r2 = 0.991) for MTC, total serum creatine, muscle SLC6A8, and serum creatinine, respectively. The interassay CV was 4.248%. Muscle total creatine and SLC6A8 were expressed as relative to muscle wet weight (mg).

Statistical Analyses

Wilks coefficients were compared (males versus females) using an independent t-test to compare PL skill levels. In addition, anthropometric data (age, height, weight, and body fat percentage) were compared between PL and CON using independent t-tests. All variables were tested for normality and homogeneity of variance using the Shapiro-Wilk's test and Levene's test of homogeneity of variance, respectively, before continuing subsequent statistical analysis. Individual MHC isoform type differences between group and sex were analyzed using a 3 × 2 × 2 (fiber type [MHC I, IIa, and IIx] × group [PL and CON] × sex [male and female]) analysis of variance (ANOVA). Relative dietary macronutrients (g·kg−1 [protein, carbohydrate, and fat]), dietary fiber (g·kg−1), MTC, SLC6A8, STC, and CRT were analyzed via separate 2 × 2 (group × sex) ANOVA. If significant interaction effects were present, pairwise comparison analyses were used with a Bonferroni adjustment for alpha inflation. Partial Eta squared (η2) was used to estimate the proportion of variance in the dependent variables explained by the independent variable. Partial Eta squared effect sizes were determined to be: weak = 0.17, medium = 0.24, strong = 0.51, and very strong = 0.70 (26). Muscle total creatine and MHC IIa content were individually compared with Wilks Coefficient using a Pearson correlation coefficient for sex-collapsed PL. In addition, if a significant (fiber type × group × sex) interaction effect for MHC IIa content and a significant (group × sex) interaction effect for MTC was detected within PL, both male and female PL were separately compared with Wilks Coefficient using Pearson correlation coefficients. Myosin heavy chain isoform contents for both group and sex are expressed as a percentage of the total fiber pool. All analyses were performed at a significance level of p < 0.05 and values are reported as mean ± SD. Confidence intervals (CI) for significant comparisons are reported as 95% CI (lower bound, upper bound).

Results

Wilks Coefficient, Subject Descriptives, and Dietary Analysis

There were no significant differences between sexes among PL for Wilks coefficient (p = 0.772). All subject descriptive and dietary data are presented in Table 2Table 3, respectively. Powerlifters consumed significantly more protein (relative to body mass) than CON (p < 0.001, η2 = 0.574, 95% CI [0.569–1.417]) when collapsed across sex. Conversely, relative protein intake displayed no significant group-collapsed sex or (group × sex) interaction effects. Dietary patterns were similar for relative dietary carbohydrate, fat, and fiber intake, displaying no significant group, sex, or interaction effects.

Table 3 - Dietary macronutrient intake.*
Mean ± SD (g·kg−1·BM) PL (n = 12) 6M/6F CON (n = 10) 5M/5F Interaction/Main effect p (<0.05); ES
Protein
 Males 2.01 ± 0.39 0.99 ± 0.62 Interaction 0.896; η2 = 0.001
 Females 1.96 ± 0.44 1.00 ± 0.43 Sex 0.930; η2 = 0.000
 M + F 1.99 ± 0.40 0.99 ± 0.50 Group <0.001; η2 = 0.574
Carbohydrate
 Males 2.98 ± 0.82 3.02 ± 1.43 Interaction 0.868; η2 = 0.002
 Females 3.13 ± 0.77 3.33 ± 1.46 Sex 0.639; η2 = 0.012
 M + F 3.05 ± 0.76 3.18 ± 1.37 Group 0.802; η2 = 0.004
Fat
 Males 1.02 ± 0.34 0.76 ± 0.34 Interaction 0.318; η2 = 0.055
 Females 0.96 ± 0.40 1.03 ± 0.42 Sex 0.521; η2 = 0.023
 M + F 0.99 ± 0.35 0.89 ± 0.39 Group 0.535; η2 = 0.022
Fiber
 Males 0.25 ± 0.14 0.22 ± 0.15 Interaction 0.632; η2 = 0.013
 Females 0.30 ± 0.10 0.21 ± 0.11 Sex 0.704; η2 = 0.008
 M + F 0.27 ± 0.12 0.21 ± 0.12 Group 0.311; η2 = 0.057
*BM = body mass CON = control; ES = effect size; F = female; PL = powerlifters; M = male.
Significant group differences (p < 0.05). Powerlifters had significantly greater relative protein intake compared with sedentary controls, regardless of sex. All data presented as means ± SD. Both p-values and effect sizes are indicated for each macronutrient by interaction (group × sex), and isolated group and sex main effects.

Homogenate Myosin Heavy Chain Fiber Type

Individual MHC isoform distributions for both sexes amongst PL and CON groups are displayed in Figure 1. Analysis revealed a significant main effect for fiber type (p < 0.001; η2 = 0.773) as well as significant fiber type × group (p < 0.001; η2 = 0.552) and fiber type × sex (p = 0.025; η2 = 0.127) interaction effects. There were no significant main effects for sex or group, nor were there significant group × sex or fiber type × group × sex interaction effects. All significant sex-collapsed differences detected by pairwise comparisons both within-and-between group are displayed in Table 4, whereas as all group-collapsed differences within-and-between sex are shown in Figure 1. It should be noted that only one male subject in PL expressed any MHC IIx content, whereas 9/10 subjects in CON expressed >10% MHC IIx (Table 4).

Figure 1.
Figure 1.:
Myosin heavy chain (MHC) isoform content via homogenate sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) compared between male and female PL, and male and female CON subjects.. All data are reported as percentage ± SD and all statistical analyses performed at a significance level of p < 0.05. *Significant group-collapsed differences within-sex, whereby both male and female MHC IIx composition was significantly lower than MHC I (male 95% CI [−33.145 to −15.697]; female 95% CI [−38.674 to −21.226]) and IIa content (male 95% CI [−32.897 to −15.450]; female 95% CI [−46.806 to −29.358]) (p < 0.001), but did not differ between sex. ¥Significant group-collapsed difference between-sex, whereby women had significantly higher MHC IIa composition relative to men (p = 0.04).
Table 4 - Individual homogenate myosin heavy chain (MHC) isoform composition in powerlifter (PL) and sedentary control (CON) subjects.*
PL Percentage MHC composition (%) CON Percentage MHC composition (%)
I IIa IIx I IIa IIx
1 50 50 0 1 38 32 30
2 39 61 0 2 45 55 0
3 55 45 0 3 33 36 31
4 49 51 0 4 39 41 20
5 55 45 0 5 34 31 35
6 38 62 0 6 23 37 40
7 39 33 27 7 24 40 35
8 50 50 0 8 28 36 36
9 50 50 0 9 35 48 17
10 51 49 0 10 39 50 11
11 54 46 0
12 52 48 0
Total PL 48 ± 6 49 ± 7 2 ± 8 Total CON 34 ± 7 41 ± 8§ 26 ± 13
*Total MHC isoform content data for each group is presented as means ± SD and all statistical analyses performed at a significance level of p < 0.05. Bolded PL have previously qualified and competed in national-level competition. All odd-numbered subjects are male & all even-numbered subjects are female.
Significant sex-collapsed differences between group MHC isoform content, whereby PL displayed higher MHC I (p < 0.001; 95% CI [7.726–21.883]) and IIa (p = 0.021; 95% CI [1.333–15.490]), whereas CON demonstrated greater MHC IIx content (p < 0.001; 95% CI [16.138–30.295]).
Significant sex-collapsed difference in within-PL MHC isoform content, whereby MHC IIx content was lower (p < 0.001) than both MHC I (95% CI [−54.514 to −37.878]) and IIa (95% CI [−55.259 to −38.624]) composition.
§Significant sex-collapsed difference within CON MHC isoform content, whereby MHC IIa was greater than MHC IIx content (p < 0.001; 95% CI [6.202–24.426]).

Creatine Analogues

All creatine analogue data are presented in Table 5. The Shapiro-Wilk's test of normality was violated for CRT and a single outlier was removed to meet assumptions before continuing subsequent analysis. Briefly, there were no significant group, sex, or interaction effects between PL and CON for MTC, SLC6A8, STC, nor CRT.

Table 5 - Creatine-associated markers between powerlifters and sedentary controls of both sexes.*
Mean ± SD PL (n = 12) 6M/6F CON (n = 10) 5M/5F Group-collapsed 11M/11F
MTC (nmol/mg)
 Males 2.395 ± 0.508 2.072 ± 0.450 2.249 ± 0.488
 Females 2.088 ± 0.653 2.131 ± 0.492 2.107 ± 0.557
 M + F 2.241 ± 0.580 2.102 ± 0.446 2.178 ± 0.516
SLC6A8 (nmol·mg−1)
 Males 0.149 ± 0.054 0.101 ± 0.067 0.127 ± 0.062
 Females 0.156 ± 0.058 0.112 ± 0.095 0.136 ± 0.076
 M + F 0.153 ± 0.054 0.107 ± 0.078 0.132 ± 0.068
STC (nmol·µL−1)
 Males 0.0917 ± 0.0481 0.0828 ± 0.0653 0.0828 ± 0.0653
 Females 0.0650 ± 0.0334 0.0870 ± 0.0768 0.0870 ± 0.0768
 M + F 0.0795 ± 0.0424 0.0847 ± 0.0668 0.0847 ± 0.0659
CRT (nmol·µL−1)
 Males 0.1142 ± 0.0363 0.0704 ± 0.0651 0.0943 ± 0.0536
 Females 0.1643 ± 0.0668 0.1385 ± 0.1356 0.1514 ± 0.0999
 M + F 0.1414 ± 0.0537 0.1007 ± 0.1015 0.1103 ± 0.0734
*CON = control; F = female; PL = powerlifters; M = male; MTC = muscle total creatine; rCRT = relative serum creatinine; rSTC = relative serum total creatine; SLC6A8 = sodium-dependent creatine transporter.
MTC, SLC6A8, STC, and STC did not differ between group nor sex (p > 0.05). MTC and SLC6A8 are expressed relative to muscle wet weight (mg). All data are presented as means ± SD.

Correlations Amongst Myosin Heavy Chain IIa, Muscle Total Creatine, and Wilks Coefficient

Pearson correlation analysis revealed a nonsignificant, negative low correlation between sex-collapsed PL Wilks coefficient and MHC IIa content (r = −0.296; p = 0.350) (Figure 2A), and a nonsignificant, positive low-moderate correlation between sex-collapsed PL Wilks coefficient and MTC (r = 0.488; p = 0.108) (Figure 2B).

Figure 2.
Figure 2.:
Pearson correlation coefficients (r) for (A) muscle total creatine content (MTC) and (B) myosin heavy chain (MHC) IIa isoform composition.

Discussion

The present study is the first to concurrently measure both MHC fiber type composition and creatine analogue content in powerlifters, while also determining differences in these profiles between sexes. In addition, these data provide novel insight into the inability of MHC IIa and MTC alone to predict Wilks coefficient as an index of powerlifting performance. As previously demonstrated in resistance-trained and powerlifting demographics, PL had a higher MHC I and IIa composition per sample relative to CON, and lower MHC IIx content (14,22,48). Notwithstanding an inability to make causal inferences, the MHC profile seen among PL (relative to control) may have been a resistance training-mediated adaptation facilitated by a group-specific greater relative protein intake (48). Incidentally, our data also provide the first MHC composition profile in female powerlifters. Female subjects interestingly had greater MHC IIa isoform content relative to men when group-collapsed; this finding is contradictory to previous data indicating a female predisposition to higher slow-twitch content (17,38). In competitive Olympic weightlifters, Serrano et al. (36) discovered higher MHC IIa isoform composition in world-level females relative to national-level males using homogenate SDS-PAGE. Nevertheless, because our data only demonstrate significant differences between group-collapsed sexes, we are unable to infer substantial discrepancies amongst PL males and females that would otherwise suggest a favorable PL female-specific profile. When further compared with the elite weightlifters examined by Serrano et al. (36), PL in the current investigation displayed clear differences in MHC I and IIa content. Sex-collapsed PL exhibited relative higher MHC I (48 ± 6 vs. 31 ± 9%) and lower MHC IIa (49 ± 7 vs. 67 ± 9) composition compared with combined national- and international-level weightlifters. Interestingly, our sex-collapsed PL demonstrated a similar MHC content to national- and international-caliber weightlifters examined by Fry et al. (13) (47 ± 2% MHC I, 47 ± 3% MHC IIa, & 2.4 ± 2% MHC IIx). However, this resemblance may be because of athlete skill, considering both the present investigation and Fry et al. (13) primarily used nationally qualifying athletes. Conversely, 6 of 21 weightlifters investigated by Serrano et al. (36) competed on the Olympic or World team and the remaining national-level athletes were required to minimally place in the top 5 at the American Open Finals meet.

It also remains a possibility that the differences between Serrano et al. (36) and the current investigation were sourced in the nature of each respective sport. Weightlifting has a high event-specific demand for muscular power (i.e., high velocity movements), whereas typical powerlifting-style training primarily enacts high force, low velocity skeletal muscle contractions (especially true for near-to-fully maximal lift exposure) (13,14). Although it is known that MHC isoform content greatly impacts skeletal muscle contractile force and power, Fry et al. (13,14) saw similar homogenate MHC IIa isoform content between powerlifters and weightlifters. Furthermore, the higher MHC IIa predisposition between these and other power/strength athletes may be expected as a likely general adaptation to high threshold motor unit recruitment somewhat independent of velocity demands (13). A noteworthy difference between the investigations conducted by Fry et al. (13,14) may be that weightlifters had comparatively higher MHC IIa cross-sectional areas and percentage areas occupied versus powerlifters. This was postulated by the author as potentially due to greater weightlifting-specific power requirements. Nevertheless, contradicting data demonstrated high-versus-low velocity training does not significantly affect fiber type-specific cross-sectional area, force, nor power among varying aged men and women (9). Consequently, we further cannot infer the higher MHC I content in the current investigation relative to Serrano et al. (36) is necessarily because of sport-specific demands. Compared with weightlifting, however, powerlifter training is well described as having periods of high-volume, submaximally loaded bouts in conjunction with low-volume, near-maximal/maximal training (2). Greater magnitudes of high-volume, submaximal training potentially elicits adaptations similar to previous research in bodybuilders, displaying a higher proportion of MHC I-expressing fibers (19). Nevertheless, these apparent discrepancies between MHC isoform content among differing athletic disciplines may simply represent a single muscular characteristic that cannot be explained by fiber type alone.

Previous data have substantially supported that increased skeletal muscle creatine via exogenous supplementation can reliably augment indices of power and strength in a multiplicity of athletes (including well-trained powerlifters) among either sex (7,32,50). Conversely, our findings indicate that several endogenous creatine-associated markers do not differ between PL and CON, nor do they differ between sexes. Typically, muscle PCr content (per volume of skeletal muscle mass) does not increase with chronic training (33). Although the current investigation did not specifically assess PCr content, these data support the notion that neither muscle nor serum total creatine differed between group nor sex. Also, although total creatine content is usually higher in fast-twitch fibers, we did not observe greater concentrations in women, regardless of their higher MHC IIa composition (40,42). As per previous fiber-type-specific assumptions, PL would ostensibly carry greater SLC6A8 concentrations due to increased MHC I content; however, we did not observe discernible differences. Furthermore, CRT did not differ between groups or sexes. This finding directly refutes previous research, whereby men typically have increased concentrations associated with greater skeletal muscle mass and subsequently increased creatine pool turnover (29). Although comparable to prior research conducted by our laboratory in resistance-trained men, the current subjects displayed a large degree of within- and between-group CRT variation (Table 5) (37). Nevertheless, serum creatinine is subject to several variables aside from skeletal muscle mass, including sex, ethnicity, protein intake, and physical activity (4).

Finally, the current investigation illustrates neither MHC IIa content (in either sex or sex-collapsed) nor MTC are significant molecular-scale predictors of powerlifting performance via Wilks coefficient. Other investigations have determined that variables including brachial index, lean body mass, and isokinetic shoulder flexion to be highly correlated to one-repetition maximum among national- and international-level powerlifting events (30). Force production is strongly related to muscle cross-sectional area and central nervous system-mediated strength adaptations may potentiate powerlifting performance through factors including greater agonist activation, and movement-specific efficiency (30). Regardless, our analysis singularly used Wilks coefficient as the surrogate performance metric. Chronic heavy resistance training results in MHC IIa-specific hypertrophy and subsequent lean body mass accrual, supporting a concomitant increase in the absolute fast-twitch fiber-associated PCr pool (40,42). Thus, because Wilks coefficient is a function of body weight, absolute adaptations to fast-twitch fiber hypertrophy and accompanying MTC may not meaningfully contribute to weight-normalized performance.

Furthermore, although homogenate SDS-PAGE is a valid assessment of MHC isoform content, it does not discern true fiber type distribution nor the detection of hybrid MHC isoforms (I/IIa, IIa/IIx, I/IIa/IIx) (36). These co-expressing fibers are common and typically seen in sedentary populations, accounting for approximately 10–50% of all fibers (1,19,20). Hybrid isoforms react readily to exercise stimuli akin to MHC IIx, ultimately transitioning to MHC I and IIa isoforms (49). Using both homogenate and single-fiber SDS-PAGE in elite weightlifters, Serrano et al. (36) demonstrated that homogenate analysis accurately identified MHC IIa content, but overestimated MHC I and MHC IIx content. The single-fiber method confidently identified the presence of hybrid MHC I/IIa and IIa/IIx fibers, which were respectively misrepresented as “pure” MHC I and IIx fibers. It is also possible that small changes in creatine-associated markers were “diluted” in muscle homogenate; considering total creatine content is highest in fast-twitch fibers, the relative high MHC I isoform contribution may have limited the ability to detect discernible MHC IIa-specific MTC differences (40,42). Furthermore, this phenomenon may explain how significantly higher MHC I content in PL did not result in concomitantly greater muscle SLC6A8 concentrations. Future use of single fiber methodology may therefore provide insight into whether MHC distribution (versus content) better predicts performance, as well as elucidating fiber type-specific creatine analogue content. Homogenate SDS-PAGE is generally regarded as a better representative of fiber type area/content (versus MHC isoform proportion) because of the influence of individual fiber size (36). Nevertheless, previous data collected from our laboratory indicates homogenate SDS-PAGE can accurately predict MHC IIa similar (within 0–4%) to the single fiber method, which was ultimately our desired correlate to powerlifting performance (36). Although use of the fine needle aspiration biopsy method limited sample yield and the ability to perform single fiber analysis, it was otherwise and justifiably appropriate in obtaining necessary subject samples for MHC IIa determination while being minimally invasive (16).

Although the current findings provide a glimpse into the relationship between competitive powerlifter performance, endogenous creatine concentrations, and fast-twitch fiber content, a primary limitation to the present study is its cross-sectional design and low sample size. Without appropriate baseline measures, we are unable to confidently conclude the current powerlifter creatine analogues markers and fiber type profiles are powerlifting-associated adaptations. Regardless, previous acute and training investigations in powerlifting and resistance-trained demographics, respectively, support that these data represent the effects of chronic powerlifting-style training (14,48). We were unable to standardize PL training programs or sampling dates with respect to a training window, potentially creating training phase-based variability. Athletes in differing training blocks may elicit exercise intensity and volume-mediated variations that potentially induced transient, inconsistent adaptations before sampling (8,24). Future research should aim to develop a longitudinal investigation with standardized powerlifting-style programming, and consistent data collection times both before and after training. Another potential limitation may be the use of the Wilks coefficient as our surrogate performance indicator. The formula has reported inherent flaws, including weight class- and sex-specific biases because of its development several years before this investigation (1988–1994); at that point, limited data were available in women and several weight class ranges have since changed (12,46). Recent data have surfaced, however, demonstrating the Wilks formula persistently better predicts powerlifting performance compared with the newly developed IPF formula; this is true despite the latter's ability to account for larger samples of (male and female) performances and different sport divisions (12). Therefore, the Wilks coefficient remains the best supported, empirically-validated method to determine the greater bodyweight-scaled powerlifting competitor. Several federations have also taken notice of the existing Wilks formula shortcomings and are determined to develop a better method (including an updated 2020 Wilks coefficient), and thus these hitherto unsupported algorithms may prove as superior performance surrogates.

Finally, our subjects were limited to local competing powerlifters; future research may benefit from recruiting specifically national- or international-level athletes, possibly displaying exaggerated adaptations. Recruiting a large sample of high-level athletes may yield differential creatine analogues compared with intermediate/advanced powerlifters through enhanced MHC IIa expression (40,42). Powerlifters in the present study demonstrated higher relative MHC IIa content vs. CON, but this trend may amplify in higher skilled athletes. A recent case study performed by our group on an elite, AAS-using powerlifter displayed a 79% MHC IIa proportion, which is among the highest reported in the literature (22,36). Unfortunately, we did not have the capacity to drug-test our subjects, nor can we confirm history of previous use or drug test results beyond 36 months for USAPL powerlifters or one year for the remaining 4 athletes. Previous data comparing long-term AAS use vs drug-naïve powerlifters suggest users may develop greater fast-twitch fiber-specific cross-sectional area, which may subsequently result in concomitantly augmented muscle creatine content (11). Notwithstanding potential AAS-mediated increases in MHC IIa and associated creatine content, previous adaptation trends in resistance training models along with profiles in elite athletes suggest a concurrent rise between MHC IIa isoform distribution and resistance training-based skill (3,11,22,36).

Overall, these data support previous research in resistance-trained and powerlifting demographics, whereby PL displayed greater slow-twitch MHC I and fast-twitch IIa isoform composition with a concomitant decrease in extremely-fast-twitch MHC IIx content (14,22,48). Contrary to previous literature, our (group-collapsed) female subjects displayed higher MHC IIa isoform content relative to males, while also encompassing the first fiber-type profile in female powerlifters. Moreover, several creatine-associated markers in powerlifters did not significantly differ from sedentary controls, supporting the notion that adaptations to total creatine content are primarily because of absolute increases in skeletal muscle mass (33). Neither MHC IIa content nor MTC alone are significant predictors of powerlifting skill as per Wilks coefficient, ultimately suggesting that these characteristics in combination with other factors, including neurological efficiency, fat free mass, or changes in enzymatic activity may better forecast powerlifting performance.

Practical Applications

The present investigation is the first to establish both a profile of creatine-associated markers in powerlifting athletes and the first to examine these markers along with MHC isoform composition in female competitive powerlifters. Our data also provide novel insight into the potential of molecular skeletal muscle characteristics to predict powerlifting performance. Ultimately, neither MHC IIa composition nor muscle total creatine content alone significantly predict powerlifting performance via Wilks coefficient. Therefore, our data suggest other biochemical, biomechanical, and neurological adaptations, potentially in combination with one or both aforementioned characteristics, may better underline powerlifting skill variation.

Acknowledgments

The authors would like to thank the team members involved in both the Exercise and Biochemical Nutrition Laboratory at Baylor University, Waco, TX, and those in the Molecular Exercise Physiology at California State University, Fullerton. Special thanks to Emma Fletcher, M.A., M.S., M.V.B., Caelin Kim, B.S., Sarah Machek B.S., and Emiliya received her B.S. since the conclusion of this study for assisting with data collection. Further thanks to USA Powerlifting Texas and the United States Powerlifting Association for assistance in subject recruitment.

This article includes data from the OpenPowerlifting project, https://www.openpowerlifting.org. You may download a copy of the data at https://gitlab.com/openpowerlifting/opl-data.

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Keywords:

fiber type; SDS-PAGE; female athletes; creatinine; muscle biopsy; SLC6A8

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