Certainty of Evidence Assessment in Systematic Reviews Published by High-Impact Sports Science Journals: A Meta-epidemiological Study

BACKGROUND

Assessing certainty of evidence is a key element of any systematic review. The aim of this meta-epidemiology study was to understand the frequency and ways with which certainty of evidence is assessed in contemporary systematic reviews published in high-impact sports science journals.

METHODS

We searched PubMed and relevant journal web sites from 1 August 2016 to 11 October 2022 for systematic reviews published in the top-ten highest-impact journals within the 2020 Journal Citation Report for the Sports Sciences category. Pairs of independent reviewers screened items using a priori established criteria.

RESULTS

Of 1250 eligible documents, 258 (20.6%) assessed the certainty of evidence, defined as using two or more distinct domains to provide an overall rating of the trustworthiness of findings across studies. Nine methods were cited for assessing certainty, with the most common being the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach (61.6%). The proportion of systematic reviews assessing certainty of evidence appeared to increase over the 6-year timeframe analyzed. Across all reviews analyzed, a large majority addressed the domains of risk of bias, imprecision, and inconsistency of the results. Other certainty domains including indirectness/applicability were less commonly assessed.

DISCUSSION

Only one in five recent contemporary systematic reviews in the field of exercise and sports science assessed certainty of evidence. Organizational and institutional education on methods for assessing evidence may help further increase uptake of these methods and improve both the quality and clinical impact of systematic reviews in the field.

Effects of plyometric- and cycle-based high-intensity interval training on body composition, aerobic capacity, and muscle function in young females: a field-based group fitness assessment

High-intensity interval training (HIIT) is an effective alternative to moderate intensity continuous training for improvements in body composition and aerobic capacity; however, there is little work comparing different modalities of HIIT. The purpose of this study was to compare the effects of plyometric- (PLYO) and cycle-oriented (CYC) HIIT on body composition, aerobic capacity, and skeletal muscle size, quality, and function in recreationally trained females. Young (21.7 ± 3.1 yrs), recreationally active females were quasi-randomized (1:1 ratio) to 8 weeks of twice weekly PLYO (n = 15) or CYC (n = 15) HIIT. Body composition (four-compartment model), VO2peak, countermovement jump performance, muscle size, and echo intensity (muscle quality), as well as strength and power of the knee extensors and plantar flexors were measured before and after training. Both groups showed a similar decrease in body fat percentage (p < 0.001;η p 2   = 0.409) and echo intensity (p < 0.001;η p 2 = 0.558), and an increase in fat-free mass (p < 0.001;η p 2   = 0.367) and VO2peak (p = 0.001;η p 2 = 0.318). Muscle size was unaffected (p > 0.05), whereas peak torque was reduced similarly in both groups (p = 0.017;η p 2 = 0.188) and rapid torque capacity was diminished only for the knee extensors after CYC (p = 0.022; d = -0.67). These results suggest that PLYO and CYC HIIT are similarly effective for improving body composition, aerobic capacity, and muscle quality, whereas muscle function may express moderate decrements in recreationally active females. ClinicalTrials.gov (NCT05821504).

Effects of a ready-to-drink thermogenic beverage on resting energy expenditure, hemodynamic function, and subjective outcomes

BACKGROUND

Thermogenic supplements are often consumed by individuals seeking to improve energy levels and reduce body fat. These supplements are sold in powdered or ready-to-drink (RTD) forms and consist of a blend of ingredients such as caffeine, green tea extract, and other botanical compounds. While there is evidence that thermogenic supplements can positively affect resting energy expenditure (REE), the effect varies based on the combination of active ingredients. Additionally, there is some concern that thermogenic supplements may cause unwanted side effects on hemodynamic variables, like heart rate (HR) and blood pressure (BP). Therefore, further investigation into the efficacy and safety of commercially available products is warranted.

METHODS

Twenty-eight individuals (14 F, 14 M; age: 23.3 ± 3.9 yrs; height: 169.4 ± 8.6 cm; body mass: 73.3 ± 13.1 kg) completed two visits in a randomized, double-blind, crossover fashion. Each visit began with baseline REE, HR, and BP assessments, which were followed by ingestion of an active RTD thermogenic beverage (RTD; OxyShred Ultra Energy) or placebo (PL). Assessments were repeated at the intervals of 35-50- and 85-100-minutes post-ingestion. In addition, subjective outcomes of energy, focus, concentration, alertness, and mood were collected five times throughout each visit. Repeated-measures analysis of variance was performed with condition and time specified as within-subjects factors and sex and resistance training (RT) status as between-subjects factors. Statistical significance was accepted at p < 0.05.

RESULTS

A significant condition × time interaction was observed for REE (p < 0.001). Higher REE values were demonstrated at 35-50 min (0.08 ± 0.02 kcal/min; p = 0.001; 5.2% difference) and 85-100 min (0.08 ± 0.02 kcal/min; p = 0.001; 5.5% difference) after RTD ingestion as compared to PL. No significant condition × time interactions were observed for respiratory quotient, HR, or BP. Condition main effects indicated lower HR (3.0 ± 0.9 bpm; p = 0.003), higher SBP (3.5 ± 1.1 mm Hg; p = 0.003) and higher DBP (3.5 ± 0.9 mm Hg; p < 0.001) in RTD as compared to PL, irrespective of time. Condition × time interactions were observed for all subjective outcomes (p ≤ 0.02). Post hoc tests indicated statistically significant benefits of the RTD over PL for energy, focus, concentration, and alertness, without significant differences for mood after correction for multiple comparisons. Sex and RT status were not involved in interactions for any outcomes, except for a Sex × RT status interaction for energy, indicating higher energy ratings in non-resistance-trained vs. resistance-trained males.

CONCLUSIONS

These data suggest that acute ingestion of a thermogenic RTD beverage significantly increases REE, and this elevated caloric expenditure is sustained for at least 100 minutes following ingestion. Furthermore, the RTD beverage increased measures of energy, focus, concentration, and alertness as compared to placebo. While minor differences in hemodynamic variables were observed between conditions, all values stayed within normal ranges. Individuals aiming to increase energy expenditure may benefit from acute ingestion of an RTD thermogenic supplement.

No Effect of Breakfast Consumption Observed for Afternoon Resistance Training Performance in Habitual Breakfast Consumers and Nonconsumers: A Randomized Crossover Trial

BACKGROUND

Pre-exercise meal frequency is commonly believed to impact exercise performance, but little is known about its impact on resistance training.

OBJECTIVE

This study investigated the impact of breakfast consumption on afternoon resistance training performance in habitual breakfast consumers and nonconsumers.

DESIGN

A randomized, crossover study was conducted in Lubbock, TX between November 2021 and May 2022.

PARTICIPANTS

Thirty-nine resistance-trained male (n = 20) and female (n = 19) adults (mean ± SD age 23.0 ± 4.7 years) who habitually consumed (≥5 d/wk; n = 19) or did not consume (≥5 d/wk; n = 20) breakfast completed the study.

INTERVENTION

After the establishment of 1-repetition maximums at the first visit, participants completed 2 additional visits, each of which included 4 sets of barbell back squat, bench press, and deadlift, using 80% of their 1-repetition maximum after either consuming breakfast and lunch or the same food at lunch only.

MAIN OUTCOME MEASURES

Repetitions, along with average and peak average concentric velocity and power, were measured for all repetitions throughout each exercise session. Visual analog scales were used to assess feelings of fatigue, energy, focus, hunger, desire to eat, and fullness throughout each exercise session.

STATISTICAL ANALYSES PERFORMED

Data were analyzed using linear mixed-effects models.

RESULTS

No interactions or main effects involving condition or habitual breakfast consumption were observed for resistance training outcomes, although sex differences were noted. Male participants performed significantly fewer repetitions on sets 2, 3, and 4 (P < .014) for total repetitions, on sets 2 and 4 for barbell back squat (P < .023), and set 4 for deadlift (P = .006), with no observed differences between sexes for bench press repetitions. Male participants displayed reductions in average power across all sets and exercises except deadlift.

CONCLUSIONS

These data suggest that alterations in pre-exercise meal frequency may not influence afternoon resistance training performance provided similar total nutritional intake is consumed.

Assessing the reliability and cross-sectional and longitudinal validity of fifteen bioelectrical impedance analysis devices

The purpose of this investigation was to expand upon the limited existing research examining the test-retest reliability, cross-sectional validity and longitudinal validity of a sample of bioelectrical impedance analysis (BIA) devices as compared with a laboratory four-compartment (4C) model. Seventy-three healthy participants aged 19-50 years were assessed by each of fifteen BIA devices, with resulting body fat percentage estimates compared with a 4C model utilising air displacement plethysmography, dual-energy X-ray absorptiometry and bioimpedance spectroscopy. A subset of thirty-seven participants returned for a second visit 12-16 weeks later and were included in an analysis of longitudinal validity. The sample of devices included fourteen consumer-grade and one research-grade model in a variety of configurations: hand-to-hand, foot-to-foot and bilateral hand-to-foot (octapolar). BIA devices demonstrated high reliability, with precision error ranging from 0·0 to 0·49 %. Cross-sectional validity varied, with constant error relative to the 4C model ranging from -3·5 (sd 4·1) % to 11·7 (sd 4·7) %, standard error of the estimate values of 3·1-7·5 % and Lin's concordance correlation coefficients (CCC) of 0·48-0·94. For longitudinal validity, constant error ranged from -0·4 (sd 2·1) % to 1·3 (sd 2·7) %, with standard error of the estimate values of 1·7-2·6 % and Lin's CCC of 0·37-0·78. While performance varied widely across the sample investigated, select models of BIA devices (particularly octapolar and select foot-to-foot devices) may hold potential utility for the tracking of body composition over time, particularly in contexts in which the purchase or use of a research-grade device is infeasible.

Anthropometric Predictors of Conventional Deadlift Kinematics and Kinetics: A Preliminary Study

The purpose of this preliminary analysis was to determine if there are relationships between anthropometric characteristics (arm length, torso length, thigh length, and shank length) and conventional deadlift (CDL) kinematics and kinetics during a 5 sets of 5 repetitions (5 × 5) CDL routine in resistance-trained males. Eleven males who had experience with the deadlift exercise were included in this analysis (age: 21.5 ± 1.4 y; height: 180.7 ± 5.7 cm; body mass: 89.9 ± 16.0 kg). Anthropometrics were measured by a 3-dimensional optical scanner. The participants underwent a 5 × 5 CDL workout using a self-selected load corresponding to a rating of perceived exertion (RPE) of 8 out of 10. Performance outcomes were measured synchronously using a 3-dimensional 12-camera motion capture system and two force platforms. Outcomes were averaged across all sets and analyzed using multiple linear regression. The selected anthropometric variables were not significantly related to the CDL performance outcomes, except for concentric ankle work. However, in the overall model, anthropometric predictors did not significantly predict ankle concentric work (p = 0.11; R ² = 0.67; R_2adj = 0.45). Independently, thigh length significantly correlated with ankle concentric work (p = 0.03). In this model, thigh length accounted for 55% of the normalized variance in ankle concentric work. The results from this preliminary study suggest that arm length, torso length, and shank length may not play a clear role in the examined CDL outcomes, but thigh length may be positively correlated with ankle concentric work during a 5 × 5 CDL routine in resistance-trained males.

Evaluation of novel beverage formulations for hydration enhancement in humans

This study evaluated the influence of novel beverage formulations on bioimpedance- and urine-based hydration markers. Thirty young healthy adults (n=16 females, n=14 males; age: 23.2±3.7 years; BMI: 24.3±3.3 kg/m²) participated in a randomized, double-blind, placebo-controlled, crossover study. Participants completed three conditions with baseline bioimpedance, urine, and body mass assessments, followed by ingestion of one liter of a test beverage over a 30-minute period. The three beverages were: active hydration formulation in still (AFstill) or sparkling (AFspark) water and a still water control. The active formulations were identical in concentrations of alpha-cyclodextrin and complexing agents. Following beverage ingestion, bioimpedance assessments were performed every 15 minutes for two hours, followed by final urinary and body mass assessments. The primary bioimpedance outcomes were phase angle at 50 kHz, resistance of the extra-cellular compartment (R₀), and resistance of the intracellular compartment (Ri). Data were analyzed using linear mixed effects models, Friedman tests, and Wilcoxon tests. Statistically significant changes in phase angle values were observed at 30 (p=0.004) and 45 minutes (p=0.024) following the initiation of beverage ingestion in the AFstill condition as compared to the reference model (i.e., control condition at baseline). Although differences between conditions were not statistically significant at later time points, the data were consistent with AFstill having greater elevations in phase angle throughout the monitoring period. At the 30-minute time point only, statistically significant differences in R₀ for AFspark (p<0.001) and in Ri for AFstill (p=0.008) were observed. When averaged across post-ingestion time points, there was a trend (p=0.08) for Ri differences between conditions. The net fluid balance was greater than zero, indicating retention of ingested fluid, for AFstill (p=0.02) and control (p=0.03), with a trend for AFspark (p=0.06). In conclusion, an active formulation containing alpha-cyclodextrin in still water demonstrated potential benefits for enhancing hydration markers in humans.

Liposomal Mineral Absorption: A Randomized Crossover Trial

Multivitamin/mineral (MVM) supplements are one of the most popular dietary supplement categories. The purpose of this analysis was to determine if a novel liposomal delivery mechanism improves mineral absorption from an MVM product. In a randomized crossover trial, 25 healthy participants (12 females, 13 males) completed two testing sessions in which blood samples were collected at baseline and 2, 4, and 6 h following the ingestion of either a liposomal MVM or a nutrient-matched standard MVM. Analysis of MVM products indicated an elemental iron content of 9.4 and 10.1 mg (~50% U.S. FDA Daily Value) and an elemental magnesium content of 22.0 and 23.3 mg (~5% U.S. FDA Daily Value) in the liposomal and standard MVM products, respectively. Blood samples were analyzed for concentrations of iron and magnesium using colorimetric assays. Changes in mineral concentrations were analyzed using linear mixed models, and pharmacokinetic parameters were compared between conditions. For iron, statistically significant condition × time interactions were observed for percent change from baseline (p = 0.002), rank of percent change from baseline (p = 0.01), and raw concentrations (p = 0.02). Follow-up testing indicated that the liposomal condition exhibited larger changes from baseline than the standard MVM condition at 4 (p = 0.0001; +14.3 ± 18.5% vs. −6.0 ± 13.1%) and 6 h (p = 0.0002; +1.0 ± 20.9% vs. −21.0 ± 15.3%) following MVM ingestion. These changes were further supported by a 50% greater mean incremental area under the curve in the liposomal condition (33.2 ± 30.9 vs. 19.8 ± 19.8 mcg/dL × 6 h; p = 0.02, Cohen’s d effect size = 0.52). In contrast, no differential effects for magnesium absorption were observed. In conclusion, iron absorption from an MVM product is enhanced by a liposomal delivery mechanism.

The influence of caffeinated and non-caffeinated multi-ingredient pre-workout supplements on resistance exercise performance and subjective outcomes

Background

There is substantial consumer and practitioner interest in an emerging supplement class known as multi-ingredient pre-workout supplements (MIPS), largely due to their prevalence in resistance training communities as well as research findings demonstrating the ergogenic impact of caffeine on muscular performance. However, limited research has examined the potential efficacy of non-caffeinated MIPS, despite their growing popularity among those who are caffeine-sensitive or who train later in the day.

Methods

Twenty-four resistance-trained college-aged males (n = 12) and females (n = 12) completed three visits in which they ingested either a caffeinated MIPS (C), an otherwise identical non-caffeinated MIPS (NC), or placebo in a double-blind, counterbalanced, crossover fashion. Squat isometric peak force (PF_iso), rate of force development (RFD), and isokinetic performance were assessed. Upper and lower body maximal muscular strength and endurance were evaluated using the bench press and leg press, respectively. Visual analog scales for energy, focus, and fatigue were completed five times throughout the testing protocol. The effects of supplementation and biological sex on all variables were examined using linear mixed effects models.

Results

Significantly greater PF_iso was observed in both C (b: 0.36 transformed units [0.09, 0.62]) and NC (b: 0.32 transformed units [95% CI: 0.05, 0.58]) conditions, relative to placebo. Early RFD (RFD₅₀) may have been higher with supplementation, particularly in females, with no effects for late RFD (RFD₂₀₀) or peak RFD. In addition, increases in subjective energy after supplement ingestion were noted for C, but not NC. No effects of supplementation on traditional resistance exercise performance or isokinetic squat performance were observed, other than a lower leg press one-repetition maximum for males in the NC condition.

Conclusions

These data indicate that acute ingestion of either a caffeinated or non-caffeinated pre-workout formulation improved maximal force production during an isometric squat test but did not provide additional benefit to leg press, bench press, or isokinetic squat performance over placebo, within the context of a laboratory environment. The consumption of a caffeinated, but not non-caffeinated, MIPS increased subjective ratings of energy over placebo when assessed as part of a testing battery.

Physiological responses to acute fasting: implications for intermittent fasting programs

Intermittent fasting (IF) is a dietary strategy that involves alternating periods of abstention from calorie consumption with periods of ad libitum food intake. There is significant interest in the body of literature describing longitudinal adaptations to IF. Less attention has been given to the acute physiological responses that occur during the fasting durations that are commonly employed by IF practitioners. Thus, the purpose of this review was to examine the physiological responses - including alterations in substrate metabolism, systemic hormones, and autophagy - that occur throughout an acute fast. Literature searches were performed to locate relevant research describing physiological responses to acute fasting and short-term starvation. A single fast demonstrated the ability to alter glucose and lipid metabolism within the initial 24 hours, but variations in protein metabolism appeared to be minimal within this time frame. The ability of an acute fast to elicit significant increases in autophagy is still unknown. The information summarized in this review can be used to help contextualize existing research and better inform development of future IF interventions.

Effect of the Repetitions-In-Reserve Resistance Training Strategy on Bench Press Performance, Perceived Effort, and Recovery in Trained Men

ABSTRACT

Mangine, GT, Serafini, PR, Stratton, MT, Olmos, AA, VanDusseldorp, TA, and Feito, Y. Effect of the repetitions-in-reserve resistance training strategy on bench press performance, perceived effort, and recovery in trained men. J Strength Cond Res 36(1): 1-9, 2022-This study examined the effects of the repetitions-in-reserve (RIR) strategy on resistance exercise performance, perceived effort, and recovery. Fourteen resistance-trained men (24.6 ± 3.0 years, 176 ± 5 cm, 85.7 ± 14.0 kg) completed 2 bench press protocols in a randomized crossover fashion. The protocols consisted of 4 sets at 80% of 1 repetition maximum (1RM) to a self-reported 3-RIR and a fifth set to failure or all 5 sets to failure (0-RIR). Barbell kinetics (velocity, rate of force development, and impulse), repetition volume, total work, and ratings of perceived exertion (RPE) were quantified on each set. Barbell kinetics were reassessed during one set of 3 repetitions at 80% 1RM completed at 24-hour, 48-hour, and 72-hour postexercise. Blood samples were collected before and after exercise at 6 hours, 24 hours, 48 hours, and 72 hours and analyzed for concentrations of creatine kinase (CK). Separate, 2-way repeated-measures analysis of variance revealed significant interactions (p < 0.001) where 3-RIR better maintained repetitions and work at greater average velocity (+0.6 m·s-1) and lower RPE (0-RIR = 10; 3-RIR = 8.2) across all sets. No differences were seen between conditions for CK at 6 hours postexercise (3-RIR: 32.2 ± 55.3%; 0-RIR: 40.8 ± 66.0%) or for CK and barbell kinetics at 24 hours to 72 hours postexercise. Although no differences were seen for recovery, the RIR strategy enabled work to be better sustained across sets at a lower perceived effort and higher average velocity. This strategy could be used to manage fatigue and better sustain effort and volume during a resistance training session.

Influence of Acute Water Ingestion and Prolonged Standing on Raw Bioimpedance and Subsequent Body Fluid and Composition Estimates

This study evaluated the influence of acute water ingestion and maintaining an upright posture on raw bioimpedance and subsequent estimates of body fluids and composition. Twenty healthy adults participated in a randomized crossover study. In both conditions, an overnight food and fluid fast was followed by an initial multi-frequency bioimpedance assessment (InBody 770). Participants then ingested 11 mL/kg of water (water condition) or did not (control condition) during a 5-minute period. Thereafter, bioimpedance assessments were performed every 10 minutes for one hour with participants remaining upright throughout. Linear mixed effects models were used to examine the influence of condition and time on raw bioimpedance, body fluids, and body composition. Water consumption increased impedance of the arms but not trunk or legs. However, drift in leg impedance was observed, with decreasing values over time in both conditions. No effects of condition on body fluids were detected, but total body water and intracellular water decreased by ~0.5 kg over time in both conditions. Correspondingly, lean body mass did not differ between conditions but decreased over the measurement duration. The increase in body mass in the water condition was detected exclusively as fat mass, with final fat mass values ~1.3 kg higher than baseline and also higher than the control condition. Acute water ingestion and prolonged standing exert practically meaningful effects on relevant bioimpedance variables quantified by a modern, vertical multi-frequency analyzer. These findings have implications for pre-assessment standardization, methodological reporting, and interpretation of assessments.

Cross-sectional and longitudinal associations between subcutaneous adipose tissue thickness and dual-energy X-ray absorptiometry fat mass

The present study examined cross-sectional and longitudinal relationships between total and segmental subcutaneous tissue thicknesses from ultrasonography (US) and total and segmental fat mass (FM) estimates from dual-energy X-ray absorptiometry (DXA). Traditional US FM estimates were also examined. Twenty resistance-trained males (mean ± SD; age: 22.0 ± 2.6 years; body mass: 74.8 ± 11.5 kg; DXA fat: 17.5 ± 4.5%) completed a 6-week supervised resistance training programme while consuming a hypercaloric diet. Pre- and post-intervention body composition was assessed by DXA and B-mode US. Data were analysed using Pearson's correlation (r), Lin's correlation coefficient (CCC), paired t-tests, Wilcoxon signed-rank tests and Bland-Altman analysis, as appropriate. Cross-sectionally, correlations were observed between total DXA FM and total subcutaneous tissue thickness (r = 0.88). Longitudinally, a correlation was observed between total DXA FM changes and total subcutaneous tissue changes (r = 0.49, CCC = 0.38). Correlations of similar magnitudes were observed for the upper body and trunk estimates, but DXA FM changes were unrelated to subcutaneous tissue changes for the lower body and arms. Cross-sectionally, US 2-compartment FM and DXA FM were correlated (r = 0.91, CCC = 0.83). Longitudinally, a weaker correlation was observed (r = 0.47, CCC = 0.33). In summary, longitudinal associations between US and DXA are weaker than cross-sectional relationships; additionally, correlations between US subcutaneous tissue and whole-body DXA FM appear to be driven by the trunk region rather than appendages. Reporting raw skinfold thicknesses rather than FM estimates alone may improve the utility of techniques based on subcutaneous tissue thickness, such as US and skinfolds.

Endocrine and Body Composition Changes Across a Competitive Season in Collegiate Speed-Power Track and Field Athletes

ABSTRACT

Mangine, GT, Eggerth, A, Gough, J, Stratton, MT, Feito, Y, and VanDusseldorp, TA. Endocrine and body composition changes across a competitive season in collegiate speed-power track and field athletes. J Strength Cond Res 35(8): 2067-2074, 2021-Maintaining lean mass is important for track and field (TF) athletes who compete in speed-power events, but little is known about how lean mass and related hormones might change over an 8- to 10-month collegiate season. Therefore, to monitor changes in free testosterone (T), cortisol (C), and body composition in TF athletes across their entire competitive season, 9 female (20.3 ± 1.2 years, 169 ± 5 cm, and 67.6 ± 8.5 kg) and 7 male (21.1 ± 2.0 years, 181 ± 9 cm, and 77.3 ± 5.9 kg) Division I TF athletes provided resting and fasted blood samples at the onset of their indoor season (baseline), before and on returning from the indoor conference championships (ICCs), at the beginning and end of a heavy midseason training week (HVY), and before leaving for the National Collegiate Athletic Association (NCAA) Championships. Body composition was also assessed at each of these periods using a 4-compartment model. Except for a 20% reduction (p = 0.030) from ICCs to the onset of HVY in men only, linear mixed models with repeated measures did not reveal any changes in hormone concentrations. Compared with baseline, an overall increase in fat-free mass was observed at HVY (∼2.74%, p = 0.023) before it reduced by 3.81% before the NCAA Championships (p = 0.022). Despite variations in training and competition, resting concentrations of hormones indicative of anabolic status remained relatively consistent over the course of an entire season in speed-power TF athletes. Coaches and athletes may consider monitoring these variables to assess the athlete's response to the changing demands of a competitive season.

Comparison of Indirect Calorimetry and Common Prediction Equations for Evaluating Changes in Resting Metabolic Rate Induced by Resistance Training and a Hypercaloric Diet

ABSTRACT

Rodriguez, C, Harty, PS, Stratton, MT, Siedler, MR, Smith, RW, Johnson, BA, Dellinger, JR, Williams, AD, White, SJ, Benavides, ML, and Tinsley, GM. Comparison of indirect calorimetry and common prediction equations for evaluating changes in resting metabolic rate induced by resistance training and a hypercaloric diet. J Strength Cond Res XX(X): 000-000, 2021-The ability to accurately identify resting metabolic rate (RMR) changes over time allows practitioners to prescribe appropriate adjustments to nutritional intake. However, there is a lack of data concerning the longitudinal utility of commonly used RMR prediction equations. The purpose of this study was to evaluate the validity of several commonly used prediction equations to track RMR changes during a hypercaloric nutritional intervention and supervised resistance exercise training program. Twenty resistance-trained men completed the study. The protocol lasted 6 weeks, and subjects underwent RMR assessments by indirect calorimetry (IC) preintervention and postintervention to obtain reference values. Existing RMR prediction equations based on body mass (BM) or dual-energy X-ray absorptiometry fat-free mass (FFM) were also evaluated. Equivalence testing was used to evaluate whether each prediction equation demonstrated equivalence with IC. Null hypothesis significance testing was also performed, and Bland-Altman analysis was used alongside linear regression to assess the degree of proportional bias. Body mass and FFM increased by 3.6 ± 1.7 kg and 2.4 ± 1.6 kg, respectively. Indirect calorimetry RMR increased by 165 ± 97 kcal·d-1, and RMR:FFM increased by 5.6 ± 5.2%. All prediction equations underestimated mean RMR changes relative to IC, with magnitudes ranging from 75 to 155 kcal·d-1, while also displaying unacceptable levels of negative proportional bias. In addition, no equation demonstrated equivalence with IC. Common RMR prediction equations based on BM or FFM did not fully detect the increase in RMR observed with resistance training plus a hypercaloric diet. Overall, the evaluated prediction equations are unsuitable for estimating RMR changes in the context of this study.

Predicting Adaptations to Resistance Training Plus Overfeeding Using Bayesian Regression: A Preliminary Investigation

Relatively few investigations have reported purposeful overfeeding in resistance-trained adults. This preliminary study examined potential predictors of resistance training (RT) adaptations during a period of purposeful overfeeding and RT. Resistance-trained males (n = 28; n = 21 completers) were assigned to 6 weeks of supervised RT and daily consumption of a high-calorie protein/carbohydrate supplement with a target body mass (BM) gain of ≥0.45 kg·wk-1. At baseline and post-intervention, body composition was evaluated via 4-component (4C) model and ultrasonography. Additional assessments of resting metabolism and muscular performance were performed. Accelerometry and automated dietary interviews estimated physical activity levels and nutrient intake before and during the intervention. Bayesian regression methods were employed to examine potential predictors of changes in body composition, muscular performance, and metabolism. A simplified regression model with only rate of BM gain as a predictor was also developed. Increases in 4C whole-body fat-free mass (FFM; (mean ± SD) 4.8 ± 2.6%), muscle thickness (4.5 ± 5.9% for elbow flexors; 7.4 ± 8.4% for knee extensors), and muscular performance were observed in nearly all individuals. However, changes in outcome variables could generally not be predicted with precision. Bayes R2 values for the models ranged from 0.18 to 0.40, and other metrics also indicated relatively poor predictive performance. On average, a BM gain of ~0.55%/week corresponded with a body composition score ((∆FFM/∆BM)*100) of 100, indicative of all BM gained as FFM. However, meaningful variability around this estimate was observed. This study offers insight regarding the complex interactions between the RT stimulus, overfeeding, and putative predictors of RT adaptations.

Explaining Discrepancies Between Total and Segmental DXA and BIA Body Composition Estimates Using Bayesian Regression

INTRODUCTION/BACKGROUND

Few investigations have sought to explain discrepancies between dual-energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA) body composition estimates. The purpose of this analysis was to explore physiological and anthropometric predictors of discrepancies between DXA and BIA total and segmental body composition estimates.

METHODOLOGY

Assessments via DXA (GE Lunar Prodigy) and single-frequency BIA (RJL Systems Quantum V) were performed in 179 adults (103 F, 76 M, age: 33.6 ± 15.3 yr; BMI: 24.9 ± 4.3 kg/m²). Potential predictor variables for differences between DXA and BIA total and segmental fat mass (FM) and lean soft tissue (LST) estimates were obtained from demographics and laboratory techniques, including DXA, BIA, bioimpedance spectroscopy, air displacement plethysmography, and 3-dimensional optical scanning. To determine meaningful predictors, Bayesian robust regression models were fit using a t-distribution and regularized hierarchical shrinkage "horseshoe" prior. Standardized model coefficients (β) were generated, and leave-one-out cross validation was used to assess model predictive performance.

RESULTS

LST hydration (i.e., total body water:LST) was a predictor of discrepancies in all FM and LST variables (|β|: 0.20-0.82). Additionally, extracellular fluid percentage was a predictor for nearly all outcomes (|β|: 0.19-0.40). Height influenced the agreement between whole-body estimates (|β|: 0.74-0.77), while the mass, length, and composition of body segments were predictors for segmental LST estimates (|β|: 0.23-3.04). Predictors of segmental FM errors were less consistent. Select sex-, race-, or age-based differences between methods were observed. The accuracy of whole-body models was superior to segmental models (leave-one-out cross-validation-adjusted R² of 0.83-0.85 for FM_TOTAL and LST_TOTAL vs. 0.20-0.76 for segmental estimates). For segmental models, predictive performance decreased in the order of: appendicular lean soft tissue, LST_LEGS, LST_TRUNK and FM_LEGS, FM_ARMS, FM_TRUNK, and LST_ARMS.

CONCLUSIONS

These findings indicate the importance of LST hydration, extracellular fluid content, and height for explaining discrepancies between DXA and BIA body composition estimates. These general findings and quantitative interpretation based on the presented data allow for a better understanding of sources of error between 2 popular segmental body composition techniques and facilitate interpretation of estimates from these technologies.

Longitudinal agreement of four bioimpedance analyzers for detecting changes in raw bioimpedance during purposeful weight gain with resistance training

BACKGROUND

Due to inherent errors involved in the transformation of raw bioelectrical variables to body fluids or composition estimates, the sole use of resistance (R), reactance (Xc), and phase angle (φ) has been advocated when quantifying longitudinal changes. The aim of this investigation was to assess the ability of four bioimpedance analyzers to detect raw bioimpedance changes induced by purposeful weight gain with resistance training.

METHODS

Twenty-one resistance trained males completed a 6-week lifestyle intervention with the aim of purposeful weight gain. Bioimpedance analysis was performed before and after the intervention using four different analyzers (MFBIA_InBody: InBody 770; MFBIA_SECA: Seca mBCA 515/514; BIS: ImpediMed SFB7; SFBIA: RJL Quantum V) for the quantification of R, Xc, and φ at the 50-kHz frequency. Repeated measures ANOVA and follow up tests were performed.

RESULTS

Analysis revealed main effects of time and method for R, Xc, and φ (p ≤ 0.02), without significant time x method interactions (p ≥ 0.07). Follow up for time main effects indicated that, on average, R decreased by 4.5-5.8%, Xc decreased by 2.3-4.0%, and φ increased by 1.8-2.6% across time for all analyzers combined. However, varying levels of disagreement in absolute values were observed for each bioelectrical variable.

CONCLUSIONS

The differences in absolute bioelectrical values suggests that analyzers should not be used interchangeably, which holds particular importance when reference values are utilized. Despite absolute differences, analyzers with varying characteristics demonstrated similar abilities to detect changes in R, Xc, and φ over time.

A Field-based Three-Compartment Model Derived from Ultrasonography and Bioimpedance for Estimating Body Composition Changes

PURPOSE

The purpose of this study was to assess the agreement between a field-based three-compartment (3CFIELD) model and a laboratory-based three-compartment (3CLAB) model for tracking body composition changes over time.

METHODS

Resistance-trained males completed a supervised nutrition and resistance training intervention. Before and after the intervention, assessments were performed via air displacement plethysmography (ADP), bioimpedance spectroscopy (BIS), portable ultrasonography (US), and bioelectrical impedance analysis (BIA). ADP body density and BIS body water were used within the reference 3CLAB model, whereas US-derived body density and BIA body water were used within the 3CFIELD model. Two-compartment model body composition estimates provided by US and BIA were also examined. Changes in fat-free mass and fat mass were analyzed using repeated-measures ANOVA, equivalence testing, Bland-Altman analysis, linear regression, and related validity analyses.

RESULTS

Significant increases in fat-free mass (3CLAB, 4.0 ± 4.5 kg; 3CFIELD, 3.9 ± 4.2 kg; US, 3.2 ± 4.3 kg; BIA, 3.9 ± 4.2 kg) and fat mass (3CLAB, 1.3 ± 2.2 kg; 3CFIELD, 1.4 ± 2.2 kg; US, 2.1 ± 2.6 kg; BIA, 1.4 ± 2.9 kg) were detected by all methods. However, only the 3CFIELD model demonstrated equivalence with the 3CLAB model. In addition, the 3CFIELD model exhibited superior performance to US and BIA individually, as indicated by the total error (3CFIELD, 1.0 kg; US, 1.8 kg; BIA, 1.6 kg), 95% limits of agreement (3CFIELD, ±2.1 kg; US, ±3.3 kg; BIA, ±3.1 kg), correlation coefficients (3CFIELD, 0.79-0.82; US, 0.49-0.55; BIA, 0.61-0.72), and additional metrics.

CONCLUSIONS

The present study demonstrated the potential usefulness of a 3CFIELD model incorporating US and BIA data for tracking body composition changes over time, as well as its superiority to US or BIA individually. As such, this accessible multicompartment model may be suitable for implementation in field or limited-resource settings.

Agreement of bioelectrical resistance, reactance, and phase angle values from supine and standing bioimpedance analyzers

OBJECTIVE

Bioimpedance devices are commonly used to assess health parameters and track changes in body composition. However, the cross-sectional agreement between different devices has not been conclusively established. Thus, the objective of this investigation was to examine the agreement between raw bioelectrical variables (resistance, reactance, and phase angle at the 50-kHz frequency) obtained from three bioimpedance analyzers.

APPROACH

Healthy male (n=76, Mean±SD; 33.8±14.5 years; 83.9±15.1 kg; 179.4±6.9 cm) and female (n=103, Mean±SD; 33.4±15.9 years; 65.6±12.1 kg; 164.9±6.4 cm) participants completed assessments using three bioimpedance devices: supine bioimpedance spectroscopy (BIS), supine single-frequency bioelectrical impedance analysis (SFBIA), and standing multi-frequency bioelectrical impedance analysis (MFBIA). Differences in raw bioelectrical variables between the devices were quantified via one-way analysis of variance for the total sample and for each sex. Equivalence testing was used to determine equivalence between methods.

MAIN RESULTS

Significant differences in all bioelectrical variables were observed between the three devices when examining the total sample and males only. The devices appeared to exhibit slightly better agreement when analyzing female participants only. Equivalence testing using the total sample as well as males and females separately revealed that resistance and phase angle were equivalent between the supine devices (BIS, SFBIA), but not with the standing analyzer (MFBIA).

SIGNIFICANCE

The present study demonstrated disagreement between different bioimpedance analyzers for quantifying raw bioelectrical variables, with the poorest agreement between devices that employed different body positions during testing. These results suggest that researchers and clinicians should employ device-specific reference values to classify participants based on raw bioelectrical variables, such as phase angle. If reference values are needed but are unavailable for a particular bioimpedance analyzer, the set of reference values produced using the most similar analyzer and reference population should be selected.

Effects of Bang® Keto Coffee Energy Drink on Metabolism and Exercise Performance in Resistance-Trained Adults: A Randomized, Double-blind, Placebo-controlled, Crossover Study

BACKGROUND

Energy drinks are often consumed by the general population, as well as by active individuals seeking to enhance exercise performance and augment training adaptations. However, limited information is available regarding the efficacy of these products. Thus, the purpose of this study was to determine the effects of a commercially available caffeine- and protein-containing energy drink on metabolism and muscular performance.

METHODS

Sixteen resistance-trained males (n = 8; mean ± SD; age: 22.4 ± 4.9 years; body mass: 78.8 ± 14.0 kg; body fat: 15.3 ± 6.4%) and females (n = 8; age: 24.5 ± 4.8 years; body mass: 67.5 ± 11.9 kg; body fat: 26.6 ± 7.1%) participated in this randomized, double-blind, placebo-controlled, crossover study. Following a familiarization visit, participants completed two identical visits to the laboratory separated by 5-10 days, each of which consisted of indirect calorimetry energy expenditure (EE) assessments before and after consumption of the beverage (Bang® Keto Coffee; 130 kcal, 300 mg caffeine, 20 g protein) or placebo (30 kcal, 11 mg caffeine, 1 g protein) as well as after exercise testing. In addition, participants' subjective feelings of energy, fatigue, and focus as well as muscular performance (leg press one-repetition maximum and repetitions to fatigue, maximal isometric and isokinetic squat testing) were assessed. Multiple repeated measures ANOVAs with Tukey post-hoc tests were used to analyze data. Estimates of effect size were quantified via partial eta squared (ηP2) and Hedge's g.

RESULTS

A significant interaction effect was identified for EE (p < 0.001, ηP2 = 0.52) but not respiratory exchange ratio (p = 0.17, ηP2 = 0.11). Following consumption of the beverage, EE was 0.18 [corrected]  kcal·min- 1 greater than placebo at the post-beverage time point (p < 0.001) and 0.08 [corrected]  kcal·min- 1 greater than placebo at the post-exercise time point (p = 0.011). However, no between-condition differences were detected for any subjective or muscular performance outcomes.

CONCLUSIONS

The results of this study suggest that consumption of the energy drink had minimal effects on lower-body muscular performance and subjective factors in the context of a laboratory setting. However, the beverage was found to significantly increase energy expenditure compared to placebo immediately following ingestion as well as during the recovery period after an exercise bout, suggesting that active individuals may improve acute metabolic outcomes via consumption of a caffeine- and protein-containing energy drink.

TRIAL REGISTRATION

This trial was prospectively registered at ClinicalTrials.gov (Identifier: NCT04180787 ; Registered 29 November 2019).

Sit-to-Stand Kinetics and Correlates of Performance in Young and Older Males

PURPOSE

To compare sit-to-stand (STS) kinetics in young (YM) and older (OM) males and determine correlates of STS performance.

METHODS

YM (n = 15, age = 20.7 ± 2.2 yrs) and OM (n = 15, age = 71.6 ± 3.9 yrs) performed a single STS task as quickly as possible on a force plate and the vertical ground reaction force (VGRF) signal was analyzed. Peak VGRF, as well as peak (100 ms rolling average), early (minimum VGRF to 50% peak VGRF), late (50% peak VGRF to peak VGRF), and overall (minimum VGRF to peak VGRF) rate of force development (RFD) were calculated. Power (absolute and relative) and velocity parameters as well as rate of electromyography rise (RER) were also obtained.

RESULTS

STS time, average power, early RFD, and lower limb lean mass were similar between groups (p > 0.05). All other power, velocity, RFD, and RER measures were lower in OM (p < 0.05; d = 0.41-2.19). Peak VGRF and all RFD measures, except late RFD, were strongly correlated with STS performance in OM, while peak VGRF and peak RFD were only moderately correlated with performance in YM.

CONCLUSIONS

Most kinetic variables, except absolute average power, were diminished in OM, and there was a preferential decrease in late RFD compared to early RFD. Peak VGRF and RFD exhibited stronger correlations with STS time and power in OM compared to YM, and early RFD appears to be more influential for STS performance than late RFD. These findings may be useful for practitioners/clinicians involved in designing interventions aimed at optimizing STS performance in older adults.

Impact of Varying Dosages of Fish Oil on Recovery and Soreness Following Eccentric Exercise

Fish oils (FOs) are rich in omega-3 long-chain polyunsaturated fatty acids, which have been purported to enhance recovery of muscular performance and reduce soreness post-exercise. However, the most effective FO dose for optimizing recovery remains unclear. The purpose of this investigation was to examine the effect of FO supplementation dosing on the recovery of measures of muscular performance, perceived soreness, and markers of muscle damage following a rigorous bout of eccentric exercise. Thirty-two college-aged resistance-trained males (~23.6 years, 71.6 kg, 172.1 cm) were supplemented with 2, 4, 6 g/day (G) FO or placebo (PL) for ~7.5 weeks. Following 7 weeks of supplementation, pre-exercise (PRE) performance assessments of vertical jump (VJ), knee extensor strength, 40-yard sprint, T-test agility, and perceived soreness were completed prior to a bout of muscle-damaging exercise and were repeated immediately post (IP), 1-, 2-, 4-, 24-, 48-, and 72-h (H) post-exercise. Repeated measures analysis of variance indicated a treatment × time interaction (p < 0.001) for VJ and perceived soreness, but no group differences were observed at any time point. VJ returned to PRE (54.8 ± 7.9 cm) by 1H (51.8 ± 6.5 cm, p = 0.112) for 6G, while no other groups returned to baseline until 48H. Lower soreness scores were observed in 6G compared to PL at 2H (mean difference [MD] = 2.74, p = 0.046), at 24H (MD: 3.45, p < 0.001), at 48H (MD = 4.45, p < 0.001), and at 72H (MD = 3.00, p = 0.003). Supplementation with 6G of FO optimized the recovery of jump performance and muscle soreness following a damaging bout of exercise.

Fat-free mass characteristics vary based on sex, race, and weight status in US adults

Common body composition estimation techniques necessitate assumptions of uniform fat-free mass (FFM) characteristics, although variation due to sex, race, and body characteristics may occur. National Health and Nutrition Examination Survey data from 1999 to 2004, during which paired dual-energy x-ray absorptiometry (DXA) and bioimpedance spectroscopy assessments were performed, were used to estimate FFM characteristics in a sample of 4619 US adults. Calculated FFM characteristics included the density and water, bone mineral, and residual content of FFM. A rapid 4-component model was also produced using DXA and bioimpedance spectroscopy data. Study variables were compared across sex, race/ethnicity, body mass index (BMI), and age categories using multiple pairwise comparisons. A general linear model was used to estimate body composition after controlling for other variables. Statistical analyses accounted for 6-year sampling weights and complex sampling design of the National Health and Nutrition Examination Survey and were based on 5 multiply imputed datasets. Differences in FFM characteristics across sex, race, and BMI were observed, with notable dissimilarities between men and women for all outcome variables. In racial/ethnic comparisons, non-Hispanic blacks most commonly presented distinct FFM characteristics relative to other groups, including greater FFM density and proportion of bone mineral. Body composition errors between DXA and the 4-component model were significantly influenced by sex, age, race, and BMI. In conclusion, FFM characteristics, which are often assumed in body composition estimation methods, vary due to sex, race/ethnicity, and weight status. The variation of FFM characteristics in diverse populations should be considered when body composition is evaluated.

Early and late rapid torque characteristics and select physiological correlates in middle-aged and older males

Purpose

The purpose of this study was to compare early and late rapid torque parameters of the plantar flexors (PFs) in middle-aged (MM) and older (OM) males, and determine the effect of normalization to peak torque (PT) and muscle cross-sectional area (CSA).

Methods

Twenty-nine healthy, MM (n = 14; 45 ± 2 yrs) and OM (n = 15; 65 ± 3 yrs) performed rapid, maximal isometric contractions of the PFs. PT, as well as rate of torque development and impulse during the early (0–50 ms; RTD_0-50, IMP_0-50) and late (100–200 ms; RTD_100-200, IMP_100-200) contraction phases were calculated. Torque at 50 (TQ₅₀), 100 (TQ₁₀₀), and 200 (TQ₂₀₀) ms was also obtained. CSA and echo-intensity (EI) of the gastrocnemii were acquired via ultrasonography. Torque variables were normalized to PT and CSA. Rate of EMG rise (RER) for the medial gastrocnemius was calculated at 30, 50 and 75 ms.

Results

TQ₁₀₀ (MM = 69.71 ± 16.85 vs. OM = 55.99 ± 18.54 Nm; p = 0.046), TQ₂₀₀ (MM = 114.76 ± 26.79 vs. OM = 91.56 ± 28.10 Nm; p = 0.031), and IMP_100-200 (MM = 4.79 ± 1.11 vs. OM = 3.83 ± 1.17 Nm·s; p = 0.032) were lower in OM. PT, TQ₅₀, RTD_0-50, IMP_0-50, RTD_100-200, RER, CSA, and EI were similar between groups (p > 0.05). No differences were found for normalized torque variables (p > 0.05). EI was moderately associated with normalized torque parameters only (r = -0.38 –-0.45). RER, at 75 ms, was moderately correlated with early, absolute torque measures and rapid torque variables made relative to PT and CSA (r = 0.41 –-0.64).

Conclusion

Late rapid torque parameters of the PFs were preferentially impaired in OM compared to MM, and PT as well as CSA appeared to mediate this result.

Physiological differences between advanced CrossFit athletes, recreational CrossFit participants, and physically-active adults

This investigation examined anthropometric, hormonal, and physiological differences between advanced (ADV; n = 8, 27.8 ± 4.2 years, 170 ± 11 cm, 79.8 ± 13.3 kg) and recreational (REC; n = 8, 33.5 ± 8.1 years, 172 ± 14 cm, 76.3 ± 19.5 kg) CrossFit (CF) trained participants in comparison to physically-active controls (CON; n = 7, 27.5 ± 6.7 years, 171 ± 14 cm, 74.5 ± 14.3 kg). ADV and REC were distinguished by their past competitive success. REC and CON were resistance-trained (>2 years) and exercised on 3-5 days·wk-1 for the past year, but CON utilized traditional resistance and cardiovascular exercise. All participants provided a fasted, resting blood sample and completed assessments of resting metabolic rate, body composition, muscle morphology, isometric mid-thigh pull strength, peak aerobic capacity, and a 3-minute maximal cycle ergometer sprint across two separate occasions (separated by 3-7 days). Blood samples were analyzed for testosterone, cortisol, and insulin-like growth factor-1. Compared to both REC and CON, one-way analysis of variance revealed ADV to possess lower body fat percentage (6.7-8.3%, p = 0.007), greater bone and non-bone lean mass (12.5-26.8%, p ≤ 0.028), muscle morphology characteristics (14.2-59.9%, p < 0.05), isometric strength characteristics (15.4-41.8%, p < 0.05), peak aerobic capacity (18.8-19.1%, p = 0.002), and 3-minute cycling performance (15.4-51.1%, p ≤ 0.023). No differences were seen between REC and CON, or between all groups for resting metabolic rate or hormone concentrations. These data suggest ADV possess several physiological advantages over REC and CON, whereas similar physiological characteristics were present in individuals who have been regularly participating in either CF or resistance and cardiovascular training for the past year.

Rate of Force Development as a Predictor of Mobility in Community-dwelling Older Adults

BACKGROUND AND PURPOSE

Rate of force development (RFD) is influential, and possibly more influential than other muscular performance parameters, for mobility in older adults. However, only a few studies have investigated this matter, and this has not been examined for the plantar flexors (PFs). The purpose of this study was to examine the contribution of PF RFD and other common tests of muscular performance to Up-and-Go (UG) performance and walking speed (WS) in older adults.

METHODS

Twenty-six (19 females) healthy, community-dwelling older adults (73.7 ± 4.9 years) were recruited from a senior citizen center for this observational study. Handgrip strength, UG performance, as well as preferred and maximal WS were obtained. Time taken to complete 5-chair rises and the number of chair rises completed in 30 seconds were recorded. Rate of force development of the PFs was obtained during a rapid, bilateral calf raise performed on a force plate. Hierarchical multiple linear regression was used to identify significant predictors, after adjusting for physical activity level and body mass index, of mobility (ie, UG, preferred and maximal WS).

RESULTS AND DISCUSSION

No muscular performance variables correlated with preferred WS. Rate of force development (adjusted R2 = 0.356; P = .008) and handgrip strength (adjusted R2 = 0.293; P = .026) were the only predictors of maximal WS and accounted for a 21.7% and 16.1% change in R2, respectively, after accounting for physical activity level and body mass index. Rate of force development was the only predictor of UG performance (adjusted R2 = 0.212; P = .006) and accounted for a 29.2% change in R2 after adjustment variables were applied.

CONCLUSIONS

Compared to common assessments of muscular performance, such as handgrip strength and chair rise performance, PF RFD was a greater predictor of mobility in older adults. These findings, in conjunction with recent reports, indicate that the assessment of RFD likely complements strength testing, thereby enabling a more robust assessment of functional decline in older adults.

Neuromuscular function of the plantar flexors and predictors of peak power in middle-aged and older males

Little evidence exists regarding the contribution of torque and velocity to the age-related decrease in peak power (PP) for the plantar flexors (PFs). A comprehensive assessment of PF neuromuscular function is necessary to elucidate age-related changes, especially between middle-aged and older adults, in order to identify early, age-related decrements. Thus, the purpose of this study was to examine neuromuscular function of the PFs in middle-aged and older males, and identify predictors of PP. Twenty-eight healthy, middle-aged (n = 13; 45.1 ± 2.7 yrs) and older (n = 15; 65.3 ± 3.2 yrs) males performed concentric isotonic PF contractions ranging in intensity from 20% to 70% isometric strength using a dynamometer. PP in addition to velocity and torque at the moment in time PP occurred, as well as the rate of velocity, torque (RTD), and power (RPD) development were recorded. The rate of electromyography rise (RER) was derived from the linear slope of the normalized electromyography signal. Isometric and concentric dynamic strength were assessed, as well as cross-sectional area and muscle quality (i.e., echo intensity) of the PFs via panoramic ultrasonography. The relationship between serum c-terminal agrin levels and select variables was examined to explore the potential role of neuromuscular junction deterioration. Appendicular lean mass and physical activity level were similar between groups (p > 0.05), and only PP (p = 0.046; d = 0.79), RPD (p = 0.026; d = 0.90), RTD (p = 0.022; d = 0.91), and RER (p = 0.010; d = 1.04) were lower in older males. When groups were collapsed, RTD was the only significant predictor of PP, while c-terminal agrin levels were not associated with any variables. Our findings indicate that PP and time-dependent parameters of muscle activation and contractile function of the PFs are dramatically diminished in older adults compared to middle-aged adults. PP is produced at the same velocity and relative intensity in middle-aged and older males, and RTD is most influential for PP. The inability of the PFs to be rapidly activated appeared to be influential for the age-related impairment in PP and time-dependent contractile parameters.