Nathaniel D.M. Jenkins

University of Iowa | UI · Department of Health and Human Physiology

This study examined the time courses of recovery for isometric peak torque and rate of torque development (RTD) after eccentric-induced muscle damage. 18 men completed 6 sets of 10 maximal eccentric isokinetic muscle actions at 30° · s(-1). Peak torque, peak RTD and RTD at 10 (RTD10), 50 (RTD50), 100 (RTD100) and 200 ms (RTD200), serum creatine kin...

Introduction: We compared absolute and normalized values for peak torque (PT), mean power (MP), rate of velocity development, and electromyography (EMG) amplitude during maximal isometric and concentric isokinetic leg extension muscle actions, as well as the %decrease in PT and %increase in MP from 1.05 to 3.14 rad·s(-1) in younger versus older me...

We examined the neuromuscular adaptations following 3 and 6 weeks of 80% versus 30% one repetition maximum (1RM) resistance training to failure in the leg extensors. Twenty-six men (age = 23.1 ± 4.7 yrs) were randomly assigned to a high- (80% 1RM; n = 13) or low-load (30% 1RM; n = 13) resistance training group and completed leg extension resistance...

Adverse childhood experiences (ACEs) are psychosocial stressors that occur during sensitive developmental windows and are associated with increased lifetime cardiovascular disease (CVD) risk in a dose-dependent manner. Vascular endothelial dysfunction is a pathophysiological mechanism that promotes hypertension and CVD, and may be a mechanism by wh...

Sarcopenia, or the age-related loss of skeletal muscle mass and function, is an increasingly prevalent condition that contributes to reduced quality of life, morbidity, and mortality in older adults. Older adults display blunted anabolic responses to otherwise anabolic stimuli—a phenomenon that has been termed anabolic resistance (AR)—which is like...

Adverse childhood experiences (ACEs) are early life psychosocial stressors that are associated with poorer mental health and increased cardiovascular disease (CVD) risk in a dose-dependent manner. We examined the feasibility of an 8-week combined aerobic and resistance exercise training program to improve systolic (SBP) and diastolic blood pressure...

New findings: What is the central question of this study? Can the near-infrared spectroscopy with vascular occlusion test (NIRS-VOT) reliably measure skeletal muscle metabolic and microvascular function in women? What is the main finding and its importance? The NIRS-VOT can be used as a reliable technique for the assessment of skeletal muscle meta...

Acute kidney injury (AKI) and ICU-acquired weakness (ICU-AW) are two frequent complications of critical illness which, until recently, have been considered unrelated processes. The adverse impact of AKI on ICU mortality is clear, but its relationship with muscle weakness – a major source of ICU morbidity – has not been fully elucidated. Furthermore...

Introduction: Adverse childhood experiences (ACEs) are potentially traumatic psychosocial stressors that occur in early life, such as abuse, neglect, and household dysfunction. We hypothesize that ACEs are an underlying risk factor for disturbed sleep and hypertension in pregnancy, which is a major cause of maternal-fetal morbidity and mortality....

Nootropics are compounds or formulae that are reported to improve memory, learning, and attention (e.g., cognitive function). The consumption of energy drinks and nootropics are increasing rapidly in the United States alongside growing interest in improving mental performance. Ingredients such as caffeine, citicoline, tyrosine, B-vitamins, and carb...

Adverse childhood experiences (ACEs) represent psychosocial stressors that occur during critical developmental periods and are associated in a dose-dependent manner with adverse cardiovascular outcomes in emerging adults, including greater age-related increases in blood pressure (BP) and increased circulating endothelin-1 (ET-1), a psychosocial str...

Introduction: Cardiovascular disease is the leading cause of death for women, and postmenopausal (PM) women are a particularly vulnerable population in terms of adverse cardiometabolic (CM) risk. Progressive aerobic exercise training (AT) is efficacious for improving CM risk factors such as cardiorespiratory fitness (CRF), body composition (e.g., r...

Introduction: Adverse family environment (AFE) in childhood may have deleterious health effects; however, whether these exposures relate to health promoting behaviors in adulthood is unknown. We hypothesize that AFE in childhood will be associated with higher sedentary behavior (SED), and lower light- (LPA) and moderate-to-vigorous (MVPA) physical...

Introduction: Early life psychosocial stress and adversity - referred to as adverse childhood experiences (ACEs) - are highly prevalent, with ~60% of American adults reporting exposure to at least one ACE. ACEs occur during critical developmental periods and are related to cardiovascular morbidity and mortality in a graded, dose-dependent manner. S...

This study examined the influence of muscle activation and motor unit number estimation (MUNE) on early phase voluntary rate of torque development (RTD) in younger (YM) and older (OM) men. Thirty-two YM (n = 17; Age = 22 yrs) and OM (n = 15; Age = 74 yrs) volunteered to participate in this study. Early phase RTD (first 50 ms of a rapid isometric co...

Introduction: Adverse Childhood Experiences (ACEs), in the forms of abuse, neglect, and violence, are associated with augmented blood pressures in young adulthood and CVD risk later in life. These effects are independent of negative health behaviors, suggesting that ACEs are biologically embedded. Sirtuin 1 (SIRT1) is a highly conserved nicotinamid...

Background & Aims: Individuals with fasting triglycerides (TG) <150 mg/dL can experience a deleterious postprandial TG response ≥220 mg/dL to a high-fat meal (HFM). The purpose of this study was to identify individuals based on fasting TG that would benefit most from additional postprandial screening. Methods: We conducted a secondary analysis of...

Background & Aims Postprandial lipemia (PPL) is predictive of cardiovascular disease risk, but the current method for assessing PPL is a burdensome process. Recently, the validity of an abbreviated fat tolerance test (AFTT) has been demonstrated. As a continuation of this research, the purpose of this study was to determine the reliability of the A...

Purpose: To examine differences in the electromyographic vs torque (EMG-T) relationship, as well as muscle strength and indicators of muscle mass and quality between young versus postmenopausal females, and explore whether the potential differences in the EMG-T relationships could be explained by differences in muscle mass. Methods: Thirty young (a...

Following critical evaluation of the available literature to date, The International Society of Sports Nutrition (ISSN) position regarding caffeine intake is as follows: 1. Supplementation with caffeine has been shown to acutely enhance various aspects of exercise performance in many but not all studies. Small to moderate benefits of caffeine use...

We examined the motor unit action potential amplitude versus recruitment threshold relationship (MUAPAMP-RT) as an indicator of MU-specific hypertrophy following high-intensity exercise training in females. Participants were assigned to either a high-intensity exercise (EX, n = 9) or control (CON, n = 18) condition and completed pre- (PRE) and post...

The consumption of a high-fat meal can induce postprandial lipemia and endothelial dysfunction. The authors assessed the impact of age and physical activity on metabolic and vascular outcomes following meal consumption in healthy adults. The authors recruited four groups: younger active (age 22.1 ± 1.4 years; n = 9), younger inactive (age 22.6 ± 3....

This study examined whether an acute bout of resistance exercise (RE) attenuated postprandial responses to a high fat meal (HFM) similarly in younger versus older adult men, and probed relationships among skeletal muscle mass (SMM), age, the metabolic load index (MLI) response, and the improvement in the MLI elicited by RE versus CON. Eleven younge...

PURPOSE: To examine the reliability of non-invasive assessments of micro- and macrovascular function and their responsiveness to a high-fat meal (HFM) in females. METHODS: During 2 visits (T1 and T2) separated by 28 ± 2 days, 11 women (age = 30.7 ± 17.2 y, BMI = 24.4 ± 3.1 kg/m2) consumed a standardized HFM (12 kcal/kg body weight; 63% fat) after...

PURPOSE: To evaluate the influence of fatigue on force variability of the vastus lateralis (VL) during repeated isometric exercise performed at 30% maximal voluntary isometric contraction force (MVIC). METHODS: Twelve resistance-trained males (23.8 ± 3.6 y; 87.3 ± 10.5 kg; 176.3 ± 6 cm) completed MVIC testing and then repeated isometric contractio...

Objectives Individuals with healthy fasting triglycerides (TG) (<150 mg/dL) can still experience a deleterious postprandial TG response (≥220 mg/dL) to a high-fat/high-sugar meal (HFHSM), which has been shown to be a stronger predictor of cardiovascular disease (CVD) than fasting TG. The purpose of this study was to identify the lower and upper lim...

Objectives: This study examined the time course of changes and patterns of responses in electromyographic amplitude (EMG AMP) and EMG mean power frequency (MPF) for the superficial quadriceps muscles during exhaustive treadmill runs within the severe exercise intensity zones (SIZ1 and SIZ2). Methods: The EMG signals for the vastus lateralis (VL)...

Pitch velocity (PV) is important for pitching success and the pelvis and trunk likely influence pitch performance. The purposes of this study were to examine differences in pelvis and trunk kinetics and kinematics in professional baseball pitchers who throw at lower- (LVPs) versus higher-velocities (HVPs), and examine the relationships among pelvis...

The consumption of a high-fat meal (HFM) can induce postprandial lipemia (PPL) resulting in endothelial dysfunction. However, there is limited research describing the influence of age and physical activity level on postprandial vascular function. PURPOSE: The purpose of this project was to determine the impact of age and physical activity on vascul...

Purpose The mediators of the perception of effort during exercise are still unclear. The aim of the present study was to examine physiological responses during runs using a rating of perceived exertion (RPE)-clamp model at the RPE corresponding to the gas exchange threshold (RPEGET) and 15% above GET (RPEGET+15%) to identify potential mediators and...

Jenkins, NDM, Miramonti, AA, Hill, EC, Smith, CM, Cochrane-Snyman, KC, Housh, TJ, and Cramer, JT. Mechanomyographic amplitude is sensitive to load-dependent neuromuscular adaptations in response to resistance training. J Strength Cond Res XX(X): 000-000, 2019-We examined the sensitivity of the mechanomyographic amplitude (MMGRMS) and frequency (MMG...

The liver enzyme cytochrome P450 1A2 (CYP1A2) is responsible for 90% of caffeine metabolism, while caffeine exerts many of its effects via antagonist binding to adenosine A2a receptors (ADORA2A). This study aimed to examine whether functional single nucleotide polymorphisms (SNPs) in 1976T > C (ADORA2A; rs5751876) and −163C > A (CYP1A2; rs762551) i...

Objective: The purpose was two-fold: (1) to examine differences in maximal voluntary isometric torque (MVIT) production, and electromyographic signal amplitude (EMGAMP) and mean power frequency (EMGMPF) values obtained during traditional (MVICTRAD), rapid (MVICRAPID), and ramp (MVICRAMP) maximal voluntary isometric contractions, and (2) to determi...

PURPOSE: To evaluate the test-retest reliability of a commercially available bioimpedance spectroscopy (BIS) device for the assessment of total body water (TBW), extracellular water (ECW), and intracellular water (ICW) content, as well as fat mass (FM), fat-free mass (FFM), and body fat percentage (BF%) in physically active males. METHODS: 16 healt...

Objectives: Large metabolic responses to high-fat meals (HFM) are known to create a deleterious physiological state. However, there is limited research describing the differential influences of age and physical activity level on meal metabolism, specifically in healthy older individuals. The goal of this project is to quantify the impact of age an...

OBJECTIVE: To examine the effects of full-body resistance exercise on endothelial function and microvascular reactivity following a high-fat meal (HFM) in healthy men. METHODS: Seven males (mean ± SD, age = 22.1 ± 2.2 y, BMI = 26.5 ± 2.0 kg/m 2) participated in this randomized, repeated measures, crossover study. Participants visited the laboratory...

To evaluate the effects of repeated, fatiguing isometric knee extension contractions performed at high-torque (HT) versus low-torque (LT) maximal voluntary isometric contraction (MVIC) on acute muscle swelling and echo intensity (EI) of the rectus femoris (RF) and vastus lateralis (VL) muscles in resistance-trained males. Twenty-two resistance-trai...

Objectives: To examine the time course of changes in electromyographic (EMG) and mechanomyographic (MMG) amplitude (AMP) and mean power frequency (MPF) responses during cycle ergometry to exhaustion performed above (CP+10%) and below (CP-10%) critical power (CP) to infer motor unit activation strategies used to maintain power output. Methods: Pa...

McKay, BD, Miramonti, AA, Gillen, ZM, Leutzinger, TJ, Mendez, AI, Jenkins, NDM, and Cramer, JT. Normative reference values for high school-aged American football players: Proagility drill and 40-yard dash split times. J Strength Cond Res XX(X): 000-000, 2018-The purpose of this short report was to provide test- and position-specific normative refer...

Introduction: Decomposition of the surface electromyographic (sEMG) signal is commonly used to examine motor unit (MU) firing behavior. However, the intra- and inter-day reliability of these measurements has yet to be quantified or reported. This investigation 1) examined the effect of input excitation on the mean firing rate (MFR) versus recruitm...

New findings: What is the central question of this study? Does a common genetic variant in the β2-adrenergic receptor (AR) have functional effects on skeletal muscle function in young, healthy men? What is the main finding and its importance? This study provides preliminary evidence that β2-AR Arg16Gly genotype has a significant effect on fat free...

Moghaddam, M, Estrada, CA, Muddle, TWD, Magrini, MA, Jenkins, NDM, and Jacobson, BH. Similar anaerobic and aerobic adaptations after 2 high-intensity interval training configurations: 10 s:5 s vs. 20 s:10 s work-to-rest ratio. J Strength Cond Res XX(X): 000-000, 2019-This study compares the effects of 2 high-intensity interval training (HIIT) confi...

Heavy resistance exercise may induce post-activation potentiation (PAP), enhancing performance of a subsequent biomechanically similar movement. Performance is enhanced when potentiation is greater than fatigue. However, previous studies typically perform 5 repetitions at a 5RM load when attempting to elicit PAP, which is very fatiguing. Thus, perf...

Objective: To determine if maximal isometric contraction (MVIC) method (i.e., ramp (MVICRAMP) versus traditional MVIC) influences (1) maximal voluntary isometric torque (MVIT) production of the knee extensors, (2) electromyographic signal amplitude (EMGAMP), and (3) EMG mean power frequency (EMGMPF) of the rectus femoris and vastus lateralis in mo...

PURPOSE: To examine the changes in mechanomyographic (MMG) amplitude following 3 and 6 weeks of 80% vs. 30% one repetition maximum (1RM) resistance training in the leg extensors. METHODS: Twenty-six healthy men (23.1 ± 4.7 years; height = 180.6 ± 6.0 cm; weight = 80.0 ± 14.1 kg) participated in this randomized, repeated measures, between-group des...

Introduction: The purpose of this investigation was to: (1) to determine the reliability of rectus femoris muscle cross sectional area (mCSA) and echo intensity (EI) obtained using panoramic ultrasound imaging during seated and supine lying positions before and after a five-minute rest period and (2), to determine the influence of body position and...

OBJECTIVE: We examined the relationships among and differences between muscle size and strength, specific tension (MQST), echo intensity (MQEI), and functional performance in physically active younger and older women. METHODS: Measurements included maximal voluntary isometric knee extension contractions (MVICs), maximal velocity sit-to-stand (STS)...

The purpose of this investigation was to evaluate the effects of repeated, high- (HT: 70% MVIC) versus low-torque (LT: 30% MVIC) isometric exercise performed to failure on motor unit (MU) recruitment and firing behavior of the vastus lateralis. Eighteen resistance trained males (23.1 ± 3.8 yrs) completed familiarization, followed by separate experi...

Previous investigations have reported a relationship between skeletal muscle phenotype and motor unit (MU) firing parameters during submaximal contractions. The purpose of the current investigation, however, was to examine the relationships between motor unit firing behavior during a maximal voluntary contraction, Myosin Heavy Chain (MHC) isoform c...

The purpose of the present report was to provide test- and position-specific normative reference values for combine test results based on a large, nationally-representative sample of high school-aged American football players in their freshman, sophomore, and junior classes. Cross-sectional anthropometric and performance data were obtained from 12...

Background Elbow injury rates among baseball pitchers are rapidly rising. However, this increase has been most dramatic among high school (HS) pitchers. Purpose To examine pitch velocity and the kinetic and kinematic characteristics of HS versus professional (PRO) pitchers to identify potential differences that may play a role in the increased ris...

Purpose: To compare the effects of a high- versus a moderate-training frequency on maximal strength and body composition. Methods: 28 young, healthy resistance-trained males were randomly assigned to either: 3x/week (3x; n=16) or 6x/week (6x; n=12). Dependent variables (DVs) assessed at baseline and after the 6-week training intervention include...

The purpose of this study was to describe the VO2 responses relative to VO2 peak at four different intensities within the severe domain and, based on the VO2 responses, identify intensity zones above critical velocity (CV). Twelve runners (mean ± SD age = 23.2 ± 3.0 years) performed an incremental treadmill test (ITT) to exhaustion to determine the...

Abstract Background We sought to determine if a pre-workout supplement (PWS), containing multiple ingredients thought to enhance blood flow, increases hyperemia associated with resistance training compared to placebo (PBO). Given the potential interaction with training loads/time-under-tension, we evaluated the hyperemic response at two different l...

Purpose: To evaluate the effects of repeated, fatiguing isometric knee extension contractions at 70% maximal voluntary isometric contraction force (MVIC) versus 30% MVIC on muscle cross-sectional area (mCSA) and echo intensity (EI) of the rectus femoris (RF) and vastus lateralis (VL) muscles in resistance trained males. Methods: Eight resistance-tr...

Introduction: Numerous recent investigations have examined the effects of high- versus low-load resistance training on skeletal muscle hypertrophy and strength. However, to date, no study has examined the potential changes in motor unit (MU) action potential (AP) morphology following either of these conditions. AP morphology has been utilized as a...

Purpose: To evaluate the effects of low- (30% one repetition maximum [1RM]) vs. high-load (80% 1RM) leg extension exercise performed to volitional failure on tissue oxygenation of the vastus lateralis (VL) in resistance-trained males. Methods: Twenty-one recreationally active males (mean ± SD; 21 ± 6 years; 86 6 10 kg; 181 ± 8 cm) visited the lab o...

Introduction: The Wingate anaerobic test (WAnT) is a laboratory based cycle ergometer test designed to measure peak power (PP), mean power, and fatigue index. At this time, the WAnT is considered to be one of the more common tests of anaerobic fitness. However, a vertical jump power assessment may be a more specific for military personnel and the a...