Performance and Endocrine Responses to Differing Ratios of Concurrent Strength and Endurance Training

Abstract

Jones, TW, Howatson, G, Russell, M, and French, DN. Performance and endocrine responses to differing ratios of concurrent strength and endurance training. J Strength Cond Res 30(3): 693–702, 2016—The present study examined functional strength and endocrine responses to varying ratios of strength and endurance training in a concurrent training regimen. Thirty resistance trained men completed 6 weeks of 3 d·wk⁻¹ of (a) strength training (ST), (b) concurrent strength and endurance training ratio 3:1 (CT3), (c) concurrent strength and endurance training ratio 1:1 (CT1), or (d) no training (CON). Strength training was conducted using whole-body multijoint exercises, whereas endurance training consisted of treadmill running. Assessments of maximal strength, lower-body power, and endocrine factors were conducted pretraining and after 3 and 6 weeks. After the intervention, ST and CT3 elicited similar increases in lower-body strength; furthermore, ST resulted in greater increases than CT1 and CON (all p ≤ 0.05). All training conditions resulted in similar increases in upper-body strength after training. The ST group observed greater increases in lower-body power than all other conditions (all p ≤ 0.05). After the final training session, CT1 elicited greater increases in cortisol than ST (p = 0.008). When implemented as part of a concurrent training regimen, higher volumes of endurance training result in the inhibition of lower-body strength, whereas low volumes do not. Lower-body power was attenuated by high and low frequencies of endurance training. Higher frequencies of endurance training resulted in increased cortisol responses to training. These data suggest that if strength development is the primary focus of a training intervention, frequency of endurance training should remain low.

Introduction

Various sports and events require contrasting physical performance phenotypes for successful performance. Training for sports and events at the extremes of the strength endurance continuum, such as powerlifting and ultra-endurance challenges, is relatively straightforward compared with sports and events that require a combination of strength and endurance capabilities. In these situations, athletes and coaches are often forced to combine training methods, which elicit contrasting and even antagonistic physiological and performance responses (¹²). In the case of “concurrent training,” the divergent stimuli of strength and endurance training can result in attenuated strength-type adaptation when compared with strength training performed in isolation. This divergent physiology is known as the interference effect or phenomenon (¹⁷).

Research has indicated that any interference experienced during a concurrent strength and endurance training regimen may be dependent in part on the volume of training performed (^{1,13,24,25,33}). Despite this, no study has specifically examined the effects of whole-body multijoint concurrent training inventions with varying training volumes and the effect that is has on muscle force characteristics. Previous work from our laboratory (²⁰) has indicated that the magnitude of interference experienced may be proportional to the frequency of endurance training performed, indicating that overall training volume and exercise stress may indeed regulate the presence of any interference experienced.

Elevated training “stress” has previously been proposed as a mechanism for interference (¹⁰) and is perhaps attributable to the experimental design of some published studies in this area. Often the concurrent training condition will perform double the overall training volume and total work to that of the strength training–alone condition, which has previously resulted in muted strength development (^6,16,20,22). In contrast, studies using lower concurrent training volumes have reported no inhibited strength development as a result of concurrent training (^24,25). These findings may support the hypothesis that total work performed in a concurrent program influences both the presence and magnitude of any interference experienced, although the underlying mechanisms are yet to be fully elucidated.

Previous research has reported a decreased testosterone:cortisol ratio after concurrent training with no such decrease in participants who performed strength training alone (^2,3,22). This may implicate elevated endocrine responses and catabolism as a contributing factor to interference. As such, it is reasonable to suggest that the higher training volumes experienced in concurrent training regimens can result in elevated physiological stress, which is reflected in the responses of primary anabolic and catabolic hormones. This shift in the endocrine milieu in favor of catabolism may contribute to attenuated strength and hypertrophic adaptation associated with concurrent training.

Previous work from our laboratory (²⁰) illustrates the value in exploring the role of training frequency in a systematic fashion. Furthermore, no research has assessed if differing ratios of strength and endurance training can influence the degree of interference experienced as a result of adaptations in the anabolic:catabolic environment. Therefore, the purpose of this research was to investigate the strength, anthropometric, and endocrine responses to a variety of concurrent strength and endurance training ratios, with incremental loads in a functional multijoint model.

Methods

Experimental Approach to the Problem

A balanced, randomized between-group study design was used to examine the effect of differing ratios of strength and endurance training in a concurrent regimen on strength, anthropometric, and endocrine responses. A 6-week training intervention was completed, during which participants were randomly assigned to 1 of 4 experimental conditions: either (a) strength training alone (ST), (b) concurrent strength and endurance training at a ratio of 3:1 (CT3), (c) concurrent strength and endurance training at a ratio of 1:1 (CT1), or (d) no training (CON). Participants in the ST group were required to perform strength training alone on all scheduled training sessions. The CT3 group completed strength training on every scheduled session with every third session immediately followed by an endurance training protocol. Elsewhere, participants designated CT1 completed an identical strength training protocol immediately followed by endurance training at every scheduled session. Those participants in the CON group performed no strength or endurance training during the entire experimental period. Because of the requirements of the separate training protocols, it was not possible to match total work performed in the respective experimental conditions. All participants were instructed to abstain from any other strength or endurance training throughout the experimental period beyond that prescribed by the investigator.

Participants completed their respective intervention 3 d·wk⁻¹ with ∼48 hours between sessions for 6 weeks resulting in a total of 18 separate training sessions in the microcycle. To assess whether the frequency and ratio of strength and endurance training performed influenced strength and changes in body composition, 1 repetition maximum (1RM), countermovement jump height (CMJ), and body composition were assessed pre-, mid-, and postintervention. To assess the effect of the designated training interventions on endocrine factors related to strength and morphological adaptation, venous blood samples were taken and subsequently analyzed for circulating testosterone and cortisol concentrations. During the investigation, venous blood samples were collected immediately before (pre) and after the cessation of exercise (post) in the initial, mid, and final compound training sessions of the 18 sessions performed.

Subjects

Before all experimental procedures, the study was approved by the Northumbria University research ethics committee. All subjects were informed of the risks and benefits of the investigation before signing an approved informed consent document to participate in the study. Thirty healthy recreationally resistance trained men (age: 23 ± 4 years; body mass: 79.2 ± 6.7 kg; height: 179.2 ± 6.7 cm; percent body fat: 16.2 ± 5.4%; sum of assessed 1RMs: 506.0 ± 11.4 kg; CMJ: 52.5 ± 7.3 cm; V[Combining Dot Above]O₂max: 50.2 ± 5.8 ml·kg⁻¹·min⁻¹) volunteered to participate in the study. Before commencing, participants were matched for age, body mass, body fat percentage, and 1RM (sum off all assessed 1RMs) load (all p > 0.05) and then randomly assigned (via block randomization) to 1 of the 4 experimental conditions. Each participant had completed >2 years of strength training activities before the start of the study and were considered recreationally “resistance trained”; all participants were conducting strength training ≥2 d·wk⁻¹; however, none were involved in a sport-specific training program. All participants were nonsmokers, free from any endocrine or metabolic contraindications, and were not following any specialized dietary interventions. In all cases, participants were asked to refrain from nutritional supplementation or pharmacological interventions for 30 days before and throughout the duration of any experimental intervention.

Procedures

Strength Training Protocol

Before the intervention, all participants completed a familiarization week involving each respective training session to habituate themselves fully with the exercise techniques used. The strength training intervention comprised 3 sessions, and each was performed on separate days with ∼48 hours between sessions. Each session was composed of differing exercises; as such, each of the sessions were designated “compound,” “pull,” and “push,” respectively, to best describe the nature of exercises performed. Full details of each session are presented in Table 1. The respective sessions were performed in the same order each week (i.e., compound, push, and then pull). Furthermore, the order of exercises within each session was consistent throughout the intervention.

Table 1: Program variables within periodized resistance training intervention.*

During familiarization, training intensity was set at 70% of 1RM for 3 sets of 10 repetitions. The first 3 weeks of the training intervention required participants to complete all sessions and exercises at 80% 1RM for 4 sets of 8 repetitions. The following and final 3 weeks of the intervention were completed at an intensity of 85% 1RM for 5 sets of 6 repetitions. These loads, volumes, and rest intervals were selected as they are deemed appropriate for eliciting adaptations in strength and hypertrophy in recreationally trained nonathletes (^27,28). Additionally, strength training programs of this nature involving exercises that stimulate large muscle masses and shorter rest periods have been shown to elicit large increases in the endocrine factors assessed within this study (^21,32). Full details of the intervention are presented in Table 1.

All strength- and endurance-based exercises commenced at the same time of the day (1,000 hours ± 1 hour) to avoid any diurnal performance or endocrine variations (¹⁵). Participants were also advised to abstain from exercise for 24 hours before a visit. Training load was modified accordingly for each exercise if a participant's 1RMs were observed to change at the mid-intervention assessments. Compliance was 100% for all participants.

Endurance Training Protocol

In all instances, endurance training was conducted immediately after strength training. The endurance training protocol required participants to run on a treadmill (H-p-cosmos Sports & Medical GmbH, hp Cosmos, Pulsar, Nussdorf-Traunstein, Germany) at 1% incline at 70% of their predetermined peak running velocity at V[Combining Dot Above]O₂max (V[Combining Dot Above]O₂max). Running velocity was modified if participant's V[Combining Dot Above]O₂max was observed to change at the mid-intervention assessments.

Whole-Body Strength Assessments—1 Repetition Maximum

One repetition maximum loads were established for all strength training exercises before the experimental intervention and after 3 and 6 weeks of training. For analysis purposes, lower-body strength was assessed via back squat and deadlift 1RM total load. To examine strength development in the upper-body musculature, bench press, bent-over row, and military press 1RM total load were analyzed. These exercises were chosen as they are considered gross motor movements that require all the major joints and muscle groups involved in the strength training intervention. All assessments were conducted in line with standardized procedures (²⁹).

Maximal Aerobic Capacity—V[Combining Dot Above]O₂max

Assessments of participant's maximal oxygen uptake and peak running velocity at V[Combining Dot Above]O₂max were conducted at baseline, after 3 weeks of training, and after the 6-week training intervention. All assessments were conducted in line with standardized procedures reported elsewhere (³⁴).

Lower-Body Power—Countermovement Jump Assessment

Lower-body power was assessed via maximal CMJ and was conducted before and after 3 and 6 weeks of training. Maximal CMJ was adopted as a proxy of lower-body power and was assessed using a contact mat (Just Jump; Probotics, Huntsville, AL, USA). After familiarization, independent trials of CMJs were conducted with 3 minutes between each individual jump; the highest jump was recorded for data analysis. When performing the test, participants positioned themselves in the center of the contact mat and placed their hands on the iliac crest where they were to remain throughout. Countermovement jump heights began from an erect standing position. When ready, participants squatted to a self-selected depth perceived as their individual optimal depth and immediately ascended to jump vertically for maximal height.

Body Ccomposition—Air Displacement Plethysmography

All participants' lean mass and percent body fat were assessed before and after 3 and 6 weeks of training. Lean mass and percent body fat were assessed using air displacement plethysmography (BodPod; Life Measurements Instruments, CA, USA) (^11,26,30). Initially, the device was calibrated using a metal cylinder of known and standardized composition. Participants were asked to disrobe to minimal clothing and place a tight fitting cap over their hair. Participants were then weighed on a calibrated scale before entering the chamber. Once 2 consistent measures of body composition were obtained, percent body fat and lean mass were calculated using associated software (⁸).

Rate of Perceived Exertion

To examine perception of physical exertion in response to the training intervention, rate of perceived exertion (RPE) was recorded during strength training. Briefly, participants were required to select a number from 6 to 20, corresponding to a statement which best described their level of exertion at that particular moment (^4,7,31).

Blood Sampling and Storage

When blood samples were collected, participants arrived at the laboratory having refrained from consuming food or caffeine for 2 hours before assessment. Venous blood samples were collected from the antecubital fossa in a branch of the basilica vein into vacutainer tubes (BD Vacutainer, NJ, USA) coated with EDTA to negate. Whole blood was subsequently centrifuged (accuSpin 3R; Fisher Scientific, Loughborough, United Kingdom) at 4° C and 1,509g for 10 minutes, after which the resultant plasma from each sample was transferred to individual Eppendorf containers for subsequent storage at −80° C. Venous blood samples were collected immediately before (pre) and after the cessation of exercise (post) in the initial, mid, and final compound training sessions (additional information presented in Table 1) of the 18 sessions performed.

Biochemical Analysis

Plasma testosterone and cortisol were measured in duplicate (testosterone: Intraclass Correlation Coefficient [ICC] = 0.89, R = 0.89, cortisol: ICC = 0.92, R = 0.95) via commercially available enzyme-linked immunosorbent assay kits (IBL International, Hamburg, Germany). In all cases, procedures were followed according to the manufacturer's instructions. For both variables, 25 μl of each standard, control, and sample were pipetted into the respective wells of the microtiter plate, after which 2,000 μl of enzyme conjugate was then pipetted into each well and the plate was covered and left to incubate at room temperature (18–25° C) for 60 minutes. After this period, the incubation solution was discarded and the microplate was washed 3 times with wash buffer and distilled water solution diluted at a ratio of 1:10. One hundred microliters of tetramethylbenzidine (TMB) substrate solution was then pipetted into each well before a 15-minute incubation period. Immediately after this incubation, 100 μl of TMB stop solution was pipetted into each well and the contents were briefly mixed by gently agitating the plate. The optical density was measured at 450 nm within 10 minutes of the stop solution being added using an Anthos 2010 microplate reader (DAZDAQ LTD, Brighton, United Kingdom [reference wavelength 600–650 nm]). For testosterone, there was a minimum detection limit of 0.2 nmol·L⁻¹, with interassay and intra-assay variations of 4.2–7.4 and 3.1–5.4, and the calibration curve revealed Pearson's correlation coefficient (r) = 0.99. For cortisol, there was a minimum detection limit of 6.8 nmol·L⁻¹, with interassay and intra-assay variations of 2.1–5.0 and 2.6–3.5, and the calibration curve revealed r = 0.99.

Statistical Analyses

Data are presented as mean ± SD. Values of RMs, CMJ, and lean mass were transformed to a percent change (Δ%) from baseline and used for analysis. Before analysis, dependent variables were verified as meeting required assumptions of parametric statistics and changes in all assessed measures were analyzed using mixed model repeated measures analysis of variance (ANOVA) tests. The ANOVA analyzed differences between 4 conditions (ST, CT3, CT1, and CON) and 3 time points (baseline, mid-intervention, and postintervention). The alpha level of 0.05 was set before data analysis. Assumptions of sphericity were assessed using Mauchly's test of sphericity; if the assumption of sphericity was violated, Greenhouse-Gessier correction was used. If significant effects between conditions or over time were observed, post hoc differences were analyzed with the use of Bonferroni correction. Statistical power of the study was calculated post hoc using G*Power statistical software (v3.1.3; Universitat Dusseldorf, Düsseldorf, Germany) using the effect size, group mean, SD, and sample size of the primary outcome measures, in this case being lower- and upper-body maximal strength and endocrine factors. Power was calculated as between 0.8 and 1 indicating sufficient statistical power (⁵).

Results

Physical Performance Measures

Participant's baseline strength and endurance physical performance capabilities were similar between experimental conditions; these data are presented in Table 2.

Table 2: Participant's baseline maximal strength, lower-body power, and maximal aerobic capacity.*

Upper- and Lower-Body Maximal Strength

A significant group × time interaction was observed (F_{(4, 36)} = 4.940, p = 0.003) for lower-body strength development, as was an effect of time (F_{(1, 36)} = 45.042, p < 0.001). All training conditions elicited increases in lower-body strength at the mid-intervention time point after 3 weeks of training (ST: 9.0 ± 4.5%, p < 0.001; CT3: 9.8 ± 11.0%, p = 0.024; CT1: 5.8 ± 3.2%, p < 0.001). Similarly, lower-body strength improved in all training conditions from baseline to postintervention (ST: 17.2 ± 7.2%, p < 0.001; CT3: 15.0 ± 11.8%, p = 0.003; CT1: 10.1 ± 4.9%, p < 0.001). The ST was the only condition to significantly increase lower-body strength from mid- to postintervention (8.3 ± 2.8%, p = 0.016; Figure 1).

Figure 1: Mean relative changes in lower-body strength (as assessed by back squat and deadlift) in response to respective training interventions in the ST (n = 8), CT3 (n = 8), CT1 (n = 8), and CON (n = 6) conditions. ST = strength training alone performed every session; CT3 = strength performed every session, strength and endurance training performed every third session; CT1 = strength and endurance training performed every session; CON = no strength or endurance training performed during experimental period. * Significant increases from baseline in all training conditions (p ≤ 0.05). ** Significant increase from mid-intervention in ST (p = 0.016). † Significantly greater increases than CON in training conditions (p ≤ 0.05). ‡ ST significantly greater than CT1 (p = 0.036).

All training conditions improved lower-body strength to a greater extent than CON at both mid- and postintervention (all p ≤ 0.05). Posttraining ST improved lower-body strength 7.1 ± 2.4% more than CT1 (p = 0.036, Figure 1).

A significant group × time interaction (F_{(5, 41)} = 2.895, p = 0.027) and an effect of time (F_{(2, 36)} = 31.510, p < 0.001) were observed for upper-body strength development. CT3 and CT1 both improved upper-body strength between baseline and mid-intervention (6.2 ± 6.9%, p = 0.024, and 7.8 ± 4.5%, p < 0.001, respectively; Figure 2). All training conditions increased upper-body strength from pre- to posttraining (all p ≤ 0.05). Upper-body strength improved in all training conditions after training interventions (ST: 10.5 ± 5.2%, p < 0.001; CT3: 10.6 ± 10.7%, p = 0.014; CT1: 12.1 ± 6.9%, p < 0.001). The ST was the only condition to improve upper-body strength from mid- to posttraining (6.9 ± 0.1%, p = 0.019).

Figure 2: Mean relative changes in upper-body strength (as assessed by bench press, bent-over row, and military press) in response to respective training interventions in the ST (n = 8), CT3 (n = 8), CT1 (n = 8), and CON (n = 6) conditions. ST = strength training alone performed every session; CT3 = strength performed every session, strength and endurance training performed every third session; CT1 = strength and endurance training performed every session; CON = no strength or endurance training performed during experimental period. * Significant increases from baseline in CT3 and CT1 (p ≤ 0.05). ** Significant increases from baseline in all training conditions (p ≤ 0.05). † Significant increase from mid-intervention in ST (p = 0.019). ‡ All training conditions greater than CON (p ≤ 0.05).

All training conditions elicited significantly greater increases in upper-body strength than CON at mid- and postintervention (all p ≤ 0.05, Figure 2).

Lower-Body Power

A significant group × time interaction (F_{(6, 52)} = 3.236, p = 0.009) and effect of time (F_{(2, 52)} = 26.086, p < 0.001) were observed for lower-body power development. Both ST and CT1 increased CMJ from baseline to mid-intervention (ST: 8.7 ± 7.0%, p = 0.003; CT1: 3.0 ± 2.3%, p = 0.002). After intervention, all training conditions elicited significant increases in CMJ from baseline (ST: 13.1 ± 7.3%, p < 0.001; CT3: 7.1 ± 3.7%, p < 0.001; CT1: 4.8 ± 2.3%, p < 0.001; Figure 3).

Figure 3: Mean relative changes in countermovement jump height in response to respective training interventions in the ST (n = 8), CT3 (n = 8), CT1 (n = 8), and CON (n = 6) conditions. ST = strength training alone performed every session; CT3 = strength performed every session, strength and endurance training performed every third session; CT1 = strength and endurance training performed every session; CON = no strength or endurance training performed during experimental period. * ST and CT1 significantly greater than baseline (p ≤ 0.05). ** ST, CT3, and CT1 significantly greater than baseline (p < 0.001). † ST significantly greater than CT1 and CON (p ≤ 0.05). ‡ ST significantly greater than CT3, CT1, and CON (all p ≤ 0.05).

Participants in the ST condition achieved significantly higher CMJ than those after CT1 (7.0 ± 3.5%) and CON (5.7 ± 4.7%) conditions after 3 weeks of training (i.e., mid-intervention) (both p = 0.04). After training (i.e., postintervention), ST elicited 6.0 ± 3.6% greater increases in CMJ than CT3, 8.3 ± 5.0% greater than CT1, and 10.9 ± 2.3% greater than CON (all p ≤ 0.05).

Strength Training Performance

During the first 3 weeks of the training intervention, all groups' ability to maintain the required training intensity was similar (F_{(3, 30)} = 1.063, p = 0.548) and did not change significantly over time (F_{(1, 30)} = 4.295, p = 0.062). Similar results were observed in the final 3 weeks of the intervention as ability to maintain designated training load was not different between conditions (F_{(3, 28)} = 1.301, p = 0.293) or over time (F_{(1, 28)} = 3.777, p = 0.052).

Testosterone

No group × time interaction was reported for circulating basal testosterone concentrations (F_{(6, 52)} = 1.820, p = 0.113; Table 3). A significant group × time interaction was, however, observed for the testosterone response to strength training (F_{(3, 26)} = 11.466, p < 0.001). Testosterone responses to the respective training interventions also changed significantly over time (F_{(1, 26)} = 130.683, p < 0.001). After the initial and mid-sessions, ST was the only condition to increase testosterone levels greater than CON (30.7 ± 5.0%, p = 0.04, and 37.1 ± 12.9%, p = 0.005, respectively). CT3 was the only condition to elicit a greater increase in testosterone than CON after the final session (42.2 ± 10.5%, p = 0.002). The ST and CT3 elicited significant increases from pretraining in both the mid and final sessions (all p ≤ 0.05). Testosterone was also increased posttraining in the CT3 condition after the final session (p = 0.01). No other increases were observed.

Table 3: Effects of respective training interventions on testosterone, cortisol, and testosterone:cortisol (T:C) ratio.*

Cortisol

No group × time interaction was observed for circulating basal cortisol concentrations (F_{(6, 52)} = 1.540, p = 0.184; Table 3). A significant group × time interaction (F_{(3, 26)} = 7.592, p = 0.001) and an effect of time (F_{(1, 26)} = 101.852, p < 0.001) were observed for cortisol responses to the respective training interventions. After the initial session, ST was the only condition to increase cortisol levels to a greater extent than CON (84.7 ± 22.1%, p = 0.014). Posttraining after the mid-intervention session, CT1 was the only condition that resulted in significantly greater cortisol increases than CON (49.2 ± 3.1%, p < 0.001). After the final session, CT1 elicited 26.6 ± 8.4% greater cortisol increases than ST (p < 0.008). All training conditions elicited significant increases in cortisol posttraining on all assessed sessions (all p ≤ 0.05).

Testosterone:Cortisol Ratio

No group × time interactions were present for basal testosterone:cortisol ratio (T:C ratio) (F_{(6, 52)} = 1.903, p = 0.098) or the T:C ratio response to training (F_{(6, 52)} = 1.124, p = 0.361).

Lean Mass

Participant's baseline lean mass was similar between experimental conditions; these data are presented in Table 4. No group × time interaction was observed for changes in participant's lean mass.

Table 4: Participant's basal lean mass.*

Body Fat Percentage

A significant group × time interaction was observed for body fat percentage (F_{(6, 52)} = 4.616, p = 0.001). After the 6-week training intervention, CT1 resulted in 2.65 ± 0.04% greater decreases in body fat percentage than CON (p < 0.001) at the postintervention time point. No other significant effects of time or group were observed for changes in body fat percentage.

Rate of Perceived Exertion

A significant group × time interaction was present for RPE (F_{(5, 52)} = 2.744, p = 0.029). At weeks 5 and 6 of the training intervention, RPE was significantly lower in the ST group than CT1 (both p ≤ 0.05) (Figure 4). No other interactions or effects were present.

Figure 4: Mean rate of perceived exertion experienced in the ST (n = 8), CT3 (n = 8), and CT1 (n = 8) conditions. ST = strength training alone performed every session; CT3 = strength performed every session, strength and endurance training performed every third session; CT1 = strength and endurance training performed every session. * ST significantly lower than CT1 (p ≤ 0.05).

Discussion

The present study sought to prioritize strength development in concurrent training regimens with varying volumes of endurance training. The primary finding of this study was that an increase in the frequency of endurance training and total training volume within the concurrent training paradigm resulted in the attenuated development of lower-body strength when compared with strength training alone. After 6 weeks of training, ST and CT3 conditions resulted in similar increases in lower-body strength, whereas the improvements of those performing both strength and endurance training collectively 3 times per week (CT1) were muted (Figure 1). These findings reflect data presented in our previous work (²⁰), in which ST and CT3 resulted in similar increases in maximal voluntary contraction, whereas increases in the CT1 condition were significantly lower. Although no other published research has examined differing frequencies of strength and endurance training on strength-related adaptation, studies using concurrent training frequencies of ≥3 d·wk⁻¹ have typically reported some manifestation of interference characteristics (^2,14,19,22). Lower concurrent training frequencies (≤2 d·wk⁻¹) have, however, resulted in similar development of strength-related phenotypes after both concurrent and strength training programs (^24,25). When combined, the findings of these studies are consistent with those of the present study. Concurrent training conducted 3 d·wk⁻¹ (CT1) resulted in inhibited gains in maximal lower-body strength, whereas performing concurrent training once per week with 2 strength-alone sessions (CT3, concurrent training frequency of 1 d·wk⁻¹) elicited similar lower-body strength increases than strength training in isolation. The findings of this study and those of previous research indicate higher training volumes, and elevated physiological stress may contribute to the presence of the interference phenomenon.

In addition to the inhibition of lower-body strength development, lower-body power development was also inhibited after 3 and 6 weeks of training in the CT1 condition when compared with strength training alone (Figure 3). Furthermore, lower volumes of endurance training also resulted in attenuated increases in lower-body power, as postintervention participants who performed strength and endurance training at a ratio of 3:1 (CT3) exhibited improvements that were 6.0 ± 3.6% (p = 0.04, smallest worthwhile change = 1.2%; ¹⁸) lower than those who performed strength training alone. As previously stated, maximal lower-body strength development was not different between ST and CT3 conditions (Figure 1), which may indicate that power phenotypes are more susceptible to interference than maximal strength indices. This suggestion is supported by previous research indicating that development of variables, including CMJ, rate of force development, and peak torques at high velocities, has been inhibited as a result of combining strength and endurance training; yet, maximal strength development remained uninhibited (^6,9,14).

Unlike lower-body strength and power development, increases in upper-body strength were similar after both strength training–alone and concurrent training conditions (CT3 and CT1). Furthermore, after 3 weeks of training, CT1 resulted in 4.2 ± 0.8% greater increases than strength training alone (Figure 2), although this was not statistically significant (p = 0.09). Previous research has also reported that concurrent training does not result in the inhibition of upper-body maximal strength (^1,3). Unlike the present study, which used steady-state running, previous research involved rowing (³) and arm cranking (¹) as the endurance training modalities. It may be argued that although aerobically demanding, the stimuli of arm cranking and rowing are further toward the strength end of the strength endurance continuum than steady-state running. As such, it is reasonable to suggest that concurrent training may not differently affect the upper-body musculature, but rather for interference to occur the assessed musculature must experience divergent contractile activity (i.e., strength and endurance stimuli) of contrasting intensities and durations. It is reasonable to suggest that the lower-body musculature was placed in a greater state of conflict than the upper body, as both training stimuli directly affected hip-dominant and lower limb muscle groups and only the strength training protocol required noteworthy contributions from the upper-body musculature. Because of the relatively low number of high-force contractions involved in strength training and the continuous lower force contractions experienced during endurance training, different patterns of motor unit activation are required. It is possible that the divergent demands placed on the neuromuscular system by strength and endurance training elicited differing alterations in motor unit recruitment in the musculature of the lower limbs; previous research has also implicated altered neural activation during high-force contractions as a potential mechanism for impaired strength development (^22,23). Moreover, the potential altered neural recruitment during rapid and high-force contractions may have contributed to the inhibition of lower-body power development as a result of both high and low frequencies of concurrent training (Figure 3).

After the final training session of the intervention, CT1 elicited greater cortisol levels than ST, which is consistent with previous research (^2,3). This may indicate that higher frequencies of concurrent training can result in elevated physiological stress, which was also reflected in participant's perceived exertion during training (Figure 4). In addition to enhanced training stress, elevations in cortisol have been implicated in catabolism and impaired hypertrophic development with concurrent training (²²). However, in the present study, increases in lean mass were similar between training conditions; as such, it is unlikely that the observed elevations in cortisol influenced muscle morphological adaptation. The variance in the findings of the present study and those of Kraemer et al. (²²) are perhaps because of the differing lengths of the respective training programs. Kraemer et al. (²²) used a 12-week intervention, whereas in the present study, participants were trained for 6 weeks. As the CT1 condition resulted in the inhibition of strength development after 6 weeks of training, it may be speculated that had the interventions been longer CT1 may have also resulted in impaired increases in lean mass.

Practical Applications

The findings of this study build on the understanding of concurrent training developed in the isolated limb model discussed in our previous work (²⁰). The data presented here indicate that if strength development is the primary goal of a training program, endurance training frequency should be kept to a minimum. It should, however, be noted that this minimal dose of endurance training should be sufficient to maintain any necessary endurance performance characteristics. Also the elevations in postexercise cortisol concentrations observed only in participants conducting strength and endurance training 3 times weekly indicate that overall training stress likely plays a key role in the inhibition of strength development. Therefore, if a concurrent training program must be performed, it is imperative that appropriate monitoring strategies are used to ensure training stress does not become too great and result in the plateau of strength development. Furthermore, if development of power-type characteristics is required, then it seems that frequency and volume of endurance training should be minimized or omitted from the program altogether. This may be achieved via appropriate program construction and periodization to allow power development to occur in periods in which endurance-type training can be kept to a minimum.

Acknowledgments

The authors would like to thank all individuals who volunteered to participate in the study. Additional thanks go to Luke Dopson, Jordan Heath, Ashkan Hakimian, Scott Keeling, and Sean Armstrong for their assistance with data collection. The results of the present study do not constitute any endorsement from the National Strength and Conditioning Association.