Sex differences in fatiguability during single-joint resistance exercise in a resistance-trained population.

The primary aim of this study was to compare the fatiguability and acute recovery from a session of unilateral biceps curl exercise between habitually resistance-trained males and females. Twenty participants performed biceps curl one repetition maximum (1RM) and maximal voluntary contraction (MVC) testing of the elbow flexors. The exercise protocol consisted of four sets of unilateral biceps curls at 50% of the measured 1RM to volitional failure. MVC force and EMG activity of the elbow flexors were assessed following each set and during the recovery phase. The outcome variables compare the repetition volume, MVC strength loss, and EMG responses between males and females during the exercise and recovery phases. Across the exercise protocol, females completed significantly more repetitions compared to males (p < 0.01, d = 1.7) and lost significantly less strength across time (76% versus 69%, d = 0.48, p = 0.046). Additionally, females maintained a higher level of normalized EMG amplitude compared to males across the protocol (p = 0.035; d = 0.510). However, there were no sex differences in the strength (p = 0.562) or EMG (p = 0.607) responses during the recovery phase. In a resistance-trained population, females were able to perform a greater number of repetitions during the biceps curl exercise compared to their male counterparts at the same relative intensity of exercise. The ability of females to maintain a greater proportion of their maximal elbow flexor strength and muscle excitation during exercise suggests females can tolerate greater upper-body resistance exercise volume than males.

Responses to Exercise at the Critical Heart Rate vs. the Power Output Associated With the Critical Heart Rate.

ABSTRACT: Succi, PJ, Dinyer-McNeely, TK, Voskuil, CC, Abel, MG, Clasey, JL, and Bergstrom, HC. Responses to exercise at the critical heart rate vs. the power output associated with the critical heart rate. J Strength Cond Res 37(12): 2362-2372, 2023-This study examined the physiological (volume of oxygen consumption [V̇ o2 ], heart rate [HR], power output [PO], respiration rate [RR], muscle oxygen saturation [%SmO 2 ]), neuromuscular (electromyographic and mechanomyographic amplitude [EMG AMP and MMG AMP] and mean power frequency [EMG MPF and MMG MPF]), and perceptual (rating of perceived exertion [RPE]) responses during exercise anchored at the critical heart rate (CHR) vs. the PO associated with CHR (PCHR). Nine subjects (mean ± SD ; age = 26 ± 3 years) performed a graded exercise test and 4 constant PO trials to exhaustion at 85-100% of peak PO (PP) to derive CHR and PCHR on a cycle ergometer. Responses were recorded during trials at CHR (173 ± 9 b·min -1 , time to exhaustion [T Lim ] = 45.5 ± 20.2 minutes) and PCHR (198 ± 58 W, T Lim = 21.0 ± 17.8 minutes) and normalized to their respective values at PP in 10% intervals. There were significant ( p ≤ 0.05) mode (CHR vs. PCHR) × time (10%-100% T Lim ) interactions for all variables ( p < 0.001-0.036) except MMG AMP ( p > 0.05). Post hoc analyses indicated differences across time for CHR V̇ o2 (%change = -22 ± 16%), PCHR V̇ o2 (19 ± 5%), CHR RR (24 ± 23%), PCHR RR (45 ± 14%), CHR PO (-33 ± 11%), PCHR HR (22 ± 5%), CHR RPE (22 ± 14%), PCHR RPE (39 ± 6%), CHR %SmO 2 (41 ± 33%), PCHR %SmO 2 (-18 ± 40%), CHR EMG AMP (-13 ± 15%), PCHR EMG AMP (13 ± 13%), CHR EMG MPF (9 ± 8%), CHR MMG MPF (7 ± 11%), and PCHR MMG MPF (-3 ± 14%). The critical heart rate was more sustainable than PCHR but required adjustments in PO which traversed intensity domains and caused dissociations of the responses previously observed in exercise anchored to PO. These dissociations indicated the demands to exercise varied with anchoring scheme and provides an important consideration for practitioners prescribing endurance exercise.

Affective and Perceptual Responses During an 8-Week Resistance Training to Failure Intervention at Low vs. High Loads in Untrained Women.

ABSTRACT: Anderson, AlOK, Voskuil, CC, Byrd, MT, Garver, MJ, Rickard, AJ, Miller, WM, Bergstrom, HC, and Dinyer McNeely, TK. Affective and perceptual responses during an 8-week resistance training to failure intervention at low vs. high loads in untrained women. J Strength Cond Res 37(3): 546-554, 2023-This study examined the effects of resistance training (RT) to failure on the perceptual and affective responses, intent-to-continue RT to failure in a self-initiated session, and affect-intent relationship. Twenty-three untrained women (mean ± SD : age 21.2 ± 2.2 years; height 167 ± 5.7 cm; body mass, 62.3 ± 16.2 kg) completed an 8-week, full-body RT to failure intervention at a low (30% 1RM; n = 11) or high (80% 1RM; n = 12) load. The Borg's rating of perceived exertion (RPE) scale was used to assess the acute (aRPE) and session (sRPE) RPE immediately after repetition failure and each training session, respectively. Immediately, 15-minute, and 60-minute postsession affective responses were assessed using the feeling scale (FS; -5 to +5), and intent to continue to RT was assessed on a scale of 0-100% intention. During week 4 (W4) and week 8 (W8), aRPE (W4: 18 ± 2, W8: 18 ± 2; p ≤ 0.032) and sRPE (W4: 17 ± 2, W8: 18 ± 1; p ≤ 0.018) were greater than that during week 1 (W1; aRPE: 17 ± 2; sRPE: 16 ± 2). The FS responses increased from immediately to 60-minute postsession during W4 ( p ≤ 0.019) and W8 ( p ≤ 0.049). The correlation between affect and intent-to-continue RT increased from W1 ( r = 0.416) to W8 ( r = 0.777). Regardless of load, untrained women reported similar perceptual, affective, and intention responses. These variables should be considered to improve RT program adoption and adherence in women.

Application of V̇ o2 to the Critical Power Model to Derive the Critical V̇ o2.

ABSTRACT: Succi, PJ, Dinyer, TK, Byrd, MT, Voskuil, CC, and Bergstrom, HC. Application of V̇ o2 to the critical power model to derive the critical V̇ o2 . J Strength Cond Res 36(12): 3374-3380, 2022-The purposes of this study were to (a) determine whether the critical power (CP) model could be applied to V̇ o2 to estimate the critical V̇ o2 (CV̇ o2 ) and (b) to compare the CV̇ o2 with the V̇ o2 at CP (V̇ o2 CP), the ventilatory threshold (VT), respiratory compensation point (RCP), and the CV̇ o2 without the V̇ o2 slow component (CV̇ o2 slow). Nine subjects performed a graded exercise test to exhaustion to determine V̇ o2 peak, VT, and RCP. The subjects performed 4 randomized, constant power output work bouts to exhaustion. The time to exhaustion (T Lim ), the total work (W Lim ), and the total volume of oxygen consumed with (TV̇ o2 ) and without the slow component (TV̇ o2 slow) were recorded during each trial. The linear regressions of the TV̇ o2 vs. T Lim , TV̇ o2 slow vs. T Lim , and W Lim vs. T Lim relationship were performed to derive the CV̇ o2 , CV̇ o2 slow, and CP, respectively. A 1-way repeated-measures analysis of variance ( p ≤ 0.05) with follow-up Sidak-Bonferroni corrected pairwise comparisons indicated that CV̇ o2 (42.49 ± 3.22 ml·kg -1 ·min -1 ) was greater than VT (30.80 ± 4.66 ml·kg -1 ·min -1 ; p < 0.001), RCP (36.74 ± 4.49 ml·kg -1 ·min -1 ; p = 0.001), V̇ o2 CP (36.76 ± 4.31 ml·kg -1 ·min -1 ; p < 0.001), and CV̇ o2 slow (38.26 ± 2.43 ml·kg -1 ·min -1 ; p < 0.001). However, CV̇ o2 slow was not different than V̇ o2 CP ( p = 0.140) or RCP ( p = 0.235). Thus, the CP model can be applied to V̇ o2 to derive the CV̇ o2 and theoretically is the highest metabolic steady state that can be maintained for an extended period without fatigue. Furthermore, the ability of the CV̇ o2 to quantify the metabolic cost of exercise and the inefficiency associated with the V̇ o2 slow component may provide a valuable tool for researchers and coaches to examine endurance exercise.

Exploring the Relationship between Ultrasonographic Measures of the Quadriceps and Knee Extensor Muscle Fitness in Endurance-Trained Individuals.

Background: B-mode ultrasonography is an accessible and cost-effective method to assess muscle size and quality through muscle thickness (MT) and echo intensity (EI), respectively. Muscle thickness and EI have demonstrated relationships with maximal strength and local muscle endurance, providing a noninvasive and efficient modality to examine muscle fitness. However, these relationships have not been quantified in the individual quadriceps muscles of habitually endurance-trained populations, which may provide information to practitioners regarding rehabilitation and performance. Methods: Twenty-three participants (males: N = 10; females: N = 13) underwent B-mode ultrasonography to assess MT, EI, and adipose tissue thickness-corrected echo intensity (cEI) in the vastus intermedius (VI), vastus lateralis (VL), and rectus femoris (RF). Muscle fitness was evaluated through maximal strength (1RM) and local muscle endurance (4 sets to failure at 50% 1RM) during dynamic knee extension. Relationships between ultrasonography outcomes and muscle fitness were examined through stepwise multiple linear regression. Results: The results indicate that VI cEI is the strongest predictor of 1RM strength (r = -0.643), while no ultrasonography-derived measures significantly predicted local muscle endurance. Conclusion: The study demonstrates that ultrasonography, specifically measures of cEI in the VI, has the greatest association with maximal strength in endurance-trained individuals. These findings suggest monitoring VI muscle size and quality may benefit practitioners who aim to improve knee extension strength for performance or following injury. In addition, the findings support the use of EI examinations in trained populations.

Exercise prescription and strategies to promote the cross-education of strength: a scoping review.

The cross-education of strength is moderated by exercise design and prescription in clinical and non-clinical populations. This review synthesizes the available evidence regarding exercise design strategies for unilateral resistance training and provides evidence-based recommendations for the prescription of unilateral training to maximize the cross-education of strength. Greater insights regarding the timing and effectiveness of cross-education interventions in clinical scenarios will strengthen the use of unilateral resistance training for individuals who may benefit from its use.

Unilateral Handgrip Holds to Failure Result in Sex-Dependent Contralateral Facilitation.

This study examined changes in maximal voluntary isometric contraction (MVIC) force following dominant (Dm) and nondominant (NDm) unilateral, handgrip isometric holds to failure (HTF) for the exercised ipsilateral (IPS) and non-exercised contralateral (CON) limbs and determined if there are sex- and hand- (Dm vs NDm) dependent responses in the HTF time, performance fatigability (PF) for the exercised IPS limb, and changes in MVIC force for the CON limb after unilateral fatigue. Ten men and 10 women (Age = 22.2 years) completed an isometric HTF at 50% MVIC for the Dm and NDm hand on separate days. Prior to, and immediately after the HTF, an MVIC was performed on the IPS and CON limbs, in a randomized order. The Dm (130.3 ± 36.8 s) HTF (collapsed across sex) was significantly longer (p = 0.002) than the NDm (112.1 ± 34.3 s). The men (collapsed across hand) demonstrated IPS (%Δ = 22.9 ± 10.8%) PF and CON facilitation (%Δ = -6.1 ± 6.9%) following the HTF, while the women demonstrated differences in PF between the Dm and NDm hands for the IPS (%Δ Dm = 28.0 ± 9.4%; NDm = 32.3% ± 10.1%; p = 0.027), but not the CON limb (%Δ Dm = -1.6 ± 5.7%; NDm = 1.7 ± 5.9%). The cross-over facilitation of the CON limb for men, but not women, following a unilateral, isometric handgrip HTF may be related to post-activation potentiation.

Applications of the Critical Power Model to Dynamic Constant External Resistance Exercise: A Brief Review of the Critical Load Test.

The study and application of the critical power (CP) concept has spanned many decades. The CP test provides estimates of two distinct parameters, CP and W', that describe aerobic and anaerobic metabolic capacities, respectively. Various mathematical models have been used to estimate the CP and W' parameters across exercise modalities. Recently, the CP model has been applied to dynamic constant external resistance (DCER) exercises. The same hyperbolic relationship that has been established across various continuous, whole-body, dynamic movements has also been demonstrated for upper-, lower-, and whole-body DCER exercises. The asymptote of the load versus repetition relationship is defined as the critical load (CL) and the curvature constant is L'. The CL and L' can be estimated from the same linear and non-linear mathematical models used to derive the CP. The aims of this review are to (1) provide an overview of the CP concept across continuous, dynamic exercise modalities; (2) describe the recent applications of the model to DCER exercise; (3) demonstrate how the mathematical modeling of DCER exercise can be applied to further our understanding of fatigue and individual performance capabilities; and (4) make initial recommendations regarding the methodology for estimating the parameters of the CL test.

The Influence of Sonographer Experience on Skeletal Muscle Image Acquisition and Analysis.

The amount of experience with ultrasonography may influence measurement outcomes while images are acquired or analyzed. The purpose of this study was to identify the interrater reliability of ultrasound image acquisition and image analysis between experienced and novice sonographers and image analysts, respectively. Following a brief hands-on training session (2 h), the experienced and novice sonographers and analysts independently performed image acquisition and analyses on the biceps brachii, vastus lateralis, and medial gastrocnemius in a sample of healthy participants (n = 17). Test-retest reliability statistics were computed for muscle thickness (transverse and sagittal planes), muscle cross-sectional area, echo intensity and subcutaneous adipose tissue thickness. The results show that image analysis experience generally has a greater impact on measurement outcomes than image acquisition experience. Interrater reliability for measurements of muscle size during image acquisition was generally good-excellent (ICC2,1: 0.82-0.98), but poor-moderate for echo intensity (ICC2,1: 0.43-0.77). For image analyses, interrater reliability for measurements of muscle size for the vastus lateralis and biceps brachii was poor-moderate (ICC2,1: 0.48-0.70), but excellent for echo intensity (ICC2,1: 0.90-0.98). Our findings have important implications for laboratories and clinics where members possess varying levels of ultrasound experience.

Interlimb Neuromuscular Responses During Fatiguing, Bilateral, Leg Extension Exercise at a Moderate Versus High Load.

This study determined the load- and limb-dependent neuromuscular responses to fatiguing, bilateral, leg extension exercise performed at a moderate (50% one-repetition maximum [1RM]) and high load (80% 1RM). Twelve subjects completed 1RM testing for the bilateral leg extension, followed by repetitions to failure at 50% and 80% 1RM, on separate days. During all visits, the electromyographic (EMG) and mechanomyographic (MMG), amplitude (AMP) and mean power frequency (MPF) signals were recorded from the vastus lateralis of both limbs. There were no limb-dependent responses for any of the neuromuscular signals and no load-dependent responses for EMG AMP, MMG AMP, or MMG MPF (p = .301-.757), but there were main effects for time that indicated increases in EMG and MMG AMP and decreases in MMG MPF. There was a load-dependent decrease in EMG MPF over time (p = .032) that suggested variability in the mechanism responsible for metabolite accumulation at moderate versus high loads. These findings suggested that common drive from the central nervous system was used to modulate force during bilateral leg extension performed at moderate and high loads.