Sex differences in muscle contraction-induced limb blood flow limitations.

PURPOSE: To determined sex differences in absolute- and %-reductions in blood flow during intermittent muscular contractions as well as relationships between blood flow reductions and time to task failure (TTF). METHODS: Thirteen males (25 ± 4 years) and 13 females (22 ± 5 years) completed intermittent isometric trapezoidal forearm flexion at 50% maximal voluntary contraction until task failure. Doppler ultrasound was used to measure brachial artery blood flow (BABF) during the 12-s plateau phase and 12-s relaxation phase. RESULTS: Target torque was less in females than males (24 ± 5 vs. 42 ± 7 Nm; p < 0.001); however, TTF was not different between sexes (F: 425 ± 187 vs. M: 401 ± 158 s; p = 0.72). Relaxation-phase BABF at end-exercise was less in females than males (435 ± 161 vs. 937 ± 281 mL/min; p < 0.001) but contraction-phase BABF was not different (127 ± 46 vs. 190 ± 99 mL/min; p = 0.42). Absolute- and %-reductions in BABF by contraction were less in females than males (309 ± 146 vs. 747 ± 210 mL/min and 69 ± 10 vs. 80% ± 6%, respectively; both p < 0.01) and were associated with target torque independent of sex (r = 0.78 and 0.56, respectively; both p < 0.01). Absolute BABF reduction per target torque (mL/min/Nm) and TTF were positively associated in males (r = 0.60; p = 0.031) but negatively associated in females (r = - 0.61; p = 0.029). CONCLUSIONS: This study provides evidence that females incur less proportional reduction in limb blood flow from muscular contraction than males at a matched relative intensity suggesting females may maintain higher levels of muscle oxygen delivery and metabolite removal than males across the contraction-relaxation cycle of intermittent exercise.

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.

Electromyographic and mechanomyographic responses of the biceps brachii during concentric and eccentric muscle actions to failure at high and low relative loads.

PURPOSE: This study examined neuromuscular responses of the biceps brachii (BB) for concentric and eccentric muscle actions during bilateral, dynamic constant external resistance (DCER), reciprocal forearm flexions and extensions to failure at high (80% 1 repetition maximum [1RM]) and low (30% 1RM) relative loads. METHODS: Nine women completed 1RM testing and repetitions to failure (RTF) at 30 and 80% 1RM. Electromyographic (EMG) and mechanomyographic (MMG) amplitude (AMP) and mean power frequency (MPF) signals were measured from the BB. Analyses included repeated measures ANOVAs (p < 0.05) and post-hoc pairwise comparisons with Bonferroni corrected alpha of p < 0.008 and p < 0.01 for between and within factor pairwise comparisons, respectively. RESULTS: EMG AMP and MPF were significantly greater for concentric than eccentric muscle actions, regardless of load or time. However, time course of change analysis revealed parallel increases in EMG AMP for concentric and eccentric muscle actions during the RTF trials at 30% 1RM, but no change at 80% 1RM. There were significant increases in MMG AMP during concentric muscle actions, but decreases or no change during eccentric muscle actions. EMG and MMG MPF decreased over time, regardless of muscle action type and loading condition. CONCLUSION: The greater EMG AMP and MPF values during concentric compared to eccentric muscle actions may reflect the difference in the efficiency characteristic of these muscle actions. The neuromuscular responses suggested that fatigue may be mediated by recruitment of additional motor units with lower firing rates during concentric muscle actions, and changes in motor unit synchronization during eccentric muscle actions.

Load-Specific Performance Fatigability, Coactivation, and Neuromuscular Responses to Fatiguing Forearm Flexion Muscle Actions in Women.

ABSTRACT: Benitez, B, Dinyer-McNeeley, TK, McCallum, L, Kwak, M, Succi, PJ, and Bergstrom, HC. Load-specific performance fatigability, coactivation, and neuromuscular responses to fatiguing forearm flexion muscle actions in women. J Strength Cond Res 37(4): 769-779, 2023-This study examined the effects of fatiguing, bilateral, dynamic constant external resistance (DCER) forearm flexion on performance fatigability, coactivation, and neuromuscular responses of the biceps brachii (BB) and triceps brachii (TB) at high (80% 1 repetition maximum [1RM]) and low (30% 1RM) relative loads in women. Ten women completed 1RM testing and repetitions to failure (RTF) at 30 and 80% 1RM. Maximal voluntary isometric force was measured before and after RTF. Electromyographic (EMG) and mechanomyographic (MMG) amplitude (AMP) and mean power frequency (MPF) signals were measured from the BB and TB. Performance fatigability was greater (p < 0.05) after RTF at 30% (%∆ = 41.56 ± 18.61%) than 80% (%∆ = 19.65 ± 8.47%) 1RM. There was an increase in the coactivation ratio (less coactivation) between the initial and final repetitions at 30%, which may reflect greater increases in agonist muscle excitation (EMG AMP) relative to the antagonist for RTF at 30% than 80% 1RM. The initial repetitions EMG AMP was greater for 80% than 30% 1RM, but there was no difference between loads for the final repetitions. For both loads, there were increases in EMG MPF and MMG AMP and decreases in MMG MPF that may suggest fatigue-dependent recruitment of higher-threshold motor units. Thus, RTF at 30 and 80% 1RM during DCER forearm flexion may not necessitate additional muscle excitation to the antagonist muscle despite greater fatigability after RTF at 30% 1RM. These specific acute performance and neuromuscular responses may provide insight into the unique mechanism underlying adaptations to training performed at varying relative loads.

Differences and Relationships Between Push-up and Sit-up Variations Among Male Law Enforcement Cadets.

ABSTRACT: Lopes dos Santos, M, Thompson, M, Dinyer-McNeely, T, Torrence, T, Lockie, RG, Orr, RM, and Dawes, JJ. Differences and relationships between push-up and sit-up variations among male law enforcement cadets. J Strength Cond Res 37(9): 1865-1869, 2023-The purpose of this study was to explore variation-dependent differences in push-up and sit-up tests, as well as to identify the relationships between push-up and sit-up variations commonly performed among tactical personnel. Data from 41 (age = 21.1 ± 1.8 years, body mass = 89.5 ± 15.5 kg) male police cadets from one US-based training academy who performed the standard push-up (PU Standard ), hand-release push-up (PU Hand-release ), standard sit-up (SU Standard ), and hands behind the head sit-up (SU BehindHead ) were examined. The maximum number of repetitions completed for each exercise was used for analyses with paired-samples t tests used to identify significant differences between outcome measures and effect sizes (ES) to determine change in magnitude. A significant difference, with a large ES, was found in the number of repetitions completed between the push-up variations ( p < 0.001, ES = 0.75) and sit-up variations ( p < 0.001, ES = 1.02). Performance in PU Standard was only able to explain 39% of the variation in PU Hand-release ( r = 0.62, r2 = 0.39, p < 0.001), and performance in the SU Standard was only able to explain 29% of the variation in SU BehindHead ( r = 0.54, r2 = 0.29, p < 0.001). The current study suggests differences in performance capabilities may be dependent on the push-up and sit-up variations chosen for testing. Although it is still unclear whether one variation would be a better predictor of police work performance, agencies must give special attention to the overall construct validity of fitness tests for entry and retention of law enforcement officers.

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.

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.