Sex-related differences in motor unit behavior are influenced by myosin heavy chain during high- but not moderate-intensity contractions.

AIMS: Motor unit recruitment and firing rate patterns of the vastus lateralis (VL) have not been compared between sexes during moderate- and high-intensity contraction intensities. Additionally, the influence of fiber composition on potential sex-related differences remains unquantified. METHODS: Eleven males and 11 females performed 40% and 70% maximal voluntary contractions (MVCs). Surface electromyographic (EMG) signals recorded from the VL were decomposed. Recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP ), initial firing rates (IFRs), mean firing rates (MFRs), and normalized EMG amplitude (N-EMGRMS ) at steady torque were analyzed. Y-intercepts and slopes were calculated for MUAPAMP , IFR, and MFR versus RT relationships. Type I myosin heavy chain isoform (MHC) was determined with muscle biopsies. RESULTS: There were no sex-related differences in MU characteristics at 40% MVC. At 70% MVC, males exhibited greater slopes (p = 0.002) for the MUAPAMP , whereas females displayed greater slopes (p = 0.001-0.007) for the IFR and MFR versus RT relationships. N-EMGRMS at 70% MVC was greater for females (p < 0.001). Type I %MHC was greater for females (p = 0.006), and was correlated (p = 0.018-0.031) with the slopes for the MUAPAMP , IFR, and MFR versus RT relationships at 70% MVC (r = -0.599-0.585). CONCLUSION: Both sexes exhibited an inverse relationship between MU firing rates and recruitment thresholds. However, the sex-related differences in MU recruitment and firing rate patterns and N-EMGRMS at 70% MVC were likely due to greater type I% MHC and smaller twitch forces of the higher threshold MUs for the females. Evidence is provided that muscle fiber composition may explain divergent MU behavior between sexes.

High-Intensity Cycling Training Necessitates Increased Neuromuscular Demand of the Vastus Lateralis During a Fatiguing Contraction.

Purpose: To examine the effects of a 5-week continuous cycling training intervention on electromyographic amplitude (EMGRMS)- and mechanomyographic amplitude (MMGRMS)-torque relationships of the vastus lateralis (VL) during a prolonged contraction. Methods: Twenty-four sedentary, young adults performed maximal voluntary contractions (MVCs) and a prolonged isometric trapezoidal contraction at the same absolute 40% MVC for the knee extensors before (PRE) and after training (POSTABS). Individual b- (slopes) and a-terms (y-intercepts) were calculated from the log-transformed electromyographic amplitude (EMGRMS)- and mechanomyographic amplitude (MMGRMS)-torque relationships during the increasing and decreasing segments of the trapezoid. EMGRMS and MMGRMS was normalized for the 45-s steady torque segment. Results: At PRE, b-terms for the EMGRMS-torque relationships during the linearly decreasing segment were greater than the increasing segment (p < .001), and decreased from PRE to POSTABS (p = .027). a-terms were greater during the linearly increasing than decreasing segment at PRE, while the a-terms for the linearly decreasing segment increased from PRE to POSTABS (p = .027). For the MMGRMS-torque relationships, b-terms during the linearly decreasing segment decreased from PRE to POSTABS (p = .013), while a-terms increased from PRE to POSTABS when collapsed across segments (p = .022). Steady torque EMGRMS increased for POSTABS (p < .001). Conclusion: Although cycling training increased aerobic endurance, incorporating resistance training may benefit athletes/individuals as the alterations in neuromuscular parameters post-training suggest a greater neural cost (EMGRMS) and mechanical output (MMGRMS) to complete the same pre-training fatiguing contraction.

Chronic training status affects muscle excitation of the vastus lateralis during repeated contractions.

This study examined electromyographic amplitude (EMGRMS)-force relationships during repeated submaximal knee extensor muscle actions among chronic aerobically-(AT), resistance-trained (RT), and sedentary (SED) individuals. Fifteen adults (5/group) attempted 20 isometric trapezoidal muscle actions at 50% of maximal strength. Surface electromyography (EMG) was recorded from vastus lateralis (VL) during the muscle actions. For the first and last successfully completed contractions, linear regression models were fit to the log-transformed EMGRMS-force relationships during the linearly increasing and decreasing segments, and the b terms (slope) and a terms (antilog of y-intercept) were calculated. EMGRMS was averaged during steady force. Only the AT completed all 20 muscle actions. During the first contraction, the b terms for RT (1.301 â€‹± â€‹0.197) were greater than AT (0.910 â€‹± â€‹0.123; p â€‹= â€‹0.008) and SED (0.912 â€‹± â€‹0.162; p â€‹= â€‹0.008) during the linearly increasing segment, and in comparison to the linearly decreasing segment (1.018 â€‹± â€‹0.139; p â€‹= â€‹0.014), respectively. For the last contraction, the b terms for RT were greater than AT during the linearly increasing (RT â€‹= â€‹1.373 â€‹± â€‹0.353; AT â€‹= â€‹0.883 â€‹± â€‹0.129; p â€‹= â€‹0.018) and decreasing (RT â€‹= â€‹1.526 â€‹± â€‹0.328; AT â€‹= â€‹0.970 â€‹± â€‹0.223; p â€‹= â€‹0.010) segments. In addition, the b terms for SED increased from the linearly increasing (0.968 â€‹± â€‹0.144) to decreasing segment (1.268 â€‹± â€‹0.126; p â€‹= â€‹0.015). There were no training, segment, or contraction differences for the a terms. EMGRMS during steady force increased from the first- ([64.08 â€‹± â€‹51.68] â€‹µV) to last-contraction ([86.73 â€‹± â€‹49.55] â€‹µV; p â€‹= â€‹0.001) collapsed across training statuses. The b terms differentiated the rate of change for EMGRMS with increments in force among training groups, indicating greater muscle excitation to the motoneuron pool was necessary for the RT than AT during the linearly increasing and decreasing segments of a repetitive task.

A noninvasive test for estimating myosin heavy chain of the vastus lateralis in females with mechanomyography.

This study examined relationships between percent myosin heavy chain (%MHC) expression and mechanomyographic amplitude (MMGRMS). Fifteen females (age ± SD=21.3 ± 5.3 yrs) completed isometric trapezoidal contractions at 30% and 70% maximal voluntary contraction (MVC). MMG was recorded from the vastus lateralis (VL). Participants gave a muscle biopsy of the VL post-testing. MMGRMS-torque relationships during the linearly varying segments were log-transformed and linear regressions were applied to calculate b terms (slopes). For the steady torque segment, MMGRMS was averaged. Correlations were performed for type I%MHC with the MMG variables. Multiple regression was utilized to examine prediction equations for type I%MHC. Type I%MHC was significantly correlated with the b terms during the increasing segment of the 70% MVC (p = 0.003; r = -0.718), and MMGRMS during steady torque at 30% (p = 0.008; r = -0.652) and 70% MVC (p = 0.040; r = -0.535). Type I%MHC reduced the linearity of the MMGRMS-torque relationship during the high-intensity linearly increasing segment, and MMGRMS at a low- and high-intensity steady torque. A combination of MMG variables estimated type I%MHC expression with 81.2% accuracy. MMG recorded during a low- and high-intensity isometric trapezoidal contraction may offer a simple, noninvasive test for estimating type I%MHC expression of the VL in sedentary females.

Testosterone and Cortisol Salivary Samples Are Stable Across Multiple Freeze-Thaw Cycles.

ABSTRACT: Sontag, SA, Cabarkapa, D, and Fry, AC. Testosterone and cortisol salivary samples are stable across multiple freeze-thaw cycles. J Strength Cond Res 37(4): 915-918, 2023-When processing salivary samples for biomarker analysis, avoiding multiple freeze-thaw cycles is generally recommended. However, confusing tissue handling instructions or challenges with collections in the field sometimes makes this problematic. Thus, the purpose of this study was to examine if the stability of salivary testosterone (T) and cortisol (C) hormones remains unchanged when exposed to multiple freeze-thaw cycles. Seven healthy recreationally active adults provided salivary samples at rest (i.e., 1600 hours) for analysis of T and C. Samples were separated into 4 aliquots for each hormone and underwent 4 freeze-thaw cycles (T1-T4 and C1-C4) before being analyzed by enzyme-linked immunosorbent assay. The overall analysis of variance model was significant for T ( p = 0.008) and nonsignificant for C ( p = 0.820). A follow-up post hoc comparison indicated significant differences in salivary hormonal concentrations between T1 and T4 ( p = 0.029), T2 and T4 ( p = 0.007), and T3 and T4 ( p = 0.032). The findings of this study indicate that salivary steroid hormones seem to be relatively stable following multiple freeze-thaw cycles. However, C seems to be more stable when exposed to multiple freeze-thaw cycles, as T concentrations did reveal a significant decrease by the fourth thaw cycle.

The influence of chronic training status on the mechanical behavior of the vastus lateralis during repetitive trapezoidal contractions.

OBJECTIVES: To examine maximal strength and fatigability of the knee extensors, and mechanomyographic amplitude (MMGRMS)-force relationships of the vastus lateralis (VL) during repetitive muscle actions for 5 aerobically-(AT), 5 resistance-trained-(RT), and 5 sedentary (SED) individuals. METHODS: Participants performed maximal voluntary contractions before (MVCPRE) and after (MVCPOST) attempting 20 isometric trapezoidal muscle actions at 50% MVCPRE. MMG was recorded from the VL. b terms (slopes) were calculated from the natural log-transformed MMGRMS-force relationships for each participant (increasing and decreasing segments). MMGRMS was averaged during steady force. RESULTS: RT had greater MVCPRE (P<0.001) and MVCPOST (P=0.001-0.004) than AT and SED. Only AT completed 20 muscle actions and exhibited no decrease in MVCPOST (P=0.149). The b terms were greater for RT than AT during the increasing segment of the first contraction (P=0.001) and decreasing segment of the last contraction (P=0.033). The b terms were also greater for RT (P=0.006) during the increasing than decreasing segment for the first contraction. MMGRMS during steady force was greater during the last contraction when collapsed across training status (P=0.021). CONCLUSION: Knee extensor MVC and fatigability, and motor unit control strategies for the VL during a series of repetitive contractions were influenced by chronic training status.

Endurance training alters motor unit activation strategies for the vastus lateralis, yet sex-related differences and relationships with muscle size remain.

PURPOSE: To examine the effects of 10 weeks of endurance cycling training on mechanomyographic amplitude (MMGRMS)-torque relationships and muscle cross-sectional area (mCSA) of the vastus lateralis (VL) for 10 sedentary males (Age ± SD; 20.2 ± 1.9 years) and 14 sedentary females (21.9 ± 5.3 years). METHODS: Participants performed maximal voluntary contractions (MVCs) and an isometric ramp up muscle action to 70% MVC of the knee extensors before (PRE) and after training at the same absolute pre-treatment submaximal torque (POSTABS). MMG was recorded from the VL and b terms were calculated from the natural log-transformed MMGRMS-torque relationships for each subject. mCSA was determined with ultrasonography. RESULTS: Cycling decreased MVCs from pre- (168.10 ± 58.49 Nm) to post-training (160.78 ± 58.39 Nm; p = 0.005) without changes in mCSA. The b terms were greater for POSTABS (0.623 ± 0.204) than PRE (0.540 ± 0.226; p = 0.012) and for males (0.717 ± 0.171) than females (0.484 ± 0.168; p = 0.003). mCSA was correlated with the b terms for PRE (p < 0.001, r = 0.674) and POSTABS (p = 0.020, r = 0.471). CONCLUSION: The decrease in MVC and increase in MMGRMS (b terms) post-training suggests increased motor unit (MU) recruitment to match pre-training torques. The greater acceleration in the b terms by males may reflect sex-related differences in fiber-type area. MMGRMS-torque relationships during a high-intensity contraction provided insight on MU activation strategies following endurance training and between sexes. Furthermore, the findings suggest a relationship between MMGRMS and muscle size.

MAPK, androgen, and glucocorticoid receptor phosphorylation following high-frequency resistance exercise non-functional overreaching.

PURPOSE: Stressful training with insufficient recovery can impair muscle performance. Expression of mitogen-activated protein kinases (MAPK) has been reported at rest following overreaching and overtraining. The acute myocellular exercise response to stressful training with insufficient recovery has not been investigated. We investigated MAPK, androgen, and glucocorticoid receptor phosphorylation following a period of stressful training. METHODS: Sixteen resistance-trained men were matched on barbell squat 1 repetition maximum strength and randomized into a group that performed normal training or stressful training with insufficient recovery. The control group (CON) performed three speed-squat training sessions on non-consecutive days, while the stressful training group (NFOR) performed 15 training sessions over 7.5 days. Resting and post-exercise skeletal muscle biopsies were obtained prior to (T1) and after the training period (T2). Samples were analyzed for total and phosphorylated androgen receptor (AR), glucocorticoid receptor (GR), and MAPKs (ERK, JNK, and p38). RESULTS: Total AR were down-regulated post-exercise at T2 in NFOR only. Phospho-AR at ser515 increased in both groups post-exercise at T1; however, ser515 only increased at T2 in NFOR. Phosphorylated ERK, JNK, and p38 increased post-exercise in CON and NFOR at T1 and T2. Post-exercise phospho-p38 was blunted in NFOR at T2 compared to T1. After the training intervention, resting phospho-p38 was higher in NFOR compared to T1. At T2, post-exercise phospho-GR at ser226 was lower compared to T1, and resting levels increased in NFOR. CONCLUSION: Steroid receptors are phosphorylated after acute resistance exercise, and in addition to MAPKs, are differentially regulated after stressful training with insufficient recovery.