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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). 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. 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

Miller, JD, Lippman, JD, Trevino, MA, and Herda, TJ. Larger motor units are recruited for high-intensity contractions than for fatiguing moderate-intensity contractions. J Strength Cond Res 34(11): 3013-3021, 2020-The purpose of this study was to investigate whether moderate-intensity contractions performed to fatigue activate the motor unit (MU) pool to the same extent as a higher-intensity contraction. Subjects (7 men, 2 women, age = 22.78 ± 4.15 years, height = 173.78 ± 14.19 cm, mass = 87.39 ± 21.19 kg) performed 3 isometric maximum voluntary contractions (MVCs), an isometric trapezoidal contraction at 90% MVC (REP90), and repetitive isometric trapezoidal contractions at 50% MVC performed to failure with the first (REP1) and final repetition (REPL) used for analysis. Surface EMG was recorded from the vastus lateralis. Action potentials were extracted into firing events of single MUs with recruitment thresholds (RTs), MU action potential amplitudes (MUAPAMP), and mean firing rates (MFRs) recorded. Linear MFR and MUAPAMP vs. RT and exponential MFR vs. MUAPAMP relationships were calculated for each subject. The level of significance was set at p ≤ 0.05. B terms for the MFR vs. MUAPAMP relationships (p = 0.001, REPL = -4.77 ± 1.82 pps·mV, REP90 = -2.63 ± 1.00 pps·mV) and predicted MFRs for MUs recruited at 40% MVC (p < 0.001, REPL = 11.14 ± 3.48 pps, REP90 = 18.38 ± 2.60 pps) were greater for REP90 than REPL indicating firing rates were greater during REP90. In addition, larger mean (p = 0.038, REPL = 0.178 ± 0.0668 mV, REP90 = 0.263 ± 0.128 mV) and maximum (p = 0.008, REPL = 0.320 ± 0.127 mV, Rep90 = 0.520 ± 0.234 mV) MUAPAMPS were recorded during REP90 than REPL. Larger MUs were recruited and similar sized MUs maintained greater firing rates during a high-intensity contraction in comparison to a moderate-intensity contraction performed at fatigue. Individuals seeking maximized activation of the MU pool should use high-intensity resistance training paradigms rather than moderate-intensity to fatigue.

Despite ample evidence that females are weaker and possess smaller muscle cross-sectional areas (CSAs) compared to males, it remains unclear if there are sex-related differences in the properties of motor units (MU). Eleven males (age 22 ± 3 years) and 12 females (age 21 ± 1 years) performed isometric trapezoid muscle actions at 10% and 70% of maximal voluntary contraction (MVC). Surface electromyography signals were recorded and decomposed into MU action potential (AP) waveforms and firing instances. Average MUAP amplitudes (MUAPAMPS), mean firing rates (MFRs), initial firing rates (IFRs), and recruitment thresholds (RT) were calculated for the 10% MVC, while MUAPAMPS, IFRs, and MFRs were regressed against RT for the 70% MVC. Ultrasonography was used to measure CSA of the first dorsal interosseous (FDI). Males had greater CSAs (p 0.05) during the 10% MVC. For the 70% MVC, the y-intercepts from the MUAPAMPS vs. RT relationships were greater (p 0.05) in the slopes. Therefore, smaller CSAs and weaker MVCs are likely the result of smaller higher-threshold MUs for females.

Previous investigations analyzing resistance training’s influence on motor unit (MU) firing rates have yielded mixed results. These mixed results may be clarified by concurrently measuring changes in MU size. Thus, this study analyzed whether post-training strength gains were due to increases in MU firing rates and/or sizes as measured indirectly via action potential amplitudes. Sixteen males (age = 20.7 ± 1.9 years) completed 8 weeks of resistance training, while eight males (age = 19.4 ± 2.5 years) served as controls. Vastus lateralis surface electromyography signals collected during submaximal isometric knee extensions were decomposed to yield an action potential amplitude (MUAPAMP), mean firing rate (MFR), and recruitment threshold (RT) for each MU. Each contraction’s average MFR and MUAPAMP, and coefficients of the linear (y-intercept and slope) MUAPAMP-RT, linear MFR-RT and exponential (A and B terms) MFR–MUAPAMP relationships were analyzed. Firing instances and action potentials were validated via reconstruct-and-test and spike-triggered averaging procedures. Vastus lateralis cross-sectional area (CSA) was analyzed with ultrasonography. Resistance training increased isometric strength from 204.6 ± 34.9 to 239.8 ± 36.3 Nm and vastus lateralis CSA from 28.7 ± 4.7 to 34.0 ± 5.0 cm2. Resistance training did not affect MFR–RT relationship parameters or average MFRs but did increase the slopes of the MUAPAMP–RT relationships (0.0067 ± 0.0041 to 0.0097 ± 0.0045 mV/%MVC) and average MUAPAMPs from 0.189 ± 0.093 to 0.249 ± 0.127 mV. MU hypertrophy altered the MFR–MUAPAMP relationships (B terms: − 3.63 ± 1.40 to − 2.66 ± 1.38 pps/mV). Resistance training induced MU and muscle hypertrophy, but did not alter firing rates. Greater MU twitch forces resulting from larger MUs firing at pre-training rates likely explain resistance training strength gains.

It is common practice to examine motor unit (MU) activity according to mean firing rate (MFR) and action potential amplitude (MUAPAMP) vs. recruitment threshold (RT) relationships during isometric trapezoidal contractions. However, it is unknown whether the rate of torque development during the linearly increasing torque phase affects the activity of MUs during such contractions. Sixteen healthy males and females performed two isometric trapezoidal muscle actions at 40% of maximum voluntary contraction (MVC), one at a rate of torque development of 5% MVC/s (SLOW40) and one at 20% MVC/s (FAST40) during the linearly increasing torque phase. Surface electromyography (EMG) was recorded from the vastus lateralis (VL) via a 5-pin surface array sensor and decomposed into action potential trains of individual MUs, yielding MFRs and MUAPAMP which were regressed against RT separately for each contraction. Surface EMG amplitude recorded from leg extensors and flexors was used to quantify muscle activation and coactivation. MFR vs. RT relationship slopes were more negative (P = 0.003) for the SLOW40 (− 0.491 ± 0.101 pps/%MVC) than FAST40 (− 0.322 ± 0.109 pps/%MVC) and the slopes of the MUAPAMP vs. RT relationships (P = 0.022, SLOW40 = 0.0057 ± 0.0021 mV/%MVC, FAST40 = 0.0041 ± 0.0023 mV/%MVC) and muscle activation of the extensors (P

The purpose of this study was to examine possible differences in motor unit action potential amplitudes (MUAPAMPS) and firing rates of the first dorsal interosseous (FDI) in male and female children aged 8–10 years. Eight male (mean ± SD, age = 8.8 ± 0.7 yrs; BMI = 16.5 ± 1.3 kg/m2) and eight female (age = 9.3 ± 0.9 yrs; BMI = 16.1 ± 1.5 kg/m2) children volunteered to complete isometric trapezoidal muscle actions of the first dorsal interosseous at 50% of maximal voluntary contraction (MVC). Electromyographic signals were decomposed to yield MUAPAMPS and mean firing rates (MFR) at the targeted force. An exponential model was fitted to the MUAPAMPS vs. recruitment threshold (RT) while linear models were fitted to the MFRs vs. RT relationships for each subject. Ultrasonography determined the muscle cross-sectional area (CSA) of the FDI. Independent samples t-tests were used to examine possible differences between the male and female children for MVC strength, CSA, and the coefficients from the MU relationships. There were no differences in MVC strength, CSA, or the MUAPAMP vs. RT relationships between the male and female children (P

It remains unclear if the sizes of higher-threshold motor units (MU) are associated with muscular strength and power. Therefore, the purpose of this study was to examine sex-related differences in muscle cross-sectional area (mCSA), percent myosin heavy chain (%MHC) isoform expression, and the MU action potential amplitudes (MUAPAMPS)-recruitment threshold (RT) relationships of the vastus lateralis and isometric peak torque, isokinetic peak torque and mean power at 1.05 rad·s−1 of the leg extensors. Surface electromyographic decomposition techniques were used to quantify MUAPAMPS recorded during isometric muscle actions at 70% of maximal voluntary contractions and regressed against RTs with the slopes calculated. Ultrasound images were used to measure mCSA. Males had greater slopes from the MUAPAMP-RT relationship than the females (P

Previous investigations report no changes in motor unit (MU) firing rates during submaximal contractions following resistance training. These investigations did not account for MU recruitment or examine firing rates as a function of recruitment threshold (REC). Therefore, MU recruitment and firing rates in chronically resistance-trained (RT) and physically active controls (CON) were examined. Surface electromyography signals were collected from the first dorsal interosseous during isometric muscle actions at 40% and 70% maximal voluntary contraction (MVC). For each MU, force at REC, mean firing rate (MFR) during the steady force, and MU action potential amplitude (MUAPAMP) were analyzed. For each individual and contraction, the MFRs were linearly regressed against REC, whereas, exponential models were applied to the MFR versus MUAPAMP and MUAPAMP versus REC relationships with the y-intercepts and slopes (linear) and A and B terms (exponential) calculated. For the 40% MVC, the RT had less negative slopes (p = 0.001) and lower y-intercepts (p = 0.006) of the MFR versus REC relationships and lower B terms (p = 0.011) of the MUAPAMP versus REC relationships. There were no differences in either relationship between groups for the 70% MVC. During the 40% MVC, the RT had a smaller range of MFRs and MUAPAMPS in comparison with the CON, likely because of reduced MU recruitment. The RT had lower MFRs and recruitment during the 40% MVC, which may indicate a leftward shift in the force–frequency relationship, and thus require less excitation to the motoneuron pool to match the same relative force.

Motor unit action potential size (MUAPsize ) versus recruitment threshold (RT) relationship analysis provides a non-invasive measure of motor unit (MU) hypertrophy; however, this method's ability to identify MU atrophy is unknown. This investigation sought to determine if MUAPsize versus RT relationship slope (APslope ) comparison could identify evidence of MU atrophy in older individuals. Surface electromyography signals were recorded from the first dorsal interosseous (FDI) of fourteen young (YG, age = 22·29 ± 2·79 years) and ten older (OG, 61·0 ± 2·0 years) subjects during a 50% maximal voluntary contraction (MVC) isometric trapezoidal muscle action. The signals were decomposed to yield a MUAPsize and RT for each MU. For each subject, the MUs recruited between 10% and 50% MVC were linearly regressed as a function of RT to calculate an individual APslope . FDI cross-sectional area (CSA) and echo intensity (EI) were quantified via ultrasonography. The mean APslope was lower for OG (0·033 ± 0·010 mV %MVC-1 ) than YG (0·056 ± 0·019 mV %MVC-1 ). OG and YG possessed similar CSAs (OG: 2·09 ± 0·31 cm2 ; YG: 2·08 ± 0·41 cm2 ); however, OG (53·25 ± 7·56 AU) had greater EI than YG (43·87 ± 7·59 AU). The lower OG mean APslope was due to smaller MUAPsizes of higher-threshold MUs, likely due to atrophy of muscle fibres that comprise those MUs. In support, similar CSA with greater EI indicated increased adipose and fibrous tissue and reduced contractile tissue in OG. Thus, MUAPsize versus RT relationship may provide a non-invasive measure of MU atrophy.

Objective To examine twitch force potentiation and twitch contraction duration, as well as electromyographic amplitude (EMGRMS) and motor unit mean firing rates (MFR) at targeted forces between young and old individuals in the first dorsal interosseous (FDI). Ultrasonography was used to assess muscle quality. Methods Twenty-two young (YG) (age = 22.6 ± 2.7 years) and 14 older (OD) (age = 62.1 ± 4.7 years) individuals completed conditioning contractions at 10% and 50% maximal voluntary contraction, (MVC) during which EMGRMS and MFRs were assessed. Evoked twitches preceded and followed the conditioning contractions. Ultrasound images were taken to quantify muscle quality (cross-sectional area [CSA] and echo intensity [EI]). Results No differences were found between young and old for CSA, pre-conditioning contraction twitch force, or MFRs (P > 0.05). However, OD individuals exhibited greater EI and contraction duration (P Conclusions Lower levels of potentiation with elongated contraction durations likely contributed to greater muscle activation during the conditioning contractions in the OD rather than altered MFRs. Ultrasonography suggested age-related changes in muscle structure contributed to altered contractile properties in the OD. Significance Greater muscle activation requirements can have negative implications on fatigue resistance at low to moderate intensities in older individuals.

Background: Skeletal muscle is overlooked in the realm of insulin resistance in children who are overweight and obese despite the fact that it accounts for the most glucose disposal.Objectives: Therefore, this study examined fasted glucose levels and muscle cross-sectional area and echo intensity (EI) via ultrasound images of the first dorsal interosseous, vastus lateralis, and rectus femoris in children who are normal weight and overweight and obese aged 8–10 years.Methods:In total, 13 males (age = 9.0 [0.7] y) and 7 females (age = 9.0 [0.8] y) volunteered for this study. Independent samplesttests and effect sizes (ESs) were used to examine potential differences in skeletal muscle composition and glucose concentrations.Results:There were no significant differences between groups for glucose concentration (P = .07, ES = 0.86); however, the children who were overweight and obese had significantly greater EI (P P r = .514 to −.593) and rectus femoris (r = .551 to −.513), but not the first dorsal interosseous.Discussion:There is evidence that adiposity-related pathways leading to insulin resistance and skeletal muscle degradation are active in young children who are overweight and obese.

New findings What is the central question of this study? Are differences in muscle size and motor unit properties between normal weight and overfat children muscle specific? What is the main finding and its importance? Muscle cross-sectional area and motor unit action potential amplitudes and firing rates were similar between overfat and normal weight children for both the medial gastrocnemius and vastus lateralis muscles. There was no evidence that the chronic mechanical overload provided by the greater body mass resulted in significant hypertrophy of contractile tissue or motor units that would be used during lower-to-moderate intensity activities. Abstract This study examined the possible differences in muscle cross-sectional area (mCSA), motor unit action potential amplitudes (MUAPAMPS ) and interspike intervals (ISIs) of the firing instances of the medial gastrocnemius (MG) and vastus lateralis (VL) between normal weight (NW) and overfat (OF) children aged 7-10 years. Fourteen NW (age = 8.6 ± 1.1 years, BMI = 15.8 ± 1.4 kg m-2 ) and 12 OF (age = 8.8 ± 0.9 years, BMI = 21.8 ± 2.4 kg m-2 ) children performed isometric trapezoidal muscle actions at 40% of maximal voluntary contraction of the plantar flexors and knee extensors. Surface electromyography was recorded from the MG and VL and decomposed into the firing events of motor units (MUs). Statistical procedures were performed on the composite recruitment thresholds (RTs), ISIs and MUAPAMPS of recorded MUs collapsed across subjects and the y-intercepts and slopes calculated from each subject's ISI and MUAPAMP vs. RT relationships. Ultrasound was used to assess mCSA, echo intensity (mEI), and subcutaneous fat (sFAT) of the MG and VL. The OF had greater mCSAs, mEI and sFAT (P = 0.004-0.024), but there were no differences in mCSA when accounting for mEI for the MG (P = 0.506) and VL (P = 0.326). The NW children had significantly greater composite MUAPAMPS for the VL and MG (P 0.05). These findings suggest that the OF children did not undergo significant muscle or MU hypertrophy that would be routinely activated during activities of daily living.

Introduction The purpose of this study was to determine the effects of vibration (VIB) on motor unit (MU) behavior of the vastus lateralis (VL) muscle during a 40% maximal voluntary contraction (MVC). Methods Eleven healthy (age 21.3 ± 2.6 years) individuals participated in the study. Surface electromyography (EMG) data were recorded from the VL during isometric trapezoidal muscle contractions at 40% MVC. Firing events of single MUs and EMG amplitude were reported for the first, middle, and final seconds of a 12-second steady force segment at 40% MVC. VIB was applied at 55 Hz to the patellar tendon for 15 minutes before and continued throughout the remainder of testing (VIB) or remained off (CON). Results There were significant increases in MU firing rates during VIB in comparison to CON and no differences in EMG amplitude between VIB and CON. Conclusion The VIB-mediated reduction in muscle spindle function altered MU behavior at 40% MVC. Muscle Nerve 55: 500-507, 2017.

Motor unit (MU) firing rates of the vastus lateralis in children and adults were examined. Seven healthy adult males (mean ± SD, age = 21 ± 2.6 yrs) and six healthy male children (mean ± SD age = 8.8 ± 1.7) volunteered. Surface electromyography (EMG) signals were recorded from 20% and 60% maximal voluntary contractions (MVC). Surface EMG signals were decomposed into firing events of individual MUs and slopes and y-intercepts were calculated for the mean firing rate (MFR, pps) at steady torque vs. recruitment thresholds (RT) relationships for each subject. Muscle cross-sectional area (mCSA) was measured, via ultrasonography, with specific torque calculated (MVC/mCSA). Adults possessed greater mCSA (p = .002; children = 11.5 ± 2.1 cm2; adults = 31.80 ± 12.15 cm2) and greater specific torque (p = .018; children = 4.63 ± 1.4 Nm/cm2; adults = 7.1 ± 1.8 Nm/cm2) compared to children. The y-intercepts were significantly (p

Previous research has indicated greater muscle activation is needed for children (CH) to match relative intensity submaximal contractions in comparison with adults (AD). However, no study has compared motor unit (MU) firing and recruitment patterns between children and adults. Therefore, MU action potential amplitudes (MUAPAMP) and firing rates were examined during two repetitive submaximal contractions of the first dorsal interosseous in children and adults. Twenty-two children (age 9.0 ± 0.8 years) and 13 adults (age 22.9 ± 4.8 years) completed three maximum voluntary contractions (MVC) and two repetitive isometric contractions at 30% MVC for 40 s. Surface electromyography (EMG) was recorded and decomposed into action potential trains. MUAPAMPS, recruitment thresholds (RTs), and mean firing rates (MFRs) were calculated, and EMG amplitude was normalized (N-EMG) to MVC. For each subject and repetition, linear MFR vs. RT and exponential MUAPAMP vs. RT and MFR vs. MUAPAMP relationships were calculated. N-EMG (P = 0.001, CH = 56.5 ± 31.7%, AD = 30.3 ± 9.1%), MFRs regardless of RT, according to greater y-intercepts of the MFR vs. RT relationships [P = 0.013, CH = 31.1 ± 5.1 pulses per second (pps), AD = 25.9 ± 4.3 pps] and MFRs of MUs with smaller action potential amplitudes (P = 0.017, CH = 29.4 ± 6.8 pps, AD = 23.5 ± 3.5 pps), were greater for children. MUAPAMPS in relation with RT were similar between groups except the highest threshold MUs (RT = 28% MVC) were greater for the adults (1.02 ± 0.43 mV) than children (0.67 ± 0.24 mV) (P = 0.010). Muscle activation and MU firing rates were greater for children, which likely indicated a greater operating point of MU control in comparison with adults during an isometric contraction performed at a relative submaximal intensity.

AIM To investigate the relationships between motor unit action potential amplitudes (MUAPAMP ), muscle cross-sectional area (mCSA) and composition (mEI), per cent myosin heavy chain (%MHC) areas and sex in the vastus lateralis (VL). METHODS Ten males and 10 females performed a submaximal isometric trapezoid muscle action that included a linearly increasing, steady torque at 40% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic decomposition techniques were utilized to determine MUAPAMPS in relation to recruitment thresholds (RT). Ultrasound images were taken to quantify muscle mCSA and mEI. Muscle biopsies were collected to calculate %MHC areas. Y-intercepts and slopes were calculated for the MUAPAMP vs RT relationships for each subject. Independent-samples t tests and ANOVA models examined sex-related differences in mCSA, mEI, slopes and y-intercepts for the MUAPAMP vs RT relationships and %MHC areas. Correlations were performed among type IIA and total type II %MHC area, mCSA and the slopes and y-intercepts for the MUAPAMP vs RT relationships. RESULTS Males exhibited greater slopes for the MUAPAMP vs RT relationships (P = .003), mCSA (P

This study examined motor unit (MU) amplitudes (APAMPS) and firing rates during moderate-intensity contractions and muscle cross-sectional area (mCSA) and echo intensity (mEI) of the vastus lateralis (VL) in chronically endurance-trained and sedentary females. Eight endurance-trained (ET) and nine sedentary controls (SED) volunteered for this study. Surface electromyographic (EMG) signals from a five-pin electrode array were recorded from the VL during isometric trapezoid muscle actions at 40% of maximal voluntary contraction (MVC). Decomposition methods were applied to the EMG signals to extract the firing events and amplitudes of single MUs. The mean firing rate (MFR) during steady force and MUAPAMP for each MU was regressed against recruitment threshold (RT, expressed as %MVC). The y-intercepts and slopes from the MFR and MUAPAMP vs. RT relationships were calculated. EMG amplitude during steady force was normalized (N-EMGRMS) to peak EMG amplitude recorded during the MVC. Ultrasonography was used to measure mCSA and mEI. Significant differences existed between the ET and SED for the slopes (P = 0.005, P = 0.001) from the MFR and MUAPAMP vs. RT relationships with no differences for the y-intercepts (P > 0.05). N-EMGRMS was significantly (P = 0.033) lower for the ET than SED. There were no differences between groups for mCSA; however, the SED possessed significantly (P = 0.001) greater mEI. Subsequently, the ET likely possessed hypertrophied and stronger MUs that allowed for lower necessary muscle activation to maintain the same relative task as the SED. The larger MUs for the ET is supported via the MFR vs. RT relationships and ultrasound data.

Fry, AC, Housh, TJ, Cramer, JB, Weir, JP, Beck, TW, Schilling, BK, Miller, JD, and Nicoll, JX. Noninvasive assessment of skeletal muscle myosin heavy chain expression in trained and untrained men. J Strength Cond Res 31(9): 2355-2362, 2017-Numerous conditions and types of physical activity (e.g., exercise, aging, and muscle-related diseases) can influence muscle fiber types and the proteins expressed. To date, muscle fibers can only be characterized by actually obtaining a tissue sample using the invasive muscle biopsy procedure. Mechanomyography (MMG) is the assessment of the vibration properties of contracting skeletal muscle and has been proposed as a possible noninvasive method for muscle fiber analysis. Therefore, the purpose of this project was to examine the feasibility of using MMG and muscle performance measures to noninvasively assess muscle fiber characteristics. Fifteen men (5 endurance-trained, 5 weight-trained, and 5 sedentary) provided muscle samples from their vastus lateralis muscle. These samples were analyzed for relative myosin heavy chain (MHC) protein expression, which is highly correlated with % muscle fiber type areas. Additionally, each subject performed several muscle performance tests, and MMG of the quadriceps was assessed during a knee extension exercise. Multiple regression was used to develop prediction equations for determining relative muscle content of MHC types I, IIa, and IIx. A combination of MMG and knee extension performance variables estimated types I, IIa, and IIx MHCs with approximately 80% accuracy. Although preliminary, these data suggest that muscle performance tests in addition to MMG assessments during a simple muscle performance task (knee extension) can be used to estimate muscle fiber type composition in a healthy male population. Such methods could ultimately be used to noninvasively monitor muscle health and fitness.

This study examined the effects of passive stretching only (PS+CON) and passive stretching with the addition of continuous vibration (VIB) during post-passive stretching tests (PS+VIB) on peak torque (PT), percent voluntary inactivation (%VI), single stimulus twitch torque (TTSINGLE), and doublet stimuli twitch torque (TTDOUBLET) of the plantar flexors at a short (20° plantar flexion (PF)) and long muscle length (15° dorsiflexion (DF)). Fourteen healthy men (age = 22 ± 3 years) performed isometric maximal voluntary contractions at PF and DF, and passive range of motion (PROM) assessments before and after 8 × 30-s passive stretches without (PS+CON) or with VIB (PS+VIB) administered continuously throughout post-passive stretching tests. The passive properties of the muscle tendon unit were assessed pre- and post-passive stretching via PROM, passive torque (PASSTQ), and musculotendinous stiffness (MTS) measurements. PT, TTSINGLE, and TTDOUBLET decreased, whereas, %VI increased following passive stretching at PF and DF (P < 0.05) with no significant differences between PS+CON and PS+VIB. PASSTQ and MTS decreased while PROM increased post-passive stretching during both trials (P < 0.05). The stretching-induced force/torque deficit and increases in %VI were evident following passive stretching at short and long muscle lengths. Although not statistically significant, effect size calculations suggested large and moderate differences in the absolute changes in PT (Cohen’s d = 1.14) and %VI (Cohen’s d = 0.54) from pre- to post-passive stretching between treatments, with PS+VIB having greater decreases of PT and higher %VI than PS+CON. The decrement in PT following passive stretching may be primarily neural in origin.

AIM To investigate the change in motor unit (MU) firing rates (FR) at de-recruitment relative to recruitment and the relation to % type I myosin heavy chain isoform content (type I %MHC) of the vastus lateralis (VL) in vivo. METHODS Ten subjects performed a 22-s submaximal isometric trapezoid muscle action that included a linearly increasing, steady force at 50% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic signals were collected from the VL and were decomposed into constituent MU action potentials trains. A tissue sample from the VL was taken to calculate type I %MHC. The y-intercepts and slopes were calculated for the changes (Δ) in FR at de-recruitment (FRDEREC ) relative to FR at recruitment (FRREC ) vs. FRREC relationship for each subject. Correlations were performed between the y-intercepts and slopes with type I %MHC. RESULTS The majority of MUs had greater FRDEREC than FRREC . The y-intercepts (r = -0.600, P = 0.067) were not significantly correlated, but the slopes (r = -0.793, P = 0.006) were significantly correlated with type I %MHC. CONCLUSION The majority of the motoneuron pool had greater FRDEREC than FRREC , however, individuals with higher type I %MHC had a greater propensity to have MUs with FRREC > FRDEREC as indicated by the slope values. Overall, the contractile properties of the muscle (MHC) could partially explain the differences in MU firing rates at de-recruitment relative to recruitment. Thus, suggesting the fatigability of the muscle influences the alterations in MU firing rates from recruitment to de-recruitment.