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The purpose of this study was to examine the acute effects of different volumes of a dynamic stretching routine on vertical jump (VJ) performance, flexibility and muscular endurance (ME). Twenty-six males (age 22.2 ± 1.3 years) performed three separate randomized conditions: (i) a control (CON) condition (5-min jog + 12 min of resting), (ii) a 5-min jog + a dynamic stretching routine (DS1; 6.7 ± 1.3 min) and (iii) a 5-min jog + a dynamic stretching routine with twice the volume (DS2; 12.1 ± 1.6 min). The dynamic stretching routine included 11 exercises targeting the hip and thigh musculature. VJ performance (jump height and velocity) and flexibility were measured prior to and following all conditions, while ME was measured following all conditions. The DS1 and DS2 conditions increased VJ height and velocity (P<0.01), while the CON condition did not change (P>0.05). When compared to the CON condition, the DS1 condition did not improve ME (P>0.05), whereas the DS2 condition resulted in a significant (15.6%) decrease in the number of repetitions completed (P<0.05). Flexibility increased following all conditions (P<0.01), while the DS1 condition was significantly greater (P<0.01) than the CON condition at post-testing. These results suggest that dynamic stretching routines lasting approximately 6-12 min performed following a 5-min jog resulted in similar increases in VJ performance and flexibility. However, longer durations of dynamic stretching routines may impair repetitive high-intensity activities., (© 2014 Scandinavian Society of Clinical Physiology and Nuclear Medicine. Published by John Wiley & Sons Ltd.)

The acute effects of stretching on peak force (Fpeak), percent voluntary activation (%VA), electromyographic (EMG) amplitude, maximum range of motion (MROM), peak passive torque, the passive resistance to stretch, and the percentage of ROM at EMG onset (%EMGonset) were examined in 18 young and 19 old men. Participants performed a MROM assessment and a maximal voluntary contraction of the plantarflexors before and immediately after 20 min of passive stretching. Fpeak (-11 %), %VA (-6 %), and MG EMG amplitude (-9 %) decreased after stretching in the young, but not the old (P > 0.05). Changes in Fpeak were related to reductions in all muscle activation variables (r = 0.56-0.75), but unrelated to changes in the passive resistance to stretch (P ≥ 0.24). Both groups experienced increases in MROM and peak passive torque and decreases in the passive resistance to stretch. However, the old men experienced greater changes in MROM (P < 0.001) and passive resistance (P = 0.02-0.06). Changes in MROM were correlated to increases in peak passive torque (r = 0.717), and the old men also experienced a nonsignificant greater (P = 0.08) increase in peak passive torque. %EMGonset did not change from pre- to post-stretching for both groups (P = 0.213), but occurred earlier in the old (P = 0.06). The stretching-induced impairments in strength and activation in the young but not the old men may suggest that the neural impairments following stretching are gamma-loop-mediated. In addition, the augmented changes in MROM and passive torque and the lack of change in %EMGonset for the old men may be a result of age-related changes in muscle-tendon behavior.

The reliability of the interpolated twitch technique during submaximal and maximal isometric muscle actions. J Strength Cond Res 27(10): 2909-2913, 2013-The purpose of this study was to examine the test-retest reliability of the percent voluntary activation (%VA) vs. force relationships. Fourteen healthy men (mean ± SD age = 21 ± 2.6 years) and 8 women (age = 21 ± 1.8 years) completed 4 maximal voluntary contractions (MVCs) and 9 randomly ordered submaximal isometric plantar flexions from 10 to 100% of the MVC. Transcutaneous electrical stimuli were delivered to the tibial nerve using a high-voltage constant-current stimulator (DS7AH; Digitimer, Herthfordshire, United Kingdom). The %VA was calculated for each maximal and submaximal MVC. Paired-samples t-tests were used to quantify systematic variability, whereas the intraclass correlation coefficients (ICCs), standard error of the mean (%SEM), and minimum differences (%MD; expressed as a percentage of the means) were used for test-retest reliability. Systematic variability was not present at any of the contraction intensities (p > 0.05). The ICCs ranged from 0.52 to 0.84, whereas the %SEM ranged from 6.75 to 38.45%, and the %MD ranged from 18.71 to 106.58%. The ICCs were ≥0.74 at contraction intensities ranging from 40 to 100% MVC (6.75-16.78% SEM), whereas the ICCs were ≤0.65 (20.95-38.45% SEM) for the contraction intensities ≤30% MVC. Although not statistically tested, the ICCs tended to be higher, whereas the %SEMs lower for contractions ≥40% MVC. Future research using %VA during submaximal contraction intensities to predict a true maximal force may want to exclude contraction intensities <40% MVC. In addition, caution is warranted when interpreting the changes in the %VA during MVCs after an experimental intervention.

We compared starters and non-starters for various isokinetic strength variables in elite women's soccer players. A convenience sample of 10 starters (mean ± s; age = 20 ± 2 years; height = 170 ± 4 cm; body mass = 65 ± 5 kg) and 7 non-starters (age = 20 ± 1 years; height = 164 ± 3 cm; body mass = 63 ± 4 kg) performed maximal voluntary muscle actions of the leg extensors (concentric) and flexors (eccentric) on an isokinetic dynamometer in order to measure concentric peak torque for the leg extensors, eccentric peak torque for the leg flexors, and the functional hamstrings:quadriceps (H:Q) ratio at 1.047 rad · s(-1) and 4.189 rad · s(-1) concentric peak torque for the leg extensors was not different between starters and non-starters. However, it was greater at 1.047 rad · s(-1) than at 4.189 rad · s(-1) in both groups. Eccentric peak torque for the leg flexors was greater for the starters versus non-starters at 4.189 rad · s(-1). Eccentric strength of the leg flexors at fast movement velocities may be used as an effective physiological profile and may discriminate between playing status in elite women's soccer players.

The purpose of this study was to investigate the consistency of commonly reported rapid force characteristics utilizing both automated and manual muscle contraction onset detection methods. Twenty-four healthy volunteers performed isometric strength testing of the plantar flexor muscle group on two nonconsecutive days. Test-retest reliability was evaluated using intraclass correlation coefficients (ICCs), standard errors of measurement (SEM), and the SEM as a percentage of the mean (SEM%) for rate of force development (RFD), relative RFD, contractile impulse, and absolute force-time values at various epoch durations using automated and manual onset detection methods. For all rapid force variables, ICC and SEM% values ranged from 0.52 to 0.96 and 7.56% to 37.56%, respectively. For the majority of these variables (20 of 23), the automated onset detection method resulted in higher ICC and lower SEM% values compared to the manual onset detection method. Regardless of onset detection methodology, the consistency of relative RFD values declined following 50% of MVC. Collectively, these findings indicated that commonly evaluated rapid muscle force variables demonstrated acceptable relative and absolute consistency values. However, these values were generally superior for the automated onset detection methodology. Additionally, the consistency of relative RFD values declines following 50% MVC and therefore should be evaluated with caution., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The purpose of this study was to examine the sensitivity and peak force prediction capability of the interpolated twitch technique (ITT) performed during submaximal and maximal voluntary contractions (MVCs) in subjects with the ability to maximally activate their plantar flexors. Twelve subjects performed two MVCs and nine submaximal contractions with the ITT method to calculate percent voluntary inactivation (%VI). Additionally, two MVCs were performed without the ITT. Polynomial models (linear, quadratic and cubic) were applied to the 10-90% VI and 40-90% VI versus force relationships to predict force. Peak force from the ITT MVC was 6.7% less than peak force from the MVC without the ITT. Fifty-eight percent of the 10-90% VI versus force relationships were best fit with nonlinear models; however, all 40-90% VI versus force relationships were best fit with linear models. Regardless of the polynomial model or the contraction intensities used to predict force, all models underestimated the actual force from 22% to 28%. There was low sensitivity of the ITT method at high contraction intensities and the predicted force from polynomial models significantly underestimated the actual force. Caution is warranted when interpreting the % VI at high contraction intensities and predicted peak force from submaximal contractions.

Avoiding the innervation zone (IZ) is important when collecting surface electromyographic data. The purposes of this study were threefold: (1) to examine the precision of two different techniques for expressing IZ location for the biceps brachii, (2) to compare these locations between men and women, and (3) to determine if IZ movement with changes in elbow joint angle is related to different anthropometric measures. Twenty-four subjects (mean+/-SD ages=21.8+/-3.5yr) performed isometric contractions of the right forearm flexors at each of three separate elbow joint angles (90 degrees, 120 degrees, and 150 degrees between the arm and forearm). During each contraction, the location of the IZ for the biceps brachii was visually identified using a linear electrode array. These IZ locations were expressed in both absolute (i.e. as a distance (mm) from the acromion process) and relative (i.e. as a percentage of humerus length) terms. The results suggested that the estimations of IZ location were more precise when expressed in relative versus absolute terms, and were generally different for men and women. The shift in IZ location with changes in elbow joint angle was not, however, related to height, weight, or humerus length.

The purposes of the present study were to (1) characterize viscoelastic creep in vivo in the human skeletal muscle-tendon unit and (2) to examine the consistency of these responses during a single 30-s stretch. Twelve volunteers (mean +/- SD = 22 +/- 3 years; height = 169 +/- 11 cm; mass = 70 +/- 17 kg) participated in two separate experimental trials. Each trial consisted of a 30-s constant-torque stretch of the plantar flexor muscles. Position (degrees) values were quantified at every 5-s period (0, 5, 10, 15, 20, 25, and 30 s) and the percent change in position was quantified for each 5-s epoch (0-5, 5-10, 10-15, 15-20, 20-25, and 25-30 s) relative to the total increase in the range of motion. In addition, the intraclass correlation coefficient (ICC) and standard errors of the measurement (SEM) were calculated for test-retest reliability. These results indicated that position increased over the entire 30-s stretch (P < 0.05), while the majority of the increases in position (73-85%) occurred during the first 15-20 s. ICC values were >or = 0.994 and SEM values (expressed as percentage of the mean) were

The purpose of this study was to use a wavelet-based signal processing technique to examine the influence of electrode placement over the innervation zone (IZ) on the shape of the electromyographic (EMG) frequency spectrum. Ten healthy males (mean+/-SD age=23.6+/-3.0 years) performed isometric muscle actions of the dominant leg extensors at 10%, 40%, 70%, and 100% of the maximum voluntary contraction (MVC). Surface EMG signals were detected simultaneously from the vastus lateralis with two bipolar electrode arrangements. One of the electrode arrangements had its center point located directly over the IZ, while the other arrangement had its center point distal to the IZ (i.e., 20 mm away). All EMG signals were processed with a wavelet-based procedure. The results showed that for all isometric torque levels, the EMG signals from the distal electrode arrangement demonstrated greater total intensity values than those for the IZ arrangement for frequencies ranging from approximately 2 to 110 Hz. There were no consistent differences, however, between the IZ and distal electrode arrangements for total EMG intensity values above 110 Hz. Thus, these findings indicated that electrode placement over the IZ affected primarily the low-, rather than the high-frequency portion of the EMG frequency spectrum.

This study examined the test-retest reliability of the slopes (b) and y-intercepts (a) of the absolute and log-transformed regression models applied to the mechanomyographic amplitude (MMG(RMS)) versus torque (TQ) relationship. Fifteen participants (mean+/-SD age=23+/-4 yrs) performed two isometric maximal voluntary contractions (MVCs) and ten randomly ordered isometric leg extensions from 5% to 95% of their MVC during three separate trials. MMG(RMS) was recorded from the vastus lateralis during each MVC. Intraclass correlation coefficients (ICCs) and standard errors of measurement (SEMs) were calculated for test-retest reliability. ICCs for the b and a terms were 0.89 and 0.90 for the log-transformed and 0.85 and 0.76 for the absolute relationships, respectively. The SEM values (expressed as a percentage of the mean) for the b and a terms were 9.7% and 16.4% for the log-transformed and 18.9% and 57.1% for the absolute relationships, respectively. These results indicated that the b and a terms from both the absolute linear and log-transformed MMG(RMS) versus TQ relationships were relatively reliable (ICCs), however, the SEMs for the log-transformed relationships were lower than the absolute linear models. Furthermore, the b term from the log-transformed relationships may provide unique information regarding the nonlinear characteristics (plateau points) of the MMG(RMS) versus TQ relationship, whereas the a term may indicate upward or downward shifts in MMG(RMS) across the TQ spectrum. Thus, the log-transformed MMG(RMS) versus TQ relationships may offer an attractive alternative method for reliably quantifying and tracking changes in the TQ-related patterns of response for MMG(RMS) on a subject-by-subject basis.

The purpose of this study was to examine the influence of repeated bouts of shiftwork on lower extremity maximal and rapid strength and reaction time in career firefighters. Thirty-five firefighters (3 females; 34.3 ± 9.1 years) performed a psychomotor vigilance test (PVT) and reactive maximal isometric strength assessment prior to and following a full shift rotation (three 24-hr on-off shifts). Reaction time (RT), maximal, absolute and normalised rapid strength (50, 100, 150, 200 ms), and PVT measures were assessed on-site. Separate linear regression models were used to evaluate the POST-PRE change in variables adjusted for BMI, age, sleep, and call duration. Early (50 ms) absolute rapid strength was the only variable significantly reduced (–25.9%; p = 0.031) following the full shift rotation. Our findings indicate that early rapid strength may be a sensitive measure in detecting work-related fatigue, despite minimal changes in sleep between work and non-work nights and a low call duration. Practitioner summary: We examined the impact of repeated shiftwork on changes in reaction time and neuromuscular function. Early rapid strength was a sensitive, portable lab assessment that feasibly measured work-related fatigue in career firefighters. Interventions that mitigate work-related fatigue may be impactful at preventing falls and/or risk of musculoskeletal injury. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine if muscular dimensional changes with increases in torque production are influenced by age- and obesity-related increases in intramuscular fat, and its relationship to percent body fat (%BF), echo intensity (EI), strength, and maximum walking speed. Sixty-six healthy men were categorized into 3 groups based on age and body mass index status (young normal weight [YNW], older normal weight [ONW], and older obese [OB]). Participants underwent %BF assessments, resting ultrasonography to determine muscle size (cross-sectional area [CSA]) and EI of the superficial quadriceps, and a 10-m maximum walking speed assessment. Maximal and submaximal (rest-100% MVC in 10% increments) isometric leg extension strength was assessed while changes in rectus femoris (RF) CSA, width, and depth were obtained with ultrasonography. Echo intensity and %BF were different among all groups (p ≤ .007), with the YNW and OB groups exhibiting the lowest and highest %BF and EI values, respectively. The RF increased in depth and decreased in width with increases in torque intensity for all groups. The ONW group demonstrated no change (-0.08%) in RF CSA across torque intensities, whereas the YNW group (-11.5%) showed the greatest decrease in CSA, and the OB group showed a more subtle decrease (-4.6%). Among older men, a greater change in RF CSA was related to poorer EI (r = -0.355) and higher %BF (r = -0.346), while a greater decrease in RF width was associated with faster walking speeds (r = -0.431). Examining muscular dimensional changes during contraction is a unique model to investigate the influence of muscle composition on functional performance. [ABSTRACT FROM AUTHOR]

This study examined the acute effects of passive stretching on electromechanical delay (EMD), peak twitch force (PTF), rate of force development (RFD), and peak-to-peak M-wave (PPM) for the soleus muscle during evoked isometric plantar flexion muscle actions. Fourteen men (mean age ± SD = 21.2 ± 2.4 years; body mass = 80.0 ± 14.9 kg; height = 176.9 ± 7.2 cm) and 20 women (20.9 ± 2.5 years; 61.3 ± 8.9 kg; 165.3 ± 7.5 cm) volunteered for the study. Five single-square, supramaximal transcutaneous electrical stimuli (each separated by 5 s) were delivered to the tibial nerve before and after passive stretching. A time × gender interaction was observed for EMD, and the post hoc dependent-samples t tests indicated that EMD increased 4% for the women (p = .023), but not for the men (p = .191). There were no other stretching-related changes for PTF, RFD, or p-p M-wave for either the men or women (p > .05). These findings tentatively suggested that mechanical factors related to the stiffness of the muscle-tendon unit may contribute to the explanation for why stretching caused an acute increase in the EMD during evoked twitches in the women, but not in the men. [ABSTRACT FROM AUTHOR]

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Neuromuscular function in young overweight/obese (OF) children is not well described. Aim: This study examined isometric and isokinetic leg extensor strength, muscle size (mCSA) and tissue composition as measured via echo intensity (mEI), and motor unit (MU) firing rates in normal weight (NW) and OF children aged 7‐10 years. Methods: Fourteen NW (eight girls and six boys, BMI: 15.8 ± 1.4 kg/m2) and 15 OF (10 girls and five boys, BMI: 20.8 ± 2.3 kg/m2) children volunteered to perform this study. Percentage body fat (%BF) was measured, and mCSA, mEI, and subcutaneous fat (sFAT) of the vastus lateralis (VL) was assessed. MU mean firing rates (MFRs) in relation to recruitment threshold (RT) of the VL were assessed during submaximal isometric contractions. Maximal isokinetic contractions were performed at 1.05 and 4.20 rad/s. The 95% confidence intervals (CI) from the statistical tests are presented. Results: The OF children had greater %BF (95% CI = −15.1 to −7.2), mCSA (95% CI = −4.1 to −1.2), mEI (95% CI = −22.3 to −5.9), and sFAT (95% CI = −0.5 to −0.1), greater rate of strength loss with increasing isokinetic velocities (95% CI = 0.4 to 5.4), and a smaller range of MFRs (95% CI = 0.007 to 7.136) at the steady torque than the NW children. Conclusions: The OF children had poorer muscle tissue composition, greater velocity‐related impairments in muscle strength, and a smaller range of MFRs at the targeted torque that may suggest altered MU recruitment strategies. Interventions in OF children should include exercises and recruit higher‐threshold MUs, such as high‐intensity resistance exercises. [ABSTRACT FROM AUTHOR]

Gerstner, GR, Giuliani, HK, Mota, JA, and Ryan, ED. Influence of muscle quality on the differences in strength from slow to fast velocities in career firefighters. J Strength Cond Res 32(10): 2982–2986, 2018—The purpose of this study was to examine the relationship between ultrasound-derived muscle quality and the %decrease in peak torque (PT) from slow to fast velocities in career firefighters. Thirty-nine male career firefighters (mean ± SD : age = 36.90 ± 6.87 years; stature = 180.93 ± 6.98 cm; mass = 109.30 ± 20.57 kg; and body mass index [BMI] = 33.24 ± 4.95 kg·m−2) performed 3 maximal concentric isokinetic contractions at a slow (1.05 rad·s−1) and fast (4.19 rad·s−1) velocity in random order. The velocity-related decrease in PT was calculated as the %decrease from 1.05 to 4.19 rad·s−1. Ultrasonography was used to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality before strength testing. The results indicated that there was a significant relationship between EI and %decrease in PT while adjusting for age (r partial = 0.331, p = 0.042), and age and BMI (r partial = 0.325, p = 0.050). These findings suggest that velocity-impaired isokinetic strength may be influenced by alterations in skeletal muscle tissue composition. Future studies are needed to determine whether the %decrease in PT is related to performance during critical firefighter tasks. [ABSTRACT FROM AUTHOR]

BACKGROUND: Declines in the ability to rapidly generate quadriceps muscle torque may underlie disability in individuals with tibiofemoral osteoarthritis. OBJECTIVE: To determine whether quadriceps rate of torque development (RTD) predicts self-reported disability and physical performance outcomes in individuals with tibiofemoral osteoarthritis. METHODS: This controlled laboratory, cross-sectional study assessed quadriceps strength and RTD in 76 individuals (55% female; mean ± SD age, 61.83 ± 7.11 years) with symptomatic and radiographic tibiofemoral osteoarthritis. Early (0-50 milliseconds), late (100-200 milliseconds), and overall peak RTDs were quantified in the symptomatic (involved) and contralateral limbs and used to calculate bilateral average values. Disability was assessed using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function subscale and 3 physical performance tests, including the (1) 20-m fast-paced walk, (2) 30-second chair stand, and (3) timed stair climb. Separate univariate regression models were used to determine the unique associations among measures of quadriceps RTD, WOMAC function score, and physical performance outcomes after accounting for quadriceps strength (change in R²). U URESULTS: Greater involved-side late RTD and greater bilateral average early RTD were associated with faster walking (change in R² = 0.05, P = .013 and change in R² = 0.05, P = .043, respectively). Greater bilateral average late RTD was associated with faster walking (change in R² = 0.20, P<.001) and faster stair climb (change in R² = 0.11, P = .001). No quadriceps RTD variable was significantly associated with WOMAC function score (change in R² range, <0.01-0.017). CONCLUSION: Involved-limb quadriceps RTD was weakly associated with physical performance outcomes, but not self-reported disability, in individuals with tibiofemoral osteoarthritis. Bilateral average quadriceps RTD was moderately associated with walking speed. LEVEL OF EVIDENCE: Prognosis, level 2b. [ABSTRACT FROM AUTHOR]

The purpose of this study was to compare the effect of whole body vibration (WBV) and local muscle vibration (LMV) on early and late quadriceps rate of torque development (RTD), and electromechanical delay (EMD) in individuals with anterior cruciate ligament reconstruction (ACLR). 20 individuals with ACLR were recruited for this study. Participants performed isometric squats while being exposed to WBV, LMV, or no vibration (control) in a randomized order during separate visits. Quadriceps early and late quadriceps RTD, and EMD were assessed during a maximal voluntary isometric knee extension prior to and immediately following WBV, LMV, or control. There was a significant condition by time interaction for early RTD (p=0.045), but not late RTD (p=0.11), or EMD of the vastus medialis (p=0.15), vastus lateralis (p=0.17), or rectus femoris (p=0.39). Post hoc analyses indicated a significant increase in early RTD following WBV (+5.59 N⋅m⋅-1⋅kg-1⋅kg-1; 95% confidence interval, 1.47 - 12.72, p=0.007). No differences were observed in the LMV or control conditions, and no difference was observed between conditions at posttest. The ability to rapidly produce knee extension torque is essential to physical function. WBV may be appropriate to aid in the restoration of RTD following ACLR. [ABSTRACT FROM AUTHOR]

Context: Individuals with anterior cruciate ligament reconstruction (ACLR) have quadriceps dysfunction that contributes to physical disability and posttraumatic knee osteoarthritis. Quadriceps function in the ACLR limb is commonly evaluated relative to the contralateral uninjured limb. Bilateral quadriceps dysfunction is common in individuals with ACLR, potentially biasing these evaluations. Objective: To compare quadriceps function between individuals with ACLR and uninjured control participants. Design: Cross-sectional study. Setting: Research laboratory. Patients or Other Participants: Twenty individuals with unilateral ACLR (age = 21.1 ± 1.7 years, mass = ±8.3 ± 14.9 kg, time since ACLR = 50.7 ± 21.3 months; females = 14; Tegner Score = 7.1 ± 0.3; 1± patellar tendon autografts, 3 hamstrings autografts, 1 allograft) matched to 20 control participants (age = 21.2 ± 1.2 years, mass = ±7.9 ± 11.3 kg; females = 14; Tegner Score = 7.1 ± 0.4) on age, sex, body mass index, and Tegner Activity Scale. Main Outcome Measure(s): Maximal voluntary isometric knee extension was performed on an isokinetic dynamometer. Peak torque (PT), rate of torque development (RTD), electromyographic (EMG) amplitude, central activation ratio (CAR), and hamstrings EMG amplitude were assessed during maximal voluntary isometric knee extension and compared between groups using independent-samples t tests. Relationships between hamstrings co-activation and quadriceps function were assessed using Pearson correlations. Results: Participants with anterior cruciate ligament reconstruction displayed lesser quadriceps PT (1.8± ± 0.74 versus 2.5± ± 0.37 Nm/kg, P=.001), RTD (39.4 ± 18.7 versus 52.9 ± 1±.4 Nm/s/kg, P = .03), EMG amplitude (0.25 ± 0.12 versus 0.37 ± 0.2± mV, P = .04), and CAR (83.3% ± 11.1% versus 93.7% ± 3.2%, P = .002) and greater hamstrings co-activation (27.2% ± 12.8% versus 14.3% ± 3.7%, P , .001) compared with control participants. Correlations were found between hamstrings co-activation and PT (r =-0.39, P = .007), RTD (r =-0.30, P = .03), and EMG amplitude (r =-0.30, P = .03). Conclusions: Individuals with ACLR possessed deficits in PT, RTD, and CAR compared with control participants. Peak torque is the net result of all agonist and antagonist activity, and lesser PT in individuals with ACLR is partially attributable to greater hamstrings co-activation. [ABSTRACT FROM AUTHOR]

Purpose Vibratory stimuli enhance muscle activity and may be used for rehabilitation and performance enhancement. Efficacy of vibration varies with the frequency of stimulation, but the optimal frequency is unclear. The purpose of this study was to examine the effects of 30 Hz and 60 Hz local muscle vibration (LMV) on quadriceps function. Methods Twenty healthy volunteers (age = 20.4 ± 1.4 years, mass = 68.1 ± 11.0 kg, height = 170.1 ± 8.8 cm, males = 9) participated. Isometric knee extensor peak torque (PT), rate of torque development (RTD), and electromyography (EMG) of the quadriceps were assessed followed by one of the three LMV treatments (30 Hz, 60 Hz, control) applied under voluntary contraction, and again immediately, 5, 15, and 30 min post-treatment in three counterbalanced sessions. Dependent variables were analyzed using condition by time repeated-measures ANOVA. Results The condition × time interaction was significant for EMG amplitude ( p = 0.001), but not for PT ( p = 0.324) or RTD ( p = 0.425). The increase in EMG amplitude following 30 Hz LMV was significantly greater than 60 Hz LMV and control. Conclusions These findings suggest that 30 Hz LMV may elicit an improvement in quadriceps activation and could be used to treat quadriceps dysfunction resulting from knee pathologies. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed to examine the effects of intermittent isometric fatigue on maximal voluntary contraction (MVC) strength, percent voluntary activation (%VA), peak twitch force (PTF), peak rate of force development (PRFD), half relaxation time (HRT). and maximal compound action potential (M-wave) amplitude of the soleus and medial gastrocnemius muscles before and after creatine (Cr) loading. Methods: Using a double-blinded, placebo-controlled, randomized design, 12 women were assigned to a Cr (n = 6; mean age ± SD = 23.3 ± 3.0 yr) or placebo (PL; n = 6; mean age ± SD = 21.3 ± 1.6 yr) group. Participants supplemented four times daily for 5 d with 5 g of Cr+ 10 g of fructose or 10 g of fructose. At baseline and after testing, an isometric MVC and the twitch interpolation procedure were used before and after a 4-min isometric fatigue protocol of the plantarflexor muscles, which consisted of six intermittent duty cycles per minute (7-s contraction, 3-s relaxation) at 70% MVC. Results: There were no interactions between the Cr and PL groups (P > 0.05) for any dependent variable. The fatigue protocol reduced voluntary strength (-17.8%, P < 0.001) and %VA (-3.7%, P = 0.005). Baseline PTF (P < 0.005) and PRFD (P < 0.001) values were less than those of all respective time points, but PTF value decreased from 3 min to 4 min and after testing (P < 0.005). HRT increased from baseline to minutes 1 and 2 and then returned to baseline at minutes 3 and 4 and after testing. The M-wave did not change (/" > 0.05). Conclusions: Five days of Cr loading did not influence isometric force, %VA, evoked twitch properties, or the central and peripheral aspects of fatigue measured in this study. [ABSTRACT FROM AUTHOR]

The article reports on research which investigated the influence of age on the viscoelastic stretch response. Researchers evaluated 22 younger and 14 older male subjects. They found that the younger men displayed a greater initial relative stress relaxation response that diminished across repeated stretches, whereas the older men experienced a smaller relative response that remained constant.

The article cites a research study which examined the relationships between absolute and body mass-normalized rapid isometric torque variables and vertical jump (VJ) performance of the leg extensors and flexors in elite National Collegiate Athletic Association Division I Football Bowl Subdivision collegiate American football players. It is concluded that strength and conditioning professionals may use these findings to identify and monitor dynamic sport performance.

The article discusses a study on the use of maximal and rapid isometric torque measurements to distinguish between starter and nonstarter level U.S. Division I college football players. Several torque characteristics were evaluated in contractions of leg flexor and extensor muscles, including peak torque, rate of torque development, and contractile impulse. The results indicate that early rapid leg flexion torque strength could be an indicator of playing level or used in training programs.

The purpose of this study was to investigate the acute effects of passive stretching on the electromechanical delay (EMD), peak twitch force (PTF), rate of force development (RFD), and compound muscle action potential (M-wave) amplitude during evoked twitches of the plantar flexor muscles. 16 men (mean age +/- SD = 21.1 +/- 1.7 years; body mass = 75.9 +/- 11.4 kg; height = 176.5 +/- 8.6 cm) participated in this study. A single, square-wave, supramaximal transcutaneous electrical stimulus was delivered to the tibial nerve before and after passive stretching. The stretching protocol consisted of nine repetitions of passive assisted stretching designed to stretch the calf muscles. Each repetition was held for 135 s separated by 5-10 s of rest. Dependent-samples t tests (pre- vs. post-stretching) were used to analyze the EMD, PTF, RFD, and M-wave amplitude data. There were significant changes (P < or = 0.05) from pre- to post-stretching for EMD (mean +/- SE = 4.84 +/- 0.31 and 6.22 +/- 0.34 ms), PTF (17.2 +/- 1.3 and 15.6 +/- 1.5), and RFD (320.5 +/- 24.5 and 279.8 +/- 28.2), however, the M-wave amplitude did not change (P > 0.05). These findings suggested that passively stretching the calf muscles affected the mechanical aspects of force production from the onset of the electrically evoked twitch to the peak twitch force. These results may help to explain the mechanisms underlying the stretching-induced force deficit that have been reported as either "mechanical" or "electrical" in origin. [ABSTRACT FROM AUTHOR]

The purposes of this study were to (1) derive and test allometric scaling models of biceps isometric strength using body mass (BM) and muscle cross-sectional area (CSA) as the scaling variables, (2) assess the influence of body mass index (BMI) by separating the cohort by BMI (normal <25 kg/m(2) vs. overweight/obese > or =25 kg/m(2)) and repeating step 1, and (3) assess the effect of BMI on isometric strength allometrically adjusted for differences in CSA by comparing scaled strength between normal weight versus overweight/obese women. The participants were 183 women (18-39 years old) who reported no strength training in the prior year. Isometric strength and CSA of the biceps were assessed on the non-dominant arm. The CSA allometric model met all statistical criteria and produced a scaling exponent of 0.44. The BM model did not meet these criteria until the entire cohort was separated by BMI. The scaling exponents for normal weight and overweight/obese women were 1.48 and 0.35, respectively. These data suggest that BMI exerted an influence on the relationship between BM and allometrically scaled isometric strength and may be explained by previous studies demonstrating greater contribution of fat mass (FM) versus fat-free mass (FFM) to BMI in overweight/obese women. As such, allometric scaling models of isometric strength, especially in populations that are heterogeneous with regard to body composition, must be carefully tested and examined across the range of BMI. Isometric strength relative to CSA was not significantly different between groups. However, allometrically scaled strength, using CSA as the criterion variable, was significantly greater in overweight/obese women compared to those of normal weight. These data suggest that isometric strength in women is not completely determined by CSA and other factors such as intramuscular fat and muscle fiber type may be confounding or contributing factors. [ABSTRACT FROM AUTHOR]

Abstract: The purposes of this study were to examine the mechanomyographic (MMG) and electromyographic (EMG) time and frequency domain responses of the vastus lateralis (VL) and rectus femoris (RF) muscles during isometric ramp contractions and compare the time–frequency of the MMG and EMG signals generated by the short-time Fourier transform (STFT) and continuous wavelet transform (CWT). Nineteen healthy subjects (mean±SD age=24±4 years) performed two isometric maximal voluntary contractions (MVCs) before and after completing 2–3, 6-s isometric ramp contractions from 5% to 100% MVC with the right leg extensors. MMG and surface EMG signals were recorded from the VL and RF muscles. Time domains were represented as root mean squared amplitude values, and time–frequency representations were generated using the STFT and CWT. Polynomial regression analyses indicated cubic increases in MMG amplitude, MMG frequency, and EMG frequency, whereas EMG amplitude increased quadratically. From 5% to 24–28% MVC, MMG amplitude remained stable while MMG frequency increased. From 24–28% to 76–78% MVC, MMG amplitude increased rapidly while MMG frequency plateaued. From 76–78% to 100% MVC, MMG amplitude plateaued (VL) or decreased (RF) while MMG frequency increased. EMG amplitude increased while EMG frequency changed only marginally across the force spectrum with no clear deflection points. Overall, these findings suggested that MMG may offer more unique information regarding the interactions between motor unit recruitment and firing rate that control muscle force production during ramp contractions than traditional surface EMG. In addition, although the STFT frequency patterns were more pronounced than the CWT, both algorithms produced similar time–frequency representations for tracking changes in MMG or EMG frequency. [Copyright &y& Elsevier]

Background The purpose of this study was to determine associations between self-reported function (International Knee Documentation Committee Index), isometric quadriceps strength and rate of torque development in individuals with a unilateral anterior cruciate ligament reconstruction. Methods Forty-one individuals [31% male, BMI mean 25 (SD 4) kg/m 2 , months post anterior cruciate ligament reconstruction mean 49 (SD 40)] completed the self-reported function and isometric quadriceps function testing. Rate of torque development was assessed at 0–100 ms (early), 100–200 ms (late) ms, and peak following the onset of contraction. Associations were examined between rate of torque development, strength, and self-reported function. Linear regression was used to determine the unique amount of variance explained by the combination of rate of torque development and strength. Findings Higher rate of torque development 100–200 ms is weakly associated with higher self-reported function in individuals with a unilateral anterior cruciate ligament reconstruction (r = 0.274, p = 0.091); however, rate of torque development 100–200 ms does not predict a significant amount of variance in self-reported function after accounting for strength (ΔR 2 = 0.003, P = 0.721). Interpretation Quadriceps strength has a greater influence on self-reported function compared to rate of torque development in individuals with an anterior cruciate ligament reconstruction with time from surgery. [ABSTRACT FROM AUTHOR]

To examine musculotendinous stiffness (MTS) and ankle joint range of motion (ROM) in men and women following an acute bout of passive stretching. Eight men (mean ± SD age = 20.3 ± 2.0 yrs; body mass = 78.8 ± 12.3 kg; height = 176.6 ± 5.8 cm) and nine women (age = 21.0 ± 2.4 yrs; body mass = 61.7 ± 7.4 kg; height = 166.3 ± 7.5 cm) volunteered for this study. To avoid any menstrual cycle differences, the women were all tested during menses. Each subjects foot was stabilized in a custom-built apparatus designed to measure plantar flexion force with a knee joint angle of 180° and ankle joint angle of 90°. The apparatus was also attached to a powered isokinetic dynamometer that stretched the plantar flexor muscles by passively dorsiflexing the foot at 5°·s-1 until a constant-torque threshold was achieved and held at a point of discomfort as acknowledged by the subject. Nine repetitions of each stretch were held for 135 s with 10 s of rest between repetitions. Before and after the stretching, a maximal, passive flexibility assessment was performed, in which subjects were asked to relax while their foot was maximally dorsiflexed at 5°·s-1. Passive torque and position were recorded from the isokinetic dynamometer. To calculate MTS, the ankle joint angle (°) and torque (Nm) signals were sampled at 1 KHz during the flexibility assessments and plotted as torque-angle curves (i.e., stress-strain curves). Each subject's curve was fit with a 4th-order polynomial regression model (Nordez et al. 2006, Clin Biomech, 21:755-760), and MTS was calculated as the slope (Nm°-1) of the tangent to the curve at a 90° joint angle. MTS decreased by 15.4% (p < 0.001) in men and 36.7% (p < 0.001) in women from pre- to post-stretching. Passive range of motion (PROM) increased by 3.1% (p = 0.018) in men and 7.3% (p = 0.001) in women from pre- to post-stretching. No differences (p > 0.05) were observed between men and women for PROM and MTS prior to and post-stretching. However, the percent change scores from pre- to post-stretching for MTS (p = 0.021) and PROM (p = 0.036) were greater for the women than the men. These findings suggested that 20 min of constant-torque passive stretching increased the PROM and decreased the MTS of the plantar flexors in both men and women, although the magnitude of the change was greater for the women. The greater decrease in MTS and increase in PROM for women may have been due to gender-specific differences in viscoelastic creep during the constant-torque stretching procedures. It has been suggested that a decrease in MTS reduces the total amount of strain through a given ROM, which may reduce the risk of strain injuries. Therefore, the findings of this study suggest that men may have to stretch for a longer duration or at a greater intensity to achieve similar increases in ROM and decrease in MTS as women.Figure 1. No Caption Available. [ABSTRACT FROM AUTHOR]

The purpose of this study was to examine the mechanomyographic (MMGRMS) and electromyographic (EMGRMS) amplitude vs. force relationships of the vastus lateralis (VL) for aerobically-trained (AT), resistance-trained (RT), and sedentary individuals (SED). Five RT (mean ± SD age = 23 ± 3 yrs; body mass = 101 ± 37 kg; height = 176 ± 8 cm), 5 AT (32 ± 5 yrs; 67 ± 4 kg; 176 ± 2 cm), and 5 SED (23 ± 4 yrs; 93 ± 32 kg; 180 ± 5 cm) men volunteered to perform two 6-s isometric ramp muscle actions from 5% to 100% of their maximal voluntary contraction (MVC) while MMG (m·s-2) and EMG (μV) signals were recorded from the VL muscle. Thigh skinfold measurements and Bergstrom muscle biopsies were taken from the VL. The muscle samples were analyzed for myosin heavy chain (MHC) isoform content. Simple linear regression models were fit to the natural log-transformed EMGRMS and MMGRMS vs. force relationships. The slope (b term) and the antilog of the Y-intercept (a term) were calculated for each relationship. For %MHC, the AT group had a greater percentage of type I fibers than the RT and SED groups (p ≤0.05). The RT group had a greater percentage of type IIa fibers than the AT group (p ≤0.05), and the SED group had a greater percentage of type IIx fibers than the AT and RT groups (p ≤0.05). For both the log-transformed EMGRMS and MMGRMS vs. force relationships, the mean a term for the AT group was higher than the RT and SED groups (p ≤0.05). For the log-transformed MMGRMS vs. force relationships, however, the b term for the AT group was lower than the RT and SED groups (p ≤0.05). There were no differences (p >0.05) among the b terms of the AT, RT, and SED groups for the log-transformed EMGRMS vs. force relationships. In addition, the mean skinfold for the SED group was higher than the AT group (p ≤0.05) (AT = 8.7 ± 2.2mm, RT = 15.4 ± 7.6mm, and SED = 25.4 ± 9.2mm). The group-related differences among the a terms for both the log-transformed EMGRMS and MMGRMS patterns might be best explained by the group differences in skinfold thickness. However, the fact that the AT group had a lower mean b term for the log-transformed MMGRMS vs. force relationships may suggest that the differences in motor control strategies between individuals with predominantly type I vs. type II fibers in the vastus lateralis could be detectible with the MMG signal, but not EMG. The lower b term for the AT group suggested that the MMG-force relationship may have been sensitive to the earlier achievement of rate coding, whereas the RT and SED groups relied more on motor unit recruitment to reach higher force levels. The b terms from the EMGRMS vs. force relationships were unable to differentiate among the groups. The log-transformed MMGRMS, but not EMGRMS, vs. force relationships may be useful as non-invasive measurements to determine the onset of rate coding, which may differ among muscles that are predominantly type I vs. type II fiber types. Therefore, the MMG signal may offer an attractive, simple technique for examining changes in the motor control strategies that govern incremental isometric force production. [ABSTRACT FROM AUTHOR]

This study examined the effects of electromyographic (EMG) recording methods and innervation zone (IZ) on the mean power frequency (MPF)–torque relationships. Nine subjects performed isometric ramp muscle actions of the leg extensors from 5% to 100% of maximal voluntary contraction with an eight channel linear electrode array over the IZ of the vastus lateralis. The slopes were calculated from the log-transformed monopolar and bipolar EMG MPF–torque relationships for each channel and subject and 95% confidence intervals (CI) were constructed around the slopes for each relationship and the composite of the slopes. Twenty-two to 55% of the subjects exhibited 95% CIs that did not include a slope of zero for the monopolar EMG MPF–torque relationships while 25–75% of the subjects exhibited 95% CIs that did not include a slope of zero for the bipolar EMG MPF–torque relationships. The composite of the slopes from the EMG MPF–torque relationships were not significantly different from zero for any method or channel, however, the method and IZ location slightly influenced the number of significant slopes on a subject-by-subject basis. The log-transform model indicated that EMG MPF–torque patterns were nonlinear regardless of recording method or distance from the IZ. [ABSTRACT FROM AUTHOR]

Objective: To determine, with more precision than in previous investigations, if the innervation zone (IZ) for the biceps brachii can move with increases in isometric torque, and if so, whether or not that movement is influenced by differences in joint angle. Methods: Twenty-three participants (mean age=21.7 years) performed isometric contractions of the forearm flexors at 20, 40, 60, 80, and 100% of their maximal voluntary contraction (MVC) at three separate elbow joint angles (90, 120, and 150°). During each contraction, 16 channels of bipolar surface electromyographic (EMG) signals were recorded from the biceps brachii using a linear electrode array. For each joint angle, movement of the IZ with increases in torque was identified by a change in the EMG channel that was over the IZ. Results: For each joint angle, the IZ shifted proximally (2.3 channels) with increases in torque from 20 to 100% MVC. With an interelectrode distance of 2.5mm, this 2.3-channel shift reflects a 4.5–7.0mm movement of the IZ. Conclusion: The IZ shifts proximally 4.5–7.0mm with increases in isometric torque, independent of joint angle. Significance: These findings suggested that if isometric contractions are performed at different torque levels, the electrodes should be placed at least 7mm from the IZ. [Copyright &y& Elsevier]

Introduction: The purposes of this study were to investigate the age-related differences in absolute and normalized plantarflexion rate of torque development (RTD) at early (0-50 ms) and late (100-200 ms) time intervals and to examine specific neural and muscular mechanisms contributing to these differences. Methods: Thirty-two young (20.0 ± 2.1 yr) and 20 older (69.5 ± 3.3 yr) recreationally active men performed rapid plantarflexion isometric muscle actions to examine absolute and normalized RTD and muscle activation using EMG at early and late time intervals. Ultrasonography was used to examine medial gastrocnemius muscle size, echo intensity (EI), and muscle architecture (fascicle length [FL] and pennation angle [PA]). Results: The older men were weaker (23.9%, P < 0.001) and had lower later absolute and normalized RTD (P = 0.001-0.034) variables when compared with the young men. The older men also had higher EI (P < 0.001), smaller PA (P = 0.004), and lower later EMG amplitude values (P = 0.009-0.046). However, there were no differences in early RTD and EMG amplitude values, muscle size, or FL between groups (P = 0.097-0.914). Lower late RTD values were related to higher EI, smaller PA, and lower EMG amplitude values (r = -0.28-0.59, P = 0.001-0.044); however, late RTD values were no longer related to PA after normalizing to peak torque. Conclusions: Age-related alterations in muscle quality (EI), architecture, and muscle activation may influence rapid torque production at late time intervals (≥100 ms) from contraction onset. These findings highlight specific neuromuscular factors that influence the age-related reductions in RTD, which has been shown to significantly influence function and performance in older adults. [ABSTRACT FROM AUTHOR]

Introduction: Quadriceps voluntary activation, assessed via the superimposed burst technique, has been extensively studied in a variety of populations as a measure of quadriceps function. However, a variety of stimulus delivery techniques have been employed, which may influence the level of voluntary activation as calculated via the central activation ratio (CAR). The purpose was to determine the effect of visual feedback, stimulus delivery, and perceived discomfort on maximal voluntary isometric contraction (MVIC) peak torque and the CAR.Methods: Quadriceps CAR was assessed in 14 individuals on two days using three stimulus delivery methods; (1) manual without visual feedback, (2) manual with visual feedback, and (3) automated with visual feedback.Results: MVIC peak torque and the CAR were not different between the automated with visual feedback (MVIC=3.25, SE=0.14Nm/kg; CAR=88.63, SE=1.75%) and manual with visual feedback (MVIC=3.26, SE=0.13Nm/kg, P=0.859; CAR=89.06, SE=1.70%, P=0.39) stimulus delivery methods. MVIC (2.99, SE=0.12Nm/kg) and CAR (85.32, SE=2.10%) were significantly lower using manual without visual feedback compared to manual with visual feedback and automated with visual feedback (CAR P<0.001; MVIC P<0.001). Perceived discomfort was lower in the second session (P<0.05).Conclusion: Utilizing visual feedback ensures participant MVIC, and may provide a more accurate assessment of quadriceps voluntary activation. [ABSTRACT FROM AUTHOR]

Abstract: The present study examined the log-transformed electromyographic amplitude (EMG) versus force relationships for the medial gastrocnemius (MG) and soleus (SOL) in high- and moderate-activated subjects. Twenty-five (age=21±2year; mass=62±12kg) participants performed six submaximal contractions (30–90% maximal voluntary contraction [MVC]) with the interpolated twitch technique (ITT) performed at 90% MVC to calculate percent voluntary activation (% VA). Sixteen participants with>90% VA at 90% MVC were categorized high-activated group; the remaining nine were the moderate-activated group. Linear regression models were fit to the log-transformed EMG–force relationships. The slope (b value) and the antilog of the Y-intercept (a value) were calculated. The b values from the MG EMG–force relationships were higher (P <0.05) for the high-activated group (1.27±0.13) than the moderate-activated group (0.88±0.06). The a values and p–p M-wave amplitude values (collapsed across twitches [superimposed and potentiated]) were larger (P <0.05) for the MG (1.17±0.40 and 8.98±0.46mV) than the SOL (0.24±0.07 and 4.48±0.20mV) when collapsed across groups. The b value from the log-transformed EMG–force relationships is an attractive model to determine if a subject has the ability to achieve high activation of their MG without muscle or nerve stimulation. [Copyright &y& Elsevier]

The article offers information on the study conducted by the authors related to effect of static stretching on muscle stiffness and strength. It states that 17 healthy men without any ongoing musculoskeletal injuries or euromuscular diseases participated in the study. Each visited the laboratory for three times in which the first one was for familiarization trial and the rest experimental trials. To determine peak torque (PT), everyone performed 5-s isometric maximal voluntary contraction (MVC) of the right leg flexor muscles before and after stretching. It mentions that musculotendinous stiffness (MTS) did not change after constant-torque (CT) stretching and suggests that CT stretching may be better than constant muscle length (CA stretching).

Abstract: This study examined the log-transformed mechanomyographic (MMGRMS) and electromyographic (EMGRMS) amplitude vs. force relationships for aerobically-trained (AT), resistance-trained (RT), and sedentary (SED) individuals. Subjects performed isometric ramp contractions from 5% to 90% maximal voluntary contraction. Muscle biopsies were collected and thigh skinfolds, MMG and EMG were recorded from the vastus lateralis muscle. Linear regression models were fit to the log-transformed EMGRMS and MMGRMS vs. force relationships. The slope (b coefficient) and the antilog of the y-intercept (a coefficient) were calculated. The AT group had the highest percentage of type I fiber area, the RT group had the highest percentage of type IIa fiber area, and the SED group had the highest percentage of type IIx fiber area. The a coefficients were higher for the AT group than the RT and SED groups in both the MMGRMS and EMGRMS vs. force relationships, whereas the b coefficients were lower for the AT group than the RT and SED groups only in the MMGRMS vs. force relationship. The group differences among the a coefficients may have reflected subcutaneous fat acting as a filter thereby reducing EMGRMS and MMGRMS. The lower b coefficients for the AT group in the MMGRMS patterns may have reflected fiber area-related differences in motor unit activation strategies. [ABSTRACT FROM AUTHOR]