Homogeneity of the Inverse Relationship Between Motor Unit Recruitment Threshold and Firing Rate Across the Lifespan

It is well understood that there is an inverse relationship between the recruitment thresholds and firing rates for a group of motor units. In other words, at any given force level, earlier-recruited, low-threshold motor units have higher firing rates than later-recruited ones. The majority of previous investigations that have examined this relationship have done so in healthy, young adults. While some evidence suggests that aging may result in decreased maximal firing rates, it is unclear if the control scheme which governs the recruitment and firing behavior of motor units is consistent across the lifespan. Therefore, the purpose of this study was to examine the motor unit recruitment threshold versus mean firing rate relationship in children and younger and older adults. Eight boys (mean ± SD age = 12 ± 2 years; body mass = 49.9 ± 13.2 kg) and five younger (age = 26 ± 3 years; body mass = 68.6 ± 5.5 kg) and six older (age = 72 ± 4 years; body mass = 79.5 ± 9.6 kg) men participated in this investigation. Subjects performed isometric, constant-force contractions at 50% of their maximal voluntary contraction (MVC) force while bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis muscle. A surface EMG signal decomposition algorithm was used to determine the recruitment thresholds and firing rates of motor units that demonstrated accuracy levels ≥ 93%. For each subject, the relationship between the recruitment thresholds and the mean firing rates was examined using linear regression. Two separate one-way analyses of variance (ANOVA) were used to examine age-related differences in the linear slope coefficients (pulses per second [pps]/% MVC) and y-intercepts (pps) from the recruitment threshold vs. firing rate calculations. The mean ± SD slope coefficients for the boys and younger and older men were -0.33 ± 0.12, -0.49 ± 0.32, and -0.60 ± 0.38 pps/%MVC, respectively. The mean ± SD for the y-intercepts for the boys and younger and older men were 21.17 ± 4.16, 26.80 ± 4.68, and 27.60 ± 6.77 pps, respectively. The ANOVAs did not reveal any significant age-related differences in the linear slope coefficients (F = 1.700, p = 0.214) or y-intercepts (F = 3.150, p = 0.070). Although larger studies with additional subjects may be required to verify these findings, the results of the present investigation suggested that the inverse relationship between motor unit recruitment thresholds and firing rates is fixed across the lifespan

Age-Related Differences in Vastus Lateralis Muscle Thickness versus Echo Intensity

Ultrasonography is frequently used in neuromuscular research to examine muscle architecture and function. In particular, measures of muscle thickness and echo intensity are often utilized to assess muscle quantity and quality, respectively. The purpose of this study was to examine differences in vastus lateralis muscle thickness versus echo intensity across the lifespan. Ultrasound images were taken of eight boys (mean ± SD age = 12 ± 2 years), eight young men (age = 26 ± 3 years), and seven old men (age = 71 ± 4 years). Muscle thickness (cm) was defined as the distance between the deep and superficial aponeuroses. Echo intensity (au) was defined as the mean of the histogram using ImageJ’s grayscale and rectangle functions. Separate one-way analyses of variance were used to examine mean differences among the age groups. There were no significant differences in muscle thickness (boys = 1.92 cm, young men = 2.13 cm, old men = 1.64 cm [F = 2.795, p = 0.085]). However, the analysis of echo intensity showed significant mean differences (boys = 68.1 au, young men = 47.5 au, old men = 65.5 au [F = 12.654, p \u3c 0.001]). Tukey post hoc analyses demonstrated significantly lower echo intensity for the young men compared to both the boys and old men. Although additional studies with larger sample sizes are needed to confirm these findings, echo intensity may be a more sensitive variable than muscle thickness for examining age-related differences in vastus lateralis muscle architecture

Effects of Diverting Activities on Recovery from Repeated Maximum Voluntary Contractions

Previous investigations have demonstrated that diverting activities (e.g., contralateral exercise and pinching one’s fingers together) can minimize the severity of muscle fatigue that occurs during exercise. In addition to their ability to resist fatigue during exercise, diverting activities may help subjects recover between work bouts. The purpose of this study was to examine the effects of mental and physical diverting activities on recovery from fatiguing isometric muscle actions. On three separate occasions, twelve men (mean ± SD age = 22 ± 1 years) and nine women (age = 22 ± 2 years) performed ten repeated, ten-second isometric maximum voluntary contractions (MVCs) of the dominant leg extensors with ten seconds of rest between each attempt (i.e., ten seconds “on,” ten seconds “off”). Following this fatiguing protocol, the subjects performed math problems (mental diverting activity), five, 20-second isometric muscle actions with the non-dominant leg extensors at a force corresponding to 50% MVC (physical diverting activity), or rested quietly (control). Immediately following this intervention period, the subjects performed a final three-second MVC, which assessed how well the leg extensors recovered from the fatiguing protocol (Post-Recovery MVC). These three data collection trials were randomly performed, and separated by at least 48 hours of rest. A two-way (time [Pre-Fatigue MVC, MVC #1-10, Post-Fatigue MVC, Post-Recovery MVC] × intervention [math problems, contralateral, and control]) repeated measures analysis of variance was used to examine the isometric force data. The mean ± SD Post-Recovery MVC values were as follows: mental diverting activities =765 ± 210 N; physical diverting activities = 797 ± 235 N; control = 790 ± 258 N. There was no significant time × intervention interaction (partial eta squared = .072), no main effect for intervention, but there was a main effect for time. The marginal mean pairwise comparisons indicated: Pre-Fatigue MVC \u3e MVC#1 – Post-Fatigue MVC; MVC #4 – Post-Fatigue MVC \u3c Post-Recovery MVC. In contrast to the results from previous studies, our results indicated that performing mental and physical diverting activities did not help the subjects recover from fatigue

Relationship Between One Repetition Maximum Strength and Peak Power Output for the Free-Weight Bench Press Exercise

Strength and conditioning coaches are often interested in improving power output in their athletes. As many coaches are aware that power is the product of force and velocity, an emphasis is often placed on training with loads that correspond to peak mechanical power output (e.g., 30-50% of the one repetition maximum [1RM]). The purpose of this study was to examine the relationship between 1RM strength and peak power output during the free-weight bench press exercise. Twenty-one healthy, recreationally resistance-trained men (mean ± SD age = 24 ± 3 years; body mass = 90.5 ± 14.6 kg; 1RM bench press = 125.4 ± 18.4 kg) volunteered for this investigation. A minimum of 48 hours following a maximal strength testing and familiarization session, the subjects performed a single repetition of the bench press using 50% of the 1RM. For each repetition, the subjects were instructed press the weight explosively throughout the concentric portion of the range of motion. A Tendo Weightlifting Analyzer was used to assess peak power output for each repetition. A linear regression analysis was used to examine the relationship between 1RM strength and peak power output. The results indicated that the coefficient of determination (R2) was 0.697, suggesting that 1RM strength explained roughly 70% of the variance in peak power output. The linear slope coefficient was 5.94 W/kg, and the y-intercept was 110.2 W. These findings demonstrated that 1RM strength and peak power output were highly correlated. Although explosive training with light loads is often stressed in strength and conditioning programs, for the free-weight bench press exercise, a relatively small degree of variance in peak power output can be explained by factors other than 1RM strength

Relationships among Maximal and Explosive Strength Production of the Leg Extensors and Vertical Jump Peak Power Output in Female Youth Volleyball Athletes

ABSTRACT Architectural and physical performance measurements are commonly implemented to identify various physical capacities in many populations. However, previous research has suggested architectural measures, notably in the leg extensors, are ineffective predictors of vertical jumping (VJ) performance. Given the functional relevance of rapid strength development on explosive dynamic tasks, further research is warranted examining, a) the presence of associations of maximal (e.g., peak torque; PT) and, in particular, explosive (e.g., rate of torque development; RTD) strength-related characteristics with jumping performance in the leg extensors, and b) the extent to which PT and RTD either uniquely, or synergistically contribute to VJ performance. The purpose of this study was to examine the relationships between isometric maximal and explosive strength measures of the leg extensors and VJ peak power (PP) output in female youth volleyball athletes. Thirty (mean ±SD, range: age= 13.73±1.11, 12-17 years, height=162.53±6.39 cm, body mass=57.84±12.05 kg) female youth competitive volleyball players reported to the laboratory on two occasions, with the first visit being a familiarization session. The second visit involved experimental testing, in which participants performed two isometric maximal voluntary contractions of the leg extensors on a dynamometer at a leg angle of 60º, followed by three countermovement VJ trials. Subjects performed countermovement jumps, starting in a standing position and feet firmly on the ground. Following the descent to the midpoint position and without pause, the subjects exploded upward as hard and fast as possible. PT and RTD were calculated as the highest 500ms epoch and the slope of the rise in torque in the first 200ms from onset, respectively. Lower-body PP was assessed using a linear velocity transducer, which was attached to the posterior side of a belt that was securely fastened to the subjects’ waistline. Pearson correlation (r) and stepwise linear regression analyses were performed to examine the relationships. Results indicated that both PT (r=0.7) and late RTD (r=0.62) were significantly correlated to PP (p≤0.01). However, linear regression analysis revealed that PT was the only variable entered into the stepwise regression model (R=0.71; R²=0.50). These findings showed that while both maximal and explosive strength variables correlated with VJ performance, only PT was necessary to effectively predict PP output with no additional explained variance from RTD. Thus, training regimens aimed at development of high force production of the leg extensors may enhance PP production during explosive vertical jump tasks more so than enhancing early rapid force production

Leg Lean Mass Adaptations Following Short-Term Barbell Training in Women

Participation in a resistance training program results in both neural adaptations and increases in lean body mass, the latter of which is considered a slow process. The majority of previous investigations that have examined the time course associated with muscular adaptations have utilized exercise machines or single-joint movements. Furthermore, the adaptations associated with resistance training in women are not as well understood as those for men. The purpose of this investigation was to examine the effects of a four week resistance training intervention on changes in leg lean mass in untrained, college-aged women. Thirty-five women (mean ± SD age = 21 ± 3 years; body mass = 61.9 ± 10.4 kg) with no previous lower-body strength training experience were randomly assigned to high volume training (n = 11), low volume training (n = 10), and control (n = 14) groups. The training groups performed the barbell back squat and deadlift twice per week for four weeks. The low volume training group performed five repetitions of two sets per exercise; the high volume group performed an additional two sets per exercise. The external loads were increased progressively during each training session. The mean ± SD external loads used in this study increased from 27.9 ± 8.2 to 51.4 ± 14.0 kg for the barbell back squat and from 34.2 ± 7.7 to 60.2 ± 10.1 kg for the deadlift. Body composition analyses were performed during pre- and post-testing with dual X-ray absorptiometry. Manufacturer provided software was used to determine the appropriate region of interest in the assessment of leg lean mass. An analysis of covariance was used to compare the post-test data, and the pre-test data were used as the covariate. The results indicated that there was a statistically significant difference among the adjusted post-test means. Specifically, when the pre-test scores served as the covariate, the mean leg lean mass for the high volume training group was 0.503 kg greater than that for the control group (p = 0.031, 95% CI for adjusted mean difference = 0.038 to 0.968 kg). The pre-test – post-test effect sizes for the high volume training and low volume training groups were 0.31 and 0.29, respectively. Collectively, these findings demonstrated that a four week, high-volume training program involving barbell back squats and deadlifts produced small increases in leg lean mass in previously untrained women

Creatine Kinase, Lactate Dehydrogenase, and Myoglobin Responses Following Repeated Wingate Anaerobic Tests Versus Barbell Back Squats: a Pilot Study

The results from many previous studies have demonstrated that muscle damage and delayed onset muscle soreness are due primarily to high force, eccentric muscle actions. However, the majority of investigators have examined these responses following single-joint, isokinetic assessments. The purpose of this study was to examine creatine kinase (CK), lactate dehydrogenase (LDH), and myoglobin responses following repeated Wingate anaerobic tests versus multiple sets of the barbell back squat exercise. Six recreationally resistance-trained men (mean ± SD age = 23 ± 2 years, body mass = 88.0 ± 17.8 kg, one repetition-maximum [1RM] back squat = 150.0 ± 28.3 kg) volunteered for this investigation, and visited the laboratory on five separate occasions. For the back squat protocol, the subjects performed six sets until volitional exhaustion using 75% of the 1RM with two minutes of rest between each set. For all repetitions, the subjects squatted to the parallel position. For the Wingate protocol, the subjects performed three tests with a 10 minute recovery period between each attempt. The order of these two protocols was randomized, and all testing for the study occurred at the same time of day. The subjects were required to refrain from vigorous physical activity during the study. Immediately prior to and 24 hours following each protocol, the subjects provided a venous blood sample from a superficial forearm vein. At the conclusion of the study, all samples were processed for CK, LDH, and myoglobin analyses. Three separate two-way (time [Pre versus Post] × protocol [squats versus Wingate anaerobic tests]) repeated measures analyses of variance were used to examine the CK, LDH, and myoglobin data. For CK, there was a significant time × protocol interaction (partial eta squared = .761). Follow-up paired samples t-tests indicated that the mean CK values increased following the squat protocol (Pre mean ± SD = 233.2 ± 97.4; Post = 457.0 ± 119.9 IU/L), but not the Wingate anaerobic tests (Pre = 218.5 ± 103.9; Post = 231.7 ± 65.3 IU/L). There was no mean difference between the protocols for both LDH and myoglobin. The primary finding from this pilot study was that performing six sets of barbell back squats using 75% of the 1RM significantly increased blood CK levels in resistance-trained men. Although additional studies with larger sample sizes are needed to confirm our findings, CK appears to be a more sensitive measure of muscle damage than both LDH and myoglobin

Isokinetic Fatigue Characteristics for the Leg Extensors versus Flexors

Maximal isokinetic muscle actions are often used in research studies to examine fatigability and even estimate muscle fiber-type. However, the majority of previous investigations have examined these topics for the leg extensors (i.e., quadriceps), and we are unaware of investigations that have specifically assessed the fatigue characteristics for the flexors (i.e., hamstrings). The purpose of this study was to compare the percent decline values for the leg extensors versus flexors for 50 and 100 repeated, maximal concentric isokinetic muscle actions. Fifteen healthy men (mean ± SD age = 23 ± 3 years; body mass = 94.1 ± 11.9 kg) with previous lower-body strength training experience volunteered to participate in this study. All of the subjects were familiarized with the testing procedures prior to data collection. For data collection, each subject performed 100 repeated, maximal concentric isokinetic muscle actions of the left leg extensors and flexors in a reciprocal manner. Each muscle action was performed at 180 degrees/second through a full 90 degree range of motion. Strong verbal encouragement was provided throughout testing. Percent decline was determined using the mean peak torque values of the initial and final three muscle actions for each muscle group (i.e., extensors versus flexors) and condition (i.e., 50 versus 100 repetitions). A two-way repeated measure analysis of variance was used to examine the data. The mean ± SD percent decline for the leg extensors was 61.8 ± 7.8 and 71.2 ± 6.5% for the 50 and 100 conditions, respectively (Cohen’s d = 1.31). For the leg flexors, these corresponding values were 48.0 ± 12.2 and 54.3 ± 11.7 % (Cohen’s d = 0.53). There was no significant muscle group × condition interaction (p= .114; partial eta squared = .169) There were, however, main effects for both factors. The bonferroni marginal mean pairwise comparisons indicated that when collapsed across condition, the leg extensors fatigued more so than the flexors (66.5 vs. 51.2%). Similarly, when collapsed across muscle group, the percent decline values were greater following 100 (62.8%) versus 50 (54.9%) repetitions. These finding demonstrated greater isokinetic fatigue characteristics for the leg extensors versus flexors. Furthermore, the additional decline in peak torque from repetitions 50-100 was more pronounced for the extensors. We speculate that these findings could be related to differences in muscle fiber-type, lower absolute strength and mass for the posterior aspect of the thigh, and/or unfamiliarity with single-joint testing of the leg flexors

Učinci odvraćajućih aktivnosti na elektromiografsku amplitudu i središnju frekvenciju

The purpose of this study was to examine the effects of diverting activities on electromyographic amplitude and mean frequency. On three separate occasions, eleven men and eight women performed two bouts of fifty consecutive maximal concentric isokinetic muscle actions of the dominant leg extensors. Between these bouts, the subjects either solved math problems, performed contralateral dynamic constant external resistance leg extensions, or rested quietly. During each muscle action, electromyographic signals were detected from the vastus lateralis, rectus femoris, and vastus medialis. The results indicated that neither the mental nor the physical diverting activities consistently affected the mean electromyographic amplitude and mean frequency values relative to the control visit of quiet resting. If mental or physical diverting activities affected muscle activation in the vastus lateralis, rectus femoris or vastus medialis, electromyographic amplitude and mean frequency values were not sensitive enough to detect it.Cilj je ovog istraživanja ispitati učinke odvraćajućih aktivnosti na amplitudu i prosječnu frekvenciju elektromiografskog signala. U tri odvojene situacije, jedanaest muškaraca i osam žena izvodilo je dvije serije od pedeset uzastopnih, maksimalnih, koncentričnih izokinetičkih mišićnih akcija ekstenzorima dominantne noge. Između serija, subjekti su ili rješavali matematičke probleme, izvodili kontralateralna dinamička opružanja s konstantnim vanjskim opterećenjem ili su se odmarali. Tijekom svake mišićne akcije bilježili su se elektromiografski signali u mišićima vastus lateralis, rectus femoris i vastus medialis. Rezultati su pokazali da ni psihičke ni fizičke odvraćajuće aktivnosti nisu konzistentno utjecale na prosječnu elektromiografsku amplitudu i frekvenciju. Ukoliko su mentalne ili fizičke odvraćajuće aktivnosti ipak nekako utjecale na mišićnu aktivaciju mišića vastus lateralis, rectus femoris ili vastus medialis, elektromiografske vrijednosti prosječne amplitude i frekvencije nisu bile dovoljno osjetljive da to otkriju

Relationships Between Anthropometric and Performance Variables in Youth: Predictors of Lower-Body Vertical Jump Peak Power

ABSTRACT Anthropometric and physical performance measurements are commonly used for identifying specific physical traits in youth. Laboratory-based tests (e.g., linear velocity transducers), while accurate, provide practical limitations due to high costs and technical necessities. Thus, commonly used field tests may be useful alternatives for assessing fitness/performance characteristics of youth. The purpose of this study was to examine the relationships between anthropometric measures and lower-and upper-body power and strength measures; and to assess the predictive ability of these measures for lower-body vertical jump peak power (PP) output in youth. Thirty-nine pre-adolescent (mean±SD, range: age=7.8±1.7, 5-12 years) children volunteered to participate in this investigation. Subjects were measured for body mass and stature on a calibrated physicians scale. Lower-body PP was assessed using a linear velocity transducer, which was attached to the posterior side of a belt that was securely fastened to the subjects’ waistline. Subjects performed countermovement jumps, starting in a standing position, with hands placed on hips and feet firmly on the ground. Following the descent to the midpoint position and without pause, the subjects exploded upward as hard and fast as possible. Broad jump testing involved subjects performing a countermovement jump in the horizontal direction, on a scaled mat. Maximum isometric hand grip strength of the dominant hand was assessed using a dynamometer. For all testing, 1-2 practice trials were performed, followed by testing consisting of 2-3 trials. The highest trial was used for analyses. Peak power values were normalized to body mass using allometric scaling procedures (PP· body mass-0.67). Pearson correlation (r) and stepwise linear regression analyses were performed to examine the relationships. Results indicated all variables (age, stature, body mass, broad jump and grip strength) were significantly correlated (r=0.38-0.64) to PP. Age was correlated to all variables (r=0.39-0.82) and stature and body mass were correlated to all variables (r=0.37-0.77) except broad jump. Broad jump was correlated only to age (r=0.39) and PP (r=0.38). Linear regression for all variables revealed that stature was the only variable entered into the model (R=0.64; R2=0.41). With the anthropometric variables removed, grip strength was the only variable entered into the model (R=0.57; R2=0.32). These findings suggest that while all the anthropometric and performance variables may be correlated to PP, only stature and grip strength were effective, and thus, necessary to predict PP abilities. Interestingly, broad jump performance was not a good predictor of lower body vertical PP