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

No Differences in Strength Improvements Following Low- or High-Volume Resistance Training

Resistance training is a widely used modality for improving muscular strength and reducing risks of injury, which is vital to counteracting physical declines associated with aging and poor health. Despite this, the minimal effective training dose for improving muscular strength has yet to be fully elucidated. PURPOSE: The purpose of this study was to examine the role of training volume (number of sets per session) on muscular strength changes following 8 weeks of progressive resistance training. METHODS: Fourteen and 12 trained males (Mean±SD; Age: 23±3y) and females (Age: 20±1y) participated in 8 weeks of supervised 3x/week progressive resistance training. Experimental sessions consisted of 3-5 repetition maximum testing both pre- and post-intervention, in accordance with the protocol outlined by the NSCA, in the following exercises: leg press (LP), bench press (BP), horizontal row (ROW), barbell Romanian deadlift (RDL), dumbbell overhead press (OHP), and lat pulldown (LAT). Following baseline strength testing, each participant was randomly allocated to either a low volume (LV; n=12 (5F)) or high volume (HV; n=14 (7F)) training group, completing 2 or 4 sets per exercise per training visit, respectively. Across all 8 weeks, participants completed each lift twice weekly, and loads were adjusted based on exercise performance using the autoregulated progressive resistance exercise protocol. Each group completed the same repetitions in their first sets, but completed the last set of every exercise until volitional failure. Percent change for each exercise was calculated as the difference between baseline strength (kgs) and post-training strength (kgs), expressed as a percentage of baseline strength. To examine the effect of group and exercise on the change in strength, a 2 (Group) × 6 (Exercise) analysis of covariance (ANCOVA) was performed, covarying for pre-test strength. In the event of a significant F test, the Bonferroni-corrected dependent-samples t-test was used. Values are presented as estimated marginal means ± standard error. RESULTS: There was no significant Group × Exercise interaction effect on percent strength change (p=0.754), nor a main effect of Group (p=0.397). However, there was a significant effect of Exercise (p\u3c0.001). Post-hoc analyses indicated, when collapsing across training groups, improvements in strength were greater in LP when compared to BP (40.6±6.8%; p\u3c0.001), RDL (26.9±6.1%; p\u3c0.001), OHP (37.4±7.9%; p\u3c0.001), and LAT (22.7±6.8%; p=0.015). Additionally, greater strength improvements were seen in ROW when compared to BP (29.7±4.5%, p\u3c0.001), RDL (16.0±4.6%, p\u3c0.001), and OHP (26.5±4.8%, p\u3c0.001). Finally, LAT experienced greater strength increases than both BP (17.8±4.5%, p\u3c0.01) and OHP (14.6±4.7%, p=0.036). There were no additional significant differences between exercises (p=0.054-0.999). CONCLUSION: Our findings suggest that a resistance training volume of as few as 2 sets per exercise twice weekly is adequate to induce muscular strength adaptations in previously trained young adults. Further examination is needed to determine if upper and lower body exercises require differing volumes to elicit similar adaptations

Does the Analysis of Separate Bands of Echo Intensity Strengthen the Relationship to Muscle Function?

Ultrasound echo intensity (EI) has been proposed as a method of assessing muscle quality through the use of non-invasive imaging. Traditionally, EI is assessed as the mean pixel brightness that ranges from 0-255 within an area of interest. However, it may be reasonable to consider that additional portions of the ultrasound EI signal (i.e., bands of signal) may provide novel insight to muscle function. The determination of which band of signal may be more related to a given functional outcome may increase the sensitivity of EI. Thus far, there is no research analyzing the association between EI bands and fatigue. PURPOSE: The purpose of this study was to compare relationships between mean echo intensity and unique bands of ultrasound signal of the vastus lateralis with metrics of whole muscle performance in healthy adults. METHODS: Twenty-four participants (mean ± age = 22 ± 3.9 yrs; BMI = 25.7 ± 3.4 kg/m2), completed two visits to the laboratory. On the first visit, subjects completed Brightness mode (B-mode) ultrasound imaging and were familiarized with the fatigue assessment. Between two and seven days later, subjects returned for the testing visit. B-mode ultrasound was used to image the vastus lateralis (VL) at 50% muscle length. The VL cross-sectional area was traced using the polygon tool. As much of the muscle was selected without selecting any of the surrounding fascia. Each pixel is assigned a brightness value from 0-255 based on gray scale; 0 representing true black and 255 is pure white. Mean EI was quantified from within the selected portion of the image. Echo intensity bands were calculated in pixel value intervals of 0-49, 50-99, 100-149, 150-199, 200-255. The percentage of pixels per band compared to the total number of pixels in each image was assessed by: (number of pixels in each band/ total number of pixels in the selected portion of the image)*100. For the fatigue assessment, participants completed 100 repeated, maximal, isokinetic muscle actions (120°/sec). Isokinetic peak torque was analyzed offline using custom written software by selecting individual torque peaks from each muscle action. Initial and final isokinetic peak torque were calculated by averaging the highest 3 of the first 5 and the highest 3 of the last 5 contractions. Isokinetic peak torque percent decline (%Decline) was calculated by: %Decline = (initial – PT – final - PT)/initial - PT. Pearson’s correlation coefficient (r) was used to assess the relationship between each EI band and %Decline as well as mean EI and %Decline. The Stieger’s Z procedure was used to compare the correlation coefficients between mean EI and each EI band. RESULTS: There were no significant correlation between mean EI and %Decline (r=0.03, p=0.88) or any of the EI bands and %Decline (r=-0.07-0.3, p=0.16-0.89). Additionally, there were no significant relationships between the mean EI and any of the EI bands (z=0.001-0.88, p=0.38-0.99). CONCLUSION: The findings suggest that unique bands of ultrasound signal do not offer different relationships compared to overall mean EI when assessing fatigue from repetitive isokinetic muscle actions

Reliability of Differing Muscle Size and Quality Analysis Techniques

Brightness-mode (B-mode) ultrasonography is a popular tool to examine anatomical cross-sectional area (ACSA) and echo intensity (EI). Muscle ACSA and EI provide valuable insight into muscle function due to their unique mechanisms which influence performance. Manually analyzing ultrasound images potentially increases variability which may increase error, thus decreasing the reliability of manual image analysis. Recently an automated program was created to improve reliability and reduce the time of ultrasound image analysis. PURPOSE: The purpose of this study was to investigate the reliability of manual compared to automatic ultrasound analyses of muscle cross-sectional area and echo intensity. METHODS: Twenty-two participants (mean ± SD age = 24 ± 4 yrs; BMI = 24.19 ± 3.26 kg/m2) volunteered for this study. The participants completed one visit to the laboratory consisting of two data collection trials separated by 10 minutes. Ultrasound scans were taken with a B-mode ultrasound imaging device and image settings were held constant (i.e., depth = 6 cm, frequency = 12 MHz, gain = 52 dB). For each trial, participants remained supine while ACSA scans of the vastus lateralis (VL) were taken at 50% the length of the proximal to distal musculo-tendon junctions. The ACSA of the VL was manually analyzed by an experienced technician with ImageJ using the polygon tool and tracing the area of interest. Echo intensity was quantified as the mean pixel brightness of the traced portion of the image. Images were automatically analyzed with the Deep Anatomical Cross-Sectional Area (DeepACSA) program which is an algorithm that is designed to automatically trace the area of interest of an ultrasound image. Test-retest reliability statistics (i.e., intraclass correlation coefficient [ICC] model 2,1, standard error of measure expressed as a percentage of the mean [SEM%], and the minimal differences [MD] values needed to be considered real) were calculated for trials 1 and 2. One-way repeated measures analysis of variance determined differences in trial 1 compared to trial 2. RESULTS: Manual analyses of ACSA (ICC2,1 = 0.98, SEM (%) = 3.39%, MD = 2.09 cm2, p = 0.046) were more reliable than automatic analyses (ICC2,1 = 0.87, SEM (%) = 12.33%, MD = 7.77 cm2, p = 0.216). Manual analyses of EI (ICC2,1 = 0.73, SEM (%) = 6.44%, MD = 10.83 cm2, p = 0.514) had similar reliability to the automatic analyses (ICC2,1 = 0.88, SEM (%) = 3.60%, MD = 6.30 cm2, p = 0.003). CONCLUSION: These results suggest that this automated analysis program may be less reliable compared to the manual analysis of muscle ACSA of the VL. Conversely, DeepACSA displayed similar reliability for EI of the VL when compared to the manual analysis

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

Test-Retest Reliability of Automatic and Manual Image Analyses of Muscle Size

Brightness-mode (B-mode) ultrasound is a non-invasive imaging modality that has risen in popularity. In research settings, B-mode ultrasound is often used to assess skeletal muscle size via the quantification of the anatomical cross-sectional area (ACSA). Typically, these images are analyzed by an experienced investigator using open-source software, though it is a time-consuming process that may introduce implicit bias into the analysis. Recently, a novel, automatic ultrasound image analysis tool has been developed which may reduce bias and increase the reliability of ultrasound ACSA image analysis. PURPOSE: The purpose of the project was to compare the test-retest reliability of manual and automatic ACSA quantification techniques. METHODS: Nine participants (mean ± SD: age = 25 ± 3 years; BMI = 23.96 ± 2.62 kg/m2) completed one laboratory visit where each participant had non-invasive ultrasound imaging performed on their rectus femoris (i.e., RF) for two data collection trials separated by 10 minutes. For each participant, ultrasound image settings were held constant (i = 6 cm, frequency = 10 MHz, gain = 52 dB). All images were manually analyzed by an experienced technician using an open-source image analysis tool. The investigator would carefully select only the surrounding muscle fascia of the RF. Automatic analyses were performed using DeepACSA, a deep learning approach for the assessment of ACSA. Both manual and automatic analyses were conducted on all images. Analysis of variance (ANOVA) was conducted to compare differences between trials and test-retest reliability (i.e., intraclass correlation coefficients [ICC] model 2,1, standard error of measure expressed as a percentage of the mean [SEM%], and the minimal differences [MD] values needed to be considered real) were calculated from the ANOVA output. RESULTS: The manual analyses of ACSA (p = 0.20, ICC2,1 = 0.84, SEM (%) = 11.67%, MD = 1.75 cm2) were more reliable than the DeepACSA analyses (p = 0.13, ICC2,1 = 0.47, SEM (%) = 30.28%, MD = 4.70 cm2). CONCLUSION: The results of the present investigation suggest that the DeepACSA approach may be less reliable compared to the manual quantification of RF muscle size. Future studies should investigate using a larger sample size and additional muscle groups

Bioimpedance Spectroscopy Compared to Ultrasound-derived Measures of Quadriceps Muscle Quality

Muscle quality is often measured using ultrasound-derived echo intensity (EI). Recent works have shown tissue frequency-dependent electrical impedance from bioimpedance spectroscopy may be a modality for assessing tissue quality. PURPOSE: The purpose of the project was to examine the association between ultrasound-derived EI of the quadriceps muscles (i.e., vastus lateralis [VL], vastus medialis [VM], vastus intermedius [VI], rectus femoris [RF]) and measures of thigh tissue frequency-dependent electrical impedance (i.e., R0, R1, C, a, fp). METHODS: Twenty-four participants (13 women; mean ± SD; age: 22 ± 4 years; BMI: 25.47 ± 3.26 kg/m2) were recruited. Participants completed one laboratory visit where quadriceps tissue quality was assessed via ultrasound and bioimpedance spectroscopy (BIS). Participants laid supine on a portable exam table to undergo imaging of the dominant leg VL using ultrasound in conjunction with a multi-frequency linear array probe (L4 – 12t – RS, 4.2-13 MHz, 47.1mm field of view). The VL was marked at the proximal and distal musculo-tendon junctions determined via ultrasound and the length was measured with a tape measure. Participants had cross-sectional scans of the VM, VL, VI, and RF at 25, 50, 75% of the length of the VL. Images were analyzed using the polygon tool in ImageJ to trace the muscles and provide EI values. Subcutaneous fat width was measured using the straight-line tool. Echo intensity was calculated using ImageJ gray-scale analysis and histogram function as well as corrected for subcutaneous fat. For statistical analyses, the average corrected EI for each muscle was created across scan sites. For BIS, participants were seated in a chair with Ag/AgCl electrodes placed above the patella and below the hip. Electrodes were placed 6cm apart and the Cole-impedance model was used to represent frequency-dependent thigh tissue data. Signals were analyzed using a custom-written software program. Pearson’s correlation coefficient (r) was used to determined associations between the VL, VM, VI, RF and BIS variables (R0, R1, C, a, fp). An alpha level of p ≤ 0.05 determined statistical significance. RESULTS: The results suggest that VL, VM, VI and RF echo intensity was significantly related to R0 (r = 0.65 – 0.81; p \u3c 0.01). For VI and RF, they were significantly related to a (r = -0.51 – -0.50; p = 0.01), but not for VL or VM (r = -0.39 - -0.22; p \u3e 0.06). Lastly, R1, C, and fp were not significantly correlated to the quadriceps muscles (r = -0.38 – 0.33; p \u3e 0.07). CONCLUSION: Our findings suggest that BIS-derived R0 may be a metric of muscle quality of the quadriceps as it was significantly related to ultrasound-derived measures of echo intensity of the VL, VM, VI, and RF. Further investigation of other muscle groups may be warranted

Acute Blood Flow Responses to Varying Blood Flow Restriction Pressures in the Lower Limbs

International Journal of Exercise Science 16(2): 118-128, 2023. The purpose of this study was to investigate lower limb blood flow responses under varying blood flow restriction (BFR) pressures based on individualized limb occlusion pressures (LOP) using a commonly used occlusion device. Twenty-nine participants (65.5% female, 23.8 ± 4.7 years) volunteered for this study. An 11.5cm tourniquet was placed around participants’ right proximal thigh, followed by an automated LOP measurement (207.1 ± 29.4mmHg). Doppler ultrasound was used to assess posterior tibial artery blood flow at rest, followed by 10% increments of LOP (10-90% LOP) in a randomized order. All data were collected during a single 90-minute laboratory visit. Friedman’s and one-way repeated-measures ANOVAs were used to examine potential differences in vessel diameter, volumetric blood flow (VolFlow), and reduction in VolFlow relative to rest (%Rel) between relative pressures. No differences in vessel diameter were observed between rest and all relative pressures (all p \u3c .05). Significant reductions from rest in VolFlow and %Rel were first observed at 50% LOP and 40% LOP, respectively. VolFlow at 80% LOP, a commonly used occlusion pressure in the legs, was not significantly different from 60% (p = .88), 70% (p = .20), or 90% (p = 1.00) LOP. Findings indicate a minimal threshold pressure of 50%LOP may be required to elicit a significant decrease in arterial blood flow at rest when utilizing the 11.5cm Delfi PTSII tourniquet system