The statistical power, or sensitivity of an experiment, is defined as the probability of rejecting a false null hypothesis. Only 3 factors can affect statistical power: (a) the significance level ...(α), (b) the magnitude or size of the treatment effect (effect size), and (c) the sample size (n). Of these 3 factors, only the sample size can be manipulated by the investigator because the significance level is usually selected before the study, and the effect size is determined by the effectiveness of the treatment. Thus, selection of an appropriate sample size is one of the most important components of research design but is often misunderstood by beginning researchers. The purpose of this tutorial is to describe procedures for estimating sample size for a variety of different experimental designs that are common in strength and conditioning research. Emphasis is placed on selecting an appropriate effect size because this step fully determines sample size when power and the significance level are fixed. There are many different software packages that can be used for sample size estimation. However, I chose to describe the procedures for the G*Power software package (version 3.1.4) because this software is freely downloadable and capable of estimating sample size for many of the different statistical tests used in strength and conditioning research. Furthermore, G*Power provides a number of different auxiliary features that can be useful for researchers when designing studies. It is my hope that the procedures described in this article will be beneficial for researchers in the field of strength and conditioning.
Skeletal muscle hypertrophy is typically considered to be a slow process. However, this is partly because the time course for hypertrophy has not been thoroughly examined. The purpose of this study ...was to use weekly testing to determine a precise time course of skeletal muscle hypertrophy during a resistance training program. Twenty-five healthy, sedentary men performed 8 weeks of high-intensity resistance training. Whole muscle cross-sectional area (CSA) of the dominant thigh was assessed using a peripheral quantitative computed tomography scanner during each week of training (W1–W8). Isometric maximum voluntary contractions (MVC) were also measured each week. After only two training sessions (W1), the mean thigh muscle CSA increased by 5.0 cm
2
(3.46%;
p
< 0.05) from the pre-testing (P1) and continued to increase with each testing session. It is possible that muscular edema may have influenced the early CSA results. To adjust for this possibility, with edema assumedly at its highest at W1, the next significant increase from W1 was at W3. W4 was the first significant increase of MVC over P1. Therefore, significant skeletal muscle hypertrophy likely occurred around weeks 3–4. Overall, from the pre-testing to W8, there was an increase of 13.9 cm
2
(9.60%). These findings suggested that training-induced skeletal muscle hypertrophy may occur early in a training program.
ABSTRACT
Introduction
This study compares the acute and chronic response of high‐load resistance training (HL) to low‐load resistance training with low blood flow restriction (LL‐BFR) pressure.
...Methods
Participants completed elbow flexion with either HL or LL‐BFR or nonexercise. In the chronic study, participants in the HL and LL‐BFR groups were trained for 8 weeks to determine differences in muscle size and strength. The acute study examined the changes in pretesting/posttesting (Pre/Post) torque, muscle swelling, and blood lactate.
Results
In the chronic study, similar changes in muscle size and strength were observed for both HL and LL‐BFR. In the acute study, Pre/Post changes in the torque, muscle swelling, and blood lactate were similar between HL and LL‐BFR.
Discussion
Our findings indicate that pressure as low as 50% arterial occlusion can produce similar changes in muscle mass and strength compared with traditional HL. Muscle Nerve 56: E126–E133, 2017
Introduction: The purpose of this study was to examine the influence of fatigue on the average firing rate versus recruitment threshold relationships for the vastus lateralis (VL) and vastus ...medialis. Methods: Nineteen subjects performed ten maximum voluntary contractions of the dominant leg extensors. Before and after this fatiguing protocol, the subjects performed a trapezoid isometric muscle action of the leg extensors, and bipolar surface electromyographic signals were detected from both muscles. These signals were then decomposed into individual motor unit action potential trains. For each subject and muscle, the relationship between average firing rate and recruitment threshold was examined using linear regression analyses. Results: For the VL, the linear slope coefficients and y‐intercepts for these relationships increased and decreased, respectively, after fatigue. For both muscles, many of the motor units decreased their firing rates. Conclusion: With fatigue, recruitment of higher threshold motor units resulted in an increase in slope for the VL. Muscle Nerve 45: 100–109, 2012
Background
It has been proposed that the increase in skeletal muscle mass observed during the initial weeks of initiating a resistance training program is concomitant with eccentric muscle damage and ...edema.
Purpose
We examined the time course of muscle hypertrophy during 4 weeks of concentric-only resistance training.
Methods
Thirteen untrained men performed unilateral concentric-only dumbbell curls and shoulder presses twice per week for 4 weeks. Sets of 8–12 repetitions were performed to failure, and training loads were increased during each session. Subjects consumed 500 ml of whole milk during training. Assessments of soreness, lean mass, echo intensity, muscle thickness, relaxed and flexed arm circumference, and isokinetic strength were performed every 72 or 96 h.
Results
Soreness, echo intensity, relaxed circumference, and peak torque data did not significantly change. Significant increases in lean mass, muscle thickness, and flexed circumference were observed within seven training sessions. Lean mass was elevated at tests #7 (+109.3 g,
p
= .002) and #8 (+116.1 g,
p
= .035), with eight different subjects showing changes above the minimal difference of 139.1 g. Muscle thickness was elevated at tests #6 (+0.23 cm,
p
= .004), #7 (+0.31 cm,
p
< .001), and #8 (+0.27 cm,
p
< .001), with ten subjects exceeding the minimal difference of 0.24 cm. There were no changes for the control arm.
Conclusion
In individuals beginning a resistance training program, small but detectable increases in hypertrophy may occur in the absence of eccentric muscle damage within seven training sessions.
Eccentric exercise results in an adaptation which attenuates muscle damage from subsequent exercise-termed the "repeated-bout effect (RBE)."
Study examined antagonist co-activation and motor-unit ...recruitment strategy, assessed via dEMG, concomitant to the RBE.
Nine participants performed 5 sub-maximal isometric trapezoid (ramp-up, hold, ramp-down) contractions at force levels corresponding to 50% and 80% of maximal isometric strength (MVC). Surface EMG signals of the biceps brachii were decomposed into individual motor-unit action potential trains. The relationship between mean firing rate (MFR) of each motor-unit and its recruitment threshold (RT) was examined using linear regression. Eccentric exercise was then performed until biceps brachii MVC had decreased by ~40%. Surface EMG of the biceps and triceps were collected during eccentric exercise. MVC, range-of-motion (ROM), and delayed onset muscle soreness (DOMS) were measured 24-hours, 72-hours, and 1-week following eccentric exercise. Three weeks later all procedures were repeated.
Changes in MVC (-32±14% vs -25±10%; p = 0.034), ROM (-11% vs 6%; p = 0.01), and DOMS (31.0±19mm vs 19±12mm; p = 0.015) were attenuated following the second bout of exercise. Triceps EMG was reduced (16.8±9.5% vs. 12.6±7.2%; p = 0.03) during the second bout of eccentric exercise. The slope (-0.60±0.13 vs -0.70±0.18; p = 0.029) and y-intercept (46.5±8.3 vs 53.3±8.8; p = 0.020) of the MFR vs. RT relationship was altered during contractions at 80% of MVC prior to the second bout of eccentric exercise. No changes were observed at 50% of MVC.
A reduction in antagonist co-activation during the second bout of eccentric exercise suggests less total force was required to move an identical external load. This finding is supported by the increased negative slope coefficient and an increased y-intercept of the linear relationship between RT and MFR.
ABSTRACT
Introduction: We examined the etiology of the electromyographic (EMG) spectral shift during dynamic fatigue. Methods: Nineteen subjects (mean ± SD age = 22.4 ± 1.6 years) performed 50 ...consecutive maximal concentric isokinetic contractions of dominant leg extensors. Surface EMG signals were detected from the vastus lateralis, rectus femoris, and vastus medialis during each contraction, processed with a wavelet analysis, and the resulting spectra were decomposed with a nonparametric spectral decomposition procedure. Results: The results indicated that the decreases in EMG frequency during the 50 contractions were generally due to reductions in high‐frequency power and increases in low‐frequency power. In addition, the spectral shifts were most pronounced for the rectus femoris, followed by the vastus lateralis, and then the vastus medialis. Conclusions: The spectral decomposition procedure is much more sensitive for tracking dynamic fatigue than is EMG mean frequency or median frequency. Muscle Nerve 50: 95–102, 2014
To examine the time course (immediate, 10, 20, and 30 min) for the acute effects of 2, 4, and 8 min of passive stretching (PS) on isometric peak torque (PT), percent voluntary activation (%VA), EMG ...amplitude, peak twitch torque (PTT), rate of twitch torque development (RTD), and range of motion (ROM) of the plantarflexors.
Thirteen volunteers (mean +/- SD age, 22 +/- 3 yr) participated in four randomly ordered experimental trials: control (CON) with no stretching, 2 min (PS2), 4 min (PS4), and 8 min (PS8) of PS. Testing was conducted before (pre), immediately after (post), and at 10, 20, and 30 min poststretching. The PS trials involved varied repetitions of 30-s passive stretches, whereas the CON trial included 15 min of resting. PT, %VA, EMG amplitude, PTT, and RTD were assessed during the twitch interpolation technique, whereas ROM was quantified as the maximum tolerable angle of passive dorsiflexion.
PT decreased (P < or = 0.05) immediately after all conditions CON (4%), PS2 (2%), PS4 (4%), and PS8 (6%) but returned to baseline at 10, 20, and 30 min poststretching. %VA and EMG amplitude were unaltered (P > 0.05) after all conditions. PTT and RTD decreased (P < or = 0.05) immediately after the PS4 (7%) and the PS8 (6%) conditions only; however, these changes were not sufficient to alter voluntary force production. There were also increases (P < or = 0.05) in ROM after the PS2 (8%), the PS4 (14%), and the PS8 (13%) conditions that returned to baseline after 10 min.
Practical durations of stretching (2, 4, or 8 min) of the plantarflexors did not decrease isometric PT compared with the CON but caused temporary improvements in the ROM, thereby questioning the overall detrimental influence of PS on performance.
Summary
The purpose of this study was to determine the muscular adaptations to low‐load resistance training performed to fatigue with and without blood flow restriction (BFR). Middle‐aged ...(42–62 years) men (n = 12) and women (n = 6) completed 18 sessions of unilateral knee extensor resistance training to volitional fatigue over 6 weeks. One limb trained under BFR, and the contralateral limb trained without BFR free flow (FF). Before and after the training, measures of anterior and lateral quadriceps muscle thickness (MTh), strength, power and endurance were assessed on each limb. The total exercise training volume was significantly greater for the FF limb compared with the BFR limb (P<0·001). Anterior quadriceps thickness and muscle function increased following the training in each limb with no differences between limbs. Lateral quadriceps MTh increased significantly more (P<0·05) in the limb trained under BFR (BFR: 3·50 ± 0·61 to 3·67 ± 0·62 cm; FF: 3·49 ± 0·73 to 3·56 ± 0·70 cm). Low‐load resistance training to volitional fatigue both with and without BFR is viable options for improving muscle function in middle‐aged individuals. However, BFR enhanced the hypertrophic effect of low‐load training and reduced the volume of exercise needed to elicit increases in muscle function.
Purpose
There have been recent attempts to characterize the mechanisms associated with fatigue-induced task failure. We compared the time to failure and the corresponding changes in the surface ...electromyogram (EMG) during sustained maximal and submaximal isometric force tasks.
Methods
EMG activity was measured from the biceps brachii of 18 male participants as they sustained either a maximal or submaximal (60 % MVC) isometric contraction of the dominant elbow flexors until force could not be maintained above 55 % MVC.
Results
Intensity-dependent patterns of change were observed for EMG amplitude and mean power frequency (MNF) between the two force tasks. Interestingly, the only significant predictor of failure time was the rate of change in EMG MNF during the submaximal task (
r
2
= 0.304). In addition, EMG amplitude at submaximal failure was significantly lower (
p
< 0.05) than the values obtained during MVC.
Conclusions
The patterns of EMG response emphasize the basis of neuromuscular fatigue and task dependency. Additionally, our data suggest that the EMG MNF should be used when monitoring the progression of local muscle fatigue.
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