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      Methods matter: the relationship between strength and hypertrophy depends on methods of measurement and analysis

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          Abstract

          Purpose

          The relationship between changes in muscle size and strength may be affected by both measurement and statistical approaches, but their effects have not been fully considered or quantified. Therefore, the purpose of this investigation was to explore how different methods of measurement and analysis can affect inferences surrounding the relationship between hypertrophy and strength gain.

          Methods

          Data from a previous study—in which participants performed eight weeks of elbow flexor training, followed by an eight-week period of detraining—were reanalyzed using different statistical models, including standard between-subject correlations, analysis of covariance, and hierarchical linear modeling.

          Results

          The associative relationship between strength and hypertrophy is highly dependent upon both method/site of measurement and analysis; large differences in variance accounted for (VAF) by the statistical models were observed (VAF = 0–24.1%). Different sites and measurements of muscle size showed a range of correlations coefficients with one another ( r = 0.326–0.945). Finally, exploratory analyses revealed moderate-to-strong relationships between within-individual strength-hypertrophy relationships and strength gained over the training period ( ρ = 0.36–0.55).

          Conclusions

          Methods of measurement and analysis greatly influence the conclusions that may be drawn from a given dataset. Analyses that do not account for inter-individual differences may underestimate the relationship between hypertrophy and strength gain, and different methods of assessing muscle size will produce different results. It is suggested that robust experimental designs and analysis techniques, which control for different mechanistic sources of strength gain and inter-individual differences (e.g., muscle moment arms, muscle architecture, activation, and normalized muscle force), be employed in future investigations.

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          Most cited references56

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          Ecological Correlations and the Behavior of Individuals

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            The adaptations to strength training : morphological and neurological contributions to increased strength.

            High-resistance strength training (HRST) is one of the most widely practiced forms of physical activity, which is used to enhance athletic performance, augment musculo-skeletal health and alter body aesthetics. Chronic exposure to this type of activity produces marked increases in muscular strength, which are attributed to a range of neurological and morphological adaptations. This review assesses the evidence for these adaptations, their interplay and contribution to enhanced strength and the methodologies employed. The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number. Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response. Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons. Indirect evidence for neurological adaptations, which encompasses learning and coordination, comes from the specificity of the training adaptation, transfer of unilateral training to the contralateral limb and imagined contractions. The apparent rise in whole-muscle specific tension has been primarily used as evidence for neurological adaptations; however, morphological factors (e.g. preferential hypertrophy of type 2 fibres, increased angle of fibre pennation, increase in radiological density) are also likely to contribute to this phenomenon. Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases. Enhanced firing frequency and spinal reflexes most likely explain this improvement, although there is contrary evidence suggesting no change in cortical or corticospinal excitability. The gains in strength with HRST are undoubtedly due to a wide combination of neurological and morphological factors. Whilst the neurological factors may make their greatest contribution during the early stages of a training programme, hypertrophic processes also commence at the onset of training.
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              A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture.

              1. In human pennate muscle, changes in anatomical cross-sectional area (CSA) or volume caused by training or inactivity may not necessarily reflect the change in physiological CSA, and thereby in maximal contractile force, since a simultaneous change in muscle fibre pennation angle could also occur. 2. Eleven male subjects undertook 14 weeks of heavy-resistance strength training of the lower limb muscles. Before and after training anatomical CSA and volume of the human quadriceps femoris muscle were assessed by use of magnetic resonance imaging (MRI), muscle fibre pennation angle (theta(p)) was measured in the vastus lateralis (VL) by use of ultrasonography, and muscle fibre CSA (CSA(fibre)) was obtained by needle biopsy sampling in VL. 3. Anatomical muscle CSA and volume increased with training from 77.5 +/- 3.0 to 85.0 +/- 2.7 cm(2) and 1676 +/- 63 to 1841 +/- 57 cm(3), respectively (+/- S.E.M.). Furthermore, VL pennation angle increased from 8.0 +/- 0.4 to 10.7 +/- 0.6 deg and CSA(fibre) increased from 3754 +/- 271 to 4238 +/- 202 microm (2). Isometric quadriceps strength increased from 282.6 +/- 11.7 to 327.0 +/- 12.4 N m. 4. A positive relationship was observed between theta(p) and quadriceps volume prior to training (r = 0.622). Multifactor regression analysis revealed a stronger relationship when theta(p) and CSA(fibre) were combined (R = 0.728). Post-training increases in CSA(fibre) were related to the increase in quadriceps volume (r = 0.749). 5. Myosin heavy chain (MHC) isoform distribution (type I and II) remained unaltered with training. 6. VL muscle fibre pennation angle was observed to increase in response to resistance training. This allowed single muscle fibre CSA and maximal contractile strength to increase more (+16 %) than anatomical muscle CSA and volume (+10 %). 7. Collectively, the present data suggest that the morphology, architecture and contractile capacity of human pennate muscle are interrelated, in vivo. This interaction seems to include the specific adaptation responses evoked by intensive resistance training.
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                Author and article information

                Contributors
                Journal
                PeerJ
                PeerJ
                peerj
                peerj
                PeerJ
                PeerJ Inc. (San Francisco, USA )
                2167-8359
                27 June 2018
                2018
                : 6
                : e5071
                Affiliations
                [1 ]Department of Biomedical Engineering, Northwestern University , Evanston, IL, United States of America
                [2 ]Department of Health Sciences, City University of New York, Herbert H. Lehman College , Bronx, NY, United States of America
                [3 ]School of Biomedical Sciences, University of Queensland , St. Lucia, Queensland, Australia
                Article
                5071
                10.7717/peerj.5071
                6026459
                0cce7260-d360-4d18-b3d1-11708db8d6a7
                ©2018 Vigotsky et al.

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited.

                History
                : 27 February 2018
                : 4 June 2018
                Funding
                The authors received no funding for this work.
                Categories
                Kinesiology
                Statistics

                hierarchical linear models,repeated measures,strength,hypertrophy,analysis of covariance,regression

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