Sports Medicine

, Volume 48, Issue 1, pp 1–6 | Cite as

Correlations Do Not Show Cause and Effect: Not Even for Changes in Muscle Size and Strength

  • Scott J. Dankel
  • Samuel L. Buckner
  • Matthew B. Jessee
  • J. Grant Mouser
  • Kevin T. Mattocks
  • Takashi Abe
  • Jeremy P. Loenneke
Current Opinion


It is well known that resistance exercise results in increased muscle strength, but the cause of the improvement is not well understood. It is generally thought that initial increases in strength are caused by neurological factors, before being predominantly driven by increases in muscle size. Despite this hypothesis, there is currently no direct evidence that training-induced increases in muscle size contribute to training-induced increases in muscle strength. The evidence used to support this hypothesis is exclusively correlational analyses and these are often an afterthought using data collected to answer a different question of interest. Not only do these studies not infer causality, but they have inherent limitations associated with measurement error and limited inter-individual variability. To answer the question as to whether training-induced increases in muscle size lead to training-induced increases in strength requires a study designed to produce differential effects on muscle size based on group membership (i.e., one group increases muscle size and one does not) and observe how this impacts muscle strength. We have performed studies in our laboratory in which muscle strength increases similarly independent of whether muscle growth is or is not present, illustrating that the increases in muscle strength are not likely driven by increases in muscle size. The hypothesis that training-induced increases in muscle size contribute to training-induced increases in muscle strength requires more appropriately designed studies, and until such studies are completed, this statement should not be made as there are no data to support this hypothesis.


Compliance with ethical standards


No sources of funding were received for the preparation of this article.

Conflict of interest

Scott J. Dankel, Samuel L. Buckner, Matthew B. Jessee, J. Grant Mouser, Kevin T. Mattocks, Takashi Abe, and Jeremy P. Loenneke have no conflicts of interest directly relevant to the content of this article.


  1. 1.
    Balshaw TG, Massey GJ, Maden-Wilkinson TM, Morales-Artacho AJ, McKeown A, Appleby CL, et al. Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training. Eur J Appl Physiol. 2017;117:631–40.CrossRefPubMedGoogle Scholar
  2. 2.
    Erskine RM, Fletcher G, Folland JP. The contribution of muscle hypertrophy to strength changes following resistance training. Eur J Appl Physiol. 2014;114:1239–49.CrossRefPubMedGoogle Scholar
  3. 3.
    Moritani T, deVries HA. Neural factors versus hypertrophy in the time course of muscle strength gain. Am J Phys Med. 1979;58:115–30.PubMedGoogle Scholar
  4. 4.
    Timson BF. Evaluation of animal models for the study of exercise-induced muscle enlargement. J Appl Physiol. 1990;69:1935–45.CrossRefPubMedGoogle Scholar
  5. 5.
    Duncan ND, Williams DA, Lynch GS. Adaptations in rat skeletal muscle following long-term resistance exercise training. Eur J Appl Physiol. 1998;77:372–8.CrossRefGoogle Scholar
  6. 6.
    Roy RR, Wilson R, Edgerton VR. Architectural and mechanical properties of the rat adductor longus: response to weight-lifting training. Anat Rec. 1997;247:170–8.CrossRefPubMedGoogle Scholar
  7. 7.
    Tamaki T, Uchiyama S, Nakano S. A weight-lifting exercise model for inducing hypertrophy in the hindlimb muscles of rats. Med Sci Sports Exerc. 1992;24:881–6.CrossRefPubMedGoogle Scholar
  8. 8.
    Wisløff U, Castagna C, Helgerud J, Jones R, Hoff J. Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. Br J Sports Med. 2004;38:285–8.CrossRefPubMedPubMedCentralGoogle Scholar
  9. 9.
    Newman AB, Kupelian V, Visser M, Simonsick EM, Goodpaster BH, Kritchevsky SB, et al. Strength, but not muscle mass, is associated with mortality in the health, aging and body composition study cohort. J Gerontol A Biol Sci Med Sci. 2006;61:72–7.CrossRefPubMedGoogle Scholar
  10. 10.
    Mattocks KT, Buckner SL, Jessee MB, Dankel SJ, Mouser JG, Loenneke JP. Practicing the test produces strength equivalent to higher volume training. Med Sci Sports Exerc. 2017. doi: 10.1249/MSS.0000000000001300 (Epub ahead of print).
  11. 11.
    Dankel SJ, Counts BR, Barnett BE, Buckner SL, Abe T, Loenneke JP. Muscle adaptations following 21 consecutive days of strength test familiarization compared with traditional training. Muscle Nerve. 2017;56:307–14.CrossRefPubMedGoogle Scholar
  12. 12.
    Mitchell CJ, Churchward-Venne TA, West DWD, Burd NA, Breen L, Baker SK, et al. Resistance exercise load does not determine training-mediated hypertrophic gains in young men. J Appl Physiol. 2012;113:71–7.CrossRefPubMedPubMedCentralGoogle Scholar
  13. 13.
    Counts BR, Buckner SL, Dankel SJ, Jessee MB, Mattocks KT, Mouser JG, et al. The acute and chronic effects of “NO LOAD” resistance training. Physiol Behav. 2016;164:345–52.CrossRefPubMedGoogle Scholar
  14. 14.
    Sale DG, Martin JE, Moroz DE. Hypertrophy without increased isometric strength after weight training. Eur J Appl Physiol. 1992;64:51–5.CrossRefGoogle Scholar
  15. 15.
    Balshaw TG, Massey GJ, Maden-Wilkinson TM, Tillin NA, Folland JP. Training-specific functional, neural, and hypertrophic adaptations to explosive- vs. sustained-contraction strength training. J Appl Physiol. 2016;120:1364–73.CrossRefPubMedGoogle Scholar
  16. 16.
    Erskine RM, Fletcher G, Hanson B, Folland JP. Whey protein does not enhance the adaptations to elbow flexor resistance training. Med Sci Sports Exerc. 2012;44:1791–800.CrossRefPubMedGoogle Scholar
  17. 17.
    Goodwin LD, Leech NL. Understanding correlation: factors that affect the size of r. J Exp Educ. 2006;74:249–66.CrossRefGoogle Scholar
  18. 18.
    Abe T, DeHoyos DV, Pollock ML, Garzarella L. Time course for strength and muscle thickness changes following upper and lower body resistance training in men and women. Eur J Appl Physiol. 2000;81:174–80.CrossRefPubMedGoogle Scholar
  19. 19.
    Weir JP. Quantifying test–retest reliability using the intraclass correlation coefficient and the SEM. J Strength Cond Res. 2005;19:231–40.PubMedGoogle Scholar
  20. 20.
    Atkinson G, Batterham AM. True and false interindividual differences in the physiological response to an intervention. Exp Physiol. 2015;100:577–88.CrossRefPubMedGoogle Scholar
  21. 21.
    Hopkins WG. Individual responses made easy. J Appl Physiol. 2015;118:1444–6.CrossRefPubMedGoogle Scholar
  22. 22.
    Ahtiainen JP, Walker S, Peltonen H, Holviala J, Sillanpää E, Karavirta L, et al. Heterogeneity in resistance training-induced muscle strength and mass responses in men and women of different ages. Age Dordr Neth. 2016;38:10.CrossRefGoogle Scholar
  23. 23.
    Bland JM, Altman DG. Statistics notes: calculating correlation coefficients with repeated observations. Part 1: correlation within subjects. BMJ. 1995;310:446.CrossRefPubMedPubMedCentralGoogle Scholar
  24. 24.
    Marzban C, Illian PR, Morison D, Mourad PD. Within-group and between-group correlation: illustration on noninvasive estimation of intracranial pressure. (2013). Accessed 29 May 2017.

Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  • Scott J. Dankel
    • 1
  • Samuel L. Buckner
    • 1
  • Matthew B. Jessee
    • 1
  • J. Grant Mouser
    • 1
  • Kevin T. Mattocks
    • 1
  • Takashi Abe
    • 1
  • Jeremy P. Loenneke
    • 1
  1. 1.Department of Health, Exercise Science, and Recreation Management, Kevser Ermin Applied Physiology LaboratoryUniversity of MississippiUniversityUSA

Personalised recommendations