Journal of Sports Science and Medicine
Journal of Sports Science and Medicine
ISSN: 1303 - 2968   
Ios-APP Journal of Sports Science and Medicine
Androit-APP Journal of Sports Science and Medicine
©Journal of Sports Science and Medicine (2022) 21, 308 - 331   DOI:

Review article
Internal Validity in Resistance Training Research: A Systematic Review
Hubert Makaruk1, , Marcin Starzak2, Maciej Płaszewski3, Jason B. Winchester4
Author Information
1 Department of Physical Education and Sport, Józef Piłsudski University of Physical Education in Warsaw, Faculty of Physical Education and Health, Poland
2 Department of Sports for All, Józef Piłsudski of Physical Education in Warsaw, Faculty of Physical Education and Health, Poland
3 Department of Rehabilitation, Józef Piłsudski University of Physical Education in Warsaw, Faculty of Physical Education and Health, Poland
4 Division of Health Sciences & Human Performance, Concordia University Chicago, USA

Hubert Makaruk
✉ 2 Akademicka, 21-500 Biała Podlaska, Poland
Publish Date
Received: 10-12-2021
Accepted: 23-05-2022
Published (online): 01-06-2022
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Ensuring internal validity is the key procedure when planning the study design. Numerous systematic reviews have demonstrated that considerations for internal validity do not receive adequate attention in the primary research in sport sciences. Therefore, the purpose of this study was to review methodological procedures in current literature where the effects of resistance training on strength, speed, and endurance performance in athletes were analyzed. A computer-based literature searches of SPORTDiscus, Scopus, Medline, and Web of Science was conducted. The internal validity of individual studies was assessed using the PEDro scale. Peer-reviewed studies were accepted only if they met all the following eligibility criteria: (a) healthy male and female athletes between the ages of 18-65 years; (b) training program based on resistance exercises; (c) training program lasted for at least 4 weeks or 12 training sessions, with at least two sessions per week; (d) the study reported maximum strength, speed, or endurance outcomes; and (e) systematic reviews, cohort studies, case-control studies, cross-sectional studies were excluded. Of the 6,516 articles identified, 133 studies were selected for rating by the PEDro scale. Sixty-eight percent of the included studies used random allocation to groups, but only one reported concealed allocation. Baseline data are presented in almost 69% of the studies. Thirty-eight percent of studies demonstrated adequate follow-up of participants. The plan to follow the intention-to-treat or stating that all participants received training intervention or control conditions as allocated were reported in only 1.5% of studies. The procedure of blinding of assessors was also satisfied in only 1.5% of the studies. The current study highlights the gaps in designing and reporting research in the field of strength and conditioning. Randomization, blinding of assessors, reporting of attrition, and intention-to-treat analysis should be more fully addressed to reduce threats to internal validity in primary research.

Key words: Evidence-based practice, research design, strength training, PEDro scale, athletes

           Key Points
  • The implementation of internal validity procedures is often not satisfied in resistance training research.
  • A high risk of bias in resistance training studies was identified in the following criteria: concealed allocations, assessor blinding, and intention-to-treat.
  • Follow-up and eligibility criteria should be widely implemented and reported for future studies.
  • The PEDro scale items may be used to improve the quality of future investigations involving resistance training.
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