BACKGROUND: The current recommendations for resistance training (RT) frequency range from 2 to 5 days per week (days/week) depending on the subjects' training status. However, the relationship between RT frequency and muscular strength remains controversial with reported variances existing across different population groups. We conducted a meta-analysis that (1) quantified the effects of low (LF; 1 day/week), medium (MF; 2 days/week), or high (HF; >= 3 days/week) RT frequency on muscular strength per exercise; (2) examined the effects of different RT frequency on one repetition maximum (1RM) strength gain profiles (multi-joint exercises and single joint exercises); (3) examined the effects of different RT frequency on 1RM strength gain when RT volume is equated; and (4) examined the effects of different RT frequency on 1RM strength gains on upper and lower body. METHODS: Computerised searches were performed using the terms 'strength training frequency', 'resistance training frequency', 'training frequency', and 'weekly training frequency'. After review, 12 studies were deemed suitable according to pre-set eligibility criteria. Primary data were pooled using a random-effects model. Outcomes analysed for main effects were pre- to post strength change with volume-equated studies that combined multi-joint and isolation exercise; isolation-only exercise and untrained subjects only. Heterogeneity between studies was assessed using I2 and Cochran's Q statistics with funnel plots used to assess publication bias and sensitivity analyses calculated for subgroups. RESULTS: Pre- versus post-training strength analysis comprised of 74 treatment groups from 12 studies. For combined multi-joint and isolation exercises, there was a trend towards higher RT frequency compared with lower frequency (mean effect size (ES) 0.09 (95% CI 0.06 to 0.24)) however not significant (p = 0.25). Volume-equated pre- to post-intervention strength gain was similar when LF was compared to HF (mean ES 0.03 (95% CI 0.20 to 0.27); p = 0.78). Upper body pre- to post-intervention strength gain was greater when HF was compared with LF (mean ES 0.48 (95% CI 0.20 to 0.76)) with significant differences between frequencies (p < 0.01). Upper body pre- to post-intervention strength gain was similar when MF was compared with LF (ES 0.12; 95% CI 0.22 to 0.47); p = 0.48). There was no significant difference in lower body mean ES between HF and LF (mean ES 0.21(95% CI 0.55 to 0.13); p = 0.22). There was a trend towards a difference in mean ES between MF and HF (mean ES 0.41(95% CI 0.26 to 1.09); however, the effect was not significant (p = 0.23). CONCLUSIONS: The existing data does not provide a strong correlation between increased weekly training frequency (HF) and maximal strength gain in upper and lower body resistance exercises for a mixed population group. When RT is volume-equated for combined multi-joint and isolation exercises, there is no significant effect of RT frequency on muscular strength gain. More investigations are required to explore the effects of varying weekly training frequencies adequately.

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