Grgic et al. 2018: Effect of Resistance Training Frequency on Hypertrophy

Purpose

The frequency-versus-volume question has long been one of the most disputed parameters in resistance training prescription. Anecdotal coaching practice has variously favored low-frequency body-part splits (training each muscle once weekly) and high-frequency full-body schedules (three or more sessions per muscle per week). Grgic and colleagues set out to test whether frequency itself — once total weekly volume is held constant — produces measurable differences in strength outcomes.

Design

This was a PRISMA-aligned systematic review and meta-analysis. The authors searched five databases for randomized trials comparing different resistance training frequencies in adults, with strength as a primary outcome. Twenty-two studies met inclusion criteria. The key analytic move was to stratify by whether weekly volume (sets per muscle group per week) was equated between the compared frequency conditions. When volume was equated, the comparison isolates the effect of frequency. When it was not, any apparent frequency effect is confounded by volume.

Key Findings

In the volume-equated subset, the pooled effect of higher frequency on strength gains was small and not statistically significant. Effect sizes for one-day-per-week versus two- or three-day-per-week conditions overlapped substantially. The authors concluded that frequency, in isolation, is not an independent driver of strength adaptation when total weekly work is matched. In the non-volume-equated comparisons, higher frequency conditions tended to produce larger gains, but this is attributable to the additional volume those conditions accumulated rather than the frequency itself.

Limitations

Most included trials ran 6 to 12 weeks, which is short relative to the typical career arc of a trained lifter. Long-term differences in fatigue management, joint stress, and adherence between frequency schemes are not well captured by short trials. Trained-subject data are sparser than novice data. The strength-outcome focus also leaves the hypertrophy question to companion analyses (notably Schoenfeld et al. 2016, which reaches a similar volume-driven conclusion). Heterogeneity in exercise selection, intensity, and proximity to failure across studies introduces noise that the meta-analytic average smooths over.

Takeaway

The practical implication is that frequency should be treated as a logistical variable — a way to distribute a target weekly volume across the week — rather than as an independent training parameter. A lifter who trains a muscle once weekly with 16 hard sets is not, on average, at a disadvantage relative to one who splits the same 16 sets across two or three sessions, provided recovery and exercise quality are acceptable. Frequency choices should be driven by recovery, schedule, exercise selection, and adherence, with volume as the primary lever for adaptation.