Strength Training and Depression: 2025 Data

What is SMD and why is −0.94 a big number?

SMD (standardized mean difference) is a measure of effect size used in meta-analyses that allows results from studies using different depression scales (BDI, PHQ-9, HAM-D) to be compared in a single metric. Cohen's benchmarks: |SMD| below 0.2 — negligible effect; 0.2–0.5 — small; 0.5–0.8 — moderate; above 0.8 — large. The value of −0.94 reported by Chang et al. (2025) represents a meaningful, clinically noticeable result.

For comparison: psychotherapy meta-analyses (e.g., cognitive-behavioural therapy) often show SMD in the range of 0.7–1.0 for depression. Strength training falls in the same zone — which is surprising for most people accustomed to thinking of the gym as a tool for the body, not the mind.

Three 2024–2025 Meta-Analyses: Key Numbers

In 2025, the journal Frontiers in Psychology published a meta-analysis by Chang et al.: 29 randomised controlled trials (RCTs), 2036 participants, with databases covering publications up to August 2024. The pooled SMD for depressive symptoms was −0.94 (95% CI: −1.16 to −0.72, p < 0.001) comparing strength training to a no-exercise control group. The authors identified mechanisms including increased BDNF levels, reduced cortisol, and changes in dopamine and serotonin systems.

Banyard et al. (2025), International Journal of Mental Health Nursing: 26 RCTs, 2681 participants. For depression — SMD = −0.97 (95% CI: −1.28 to −0.66); for anxiety — SMD = −0.66 (95% CI: −1.09 to −0.23). The authors specifically noted that aerobic, strength, and combined exercise all produced significant improvement on both outcomes with no statistically significant differences between formats.

Wang, Liu, and Pan (2025), BMC Sports Science, Medicine and Rehabilitation (PMID: 39849618): 27 RCTs, 2342 participants — studying combined (aerobic plus resistance) protocols for depression. Pooled SMD — −1.39 (95% CI: −1.80 to −0.96, p < 0.001). The optimal regimen according to the authors: 3–4 sessions per week, protocol duration of 9–24 weeks, more than 180 minutes of exercise per week in total; the greatest effect was observed in middle-aged and older adults with moderate depression.

What works better — strength training or cardio?

The data do not point to a clear winner. Banyard et al. (2025) found no statistically significant differences between aerobic and strength training in reducing depressive symptoms: both formats produced a large effect. This is consistent with earlier findings: physical exercise in general is a powerful intervention, and the type of exercise may be less critical than the fact of doing it regularly.

The combined protocol (aerobic plus strength training) in Wang et al. (2025) showed a somewhat larger overall SMD (−1.39), but the sample design there differs from trials of purely strength-based regimens. For young people up to age 26, the meta-analysis by Marinelli et al. (Early Intervention in Psychiatry, 2024, 10 RCTs) also confirmed a significant effect of combined and strength protocols on clinically elevated anxiety and depression — with high quality of evidence for depressive outcomes.

Why exercise works — possible mechanisms

The exact mechanism has not been established, but several candidates recur across reviews. First, BDNF (brain-derived neurotrophic factor): strength training increases its levels in people with depression, and rising BDNF correlates with symptom reduction (Yuping et al., 2024, Depression and Anxiety). Second, cortisol: regular exercise reduces chronic activation of the HPA stress axis, which itself improves mood. Third, dopamine and serotonin: exercise affects the synthesis and sensitivity of monoamines — the same neurotransmitters targeted by antidepressants. Finally, a structural effect: neuroimaging studies detect growth in hippocampal volume with regular exercise — a brain region that is reduced in depression.

Caveats: what to keep in mind

Before treating these numbers as grounds to discontinue medications or psychotherapy, several caveats are necessary. All three meta-analyses include trials with different protocols, rating scales, and populations — heterogeneity is high. Effect sizes vary considerably across subgroups. RCTs of physical exercise are difficult to blind, which creates a risk of placebo effects and effects from social interaction in structured programmes.

For clinical depression, strength training is not a substitute for professional care. The data position it as an evidence-based adjunct that today has a stronger evidence base than many other popular "complementary" interventions.