Co-Contraction Training Induces Muscle Hypertrophy but Not Strength Gains in Older Adults: A Controlled Intervention...

Aging is associated with neuromuscular decline, impairing function and independence. Resistance training mitigates these effects; however, in some contexts, barriers such as access to facilities, transportation, and adherence may limit participation, particularly among socioeconomically disadvantaged and community-dwelling older adults. Maximal voluntary co-contraction resistance training has emerged as a low-cost, equipment-free alternative; however, its effectiveness compared to traditional resistance training remains unclear.

To compare the effects of an 8-week co-contraction resistance training program with traditional resistance training and a control group on functional capacity, strength, and muscle morphology in community-dwelling older adults.

Thirty-four physically active older adults (60-85 years) were randomized into co-contraction (n = 12), traditional (n = 11), or control (n = 11) groups. The co-contraction group performed bilateral isometric co-contractions of the knee extensors and flexors; the traditional group trained on resistance machines at 70% one-repetition maximum. Both training programs were conducted twice weekly for 8 weeks. Muscle strength (isometric and isokinetic torque), morphology (muscle thickness and echo intensity), and functional capacity (sit-to-stand and gait) were assessed pre- and post-intervention. Multivariate and univariate analyses of variance were applied.

The traditional group improved knee extensor strength (p < 0.001), whereas the co-contraction group showed a reduction (p = 0.042). Morphological increases were observed in both interventions: up to 15% in the traditional group and 10% in the co-contraction group. Functional capacity improved across all groups (p < 0.05), with no between-group differences.

Co-contraction resistance training appears feasible and induced muscle hypertrophy in older adults, but without corresponding strength gains. These findings suggest it may be a viable option in contexts where access to resistance machines is limited. Nevertheless, further research is needed to confirm efficacy and define optimal program parameters (frequency and volume) for effective implementation in this population. Trial Registration: ReBEC: U1111-1279-4670.