Effect of 12-week unstable surfaces resistance training on muscle stiffness, muscle co-activation, and balance in older individuals

Objective This randomized controlled trial evaluated the effects of unstable resistance training (URT) and stable resistance training (SRT) on muscle stiffness, co-activation, balance, and pain in older adults. To date, no study has investigated dose-matched instability-based resistance training in this population with prespecified endpoints of co-activation and muscle stiffness. Methods Forty-six older adults were randomly assigned to either the URT ( n = 23) or SRT ( n = 23) group for a 12-week intervention. Both groups performed seven lower-limb exercises with matched volume and intensity, with the URT group utilizing instability devices. Outcomes included muscle–tendon stiffness in the quadriceps and hamstrings, co-activation during squat-down (SD), stand-up (SU), and transition (SUSD). Balance was assessed using the Y-Balance Test (YBT) and Balance Error Scoring System (BESS), and pain was measured with the visual analogue scale (VAS). Results Compared with SRT, URT resulted in greater reductions in co-activation during SD ( p = 0.035), SU ( p = 0.003), and SUSD ( p = 0.001). URT also produced larger improvements in balance, as indicated by BESS ( p = 0.009), YBT-right ( p = 0.008), and YBT-left ( p < 0.001). Increases in muscle stiffness with URT were limited to the vastus medialis, quadriceps tendon, and hamstring tendon (all p < 0.05), whereas SRT produced broader increases in muscle–tendon stiffness (all p < 0.05). VAS scores decreased in both groups, with a greater reduction after URT ( p = 0.009). Conclusion In older adults, URT more effectively reduced co-activation and enhanced balance under matched loads, while SRT increased muscle stiffness across a broader range of muscles. Given that both interventions reduced pain, the choice of intervention should depend on whether the primary objective is to improve coordination and balance or to increase muscle stiffness. Trial registration UMIN Clinical Trials Registry, retrospectively registered. Registration number XXXX.