Effects on Force, Velocity, Power, and Muscle Activation of Resistances with Variable Inertia Generated by Programmable Electromechanical Motors During Explosive Chest Press Exercises

Strength training machines incorporating advanced electro-mechanical technologies can produce hybrid resistances with variable inertia, such as a resistance that progressively changes from gravitational (inertial) to pneumatic (non-inertial) across the range of motion (ROM). To explore the biomechanical effects of these innovative resistances, a robotic chest press machine was programmed to offer three distinct inertial profiles: gravitational-type constant inertia throughout the ROM (I_FULL); no inertia (I_ZERO); and linearly descending inertia across the ROM (I_VAR). Ten healthy adults performed five maximal-effort, explosive chest press movements under each inertial profile at 30, 50 and 70% of their one-repetition maximum. During each trial, muscle activity of the pectoralis major, anterior deltoid, and triceps brachii was recorded, along with force, velocity and power outputs from the machine. Statistical non-parametric maps based on two-way repeated measures ANOVA were used to assess the effects of load level and inertial profile on the collected time series. Higher load levels consistently led to increased force and reduced velocity and power outcomes over large parts of the ROM. Compared to I_FULL, I_ZERO allowed for greater velocity at the expense of lower force throughout the ROM, while I_VAR produced higher force and power outputs despite having lower velocity than I_ZERO. Additionally, both I_ZERO and I_VAR significantly increased triceps brachii activity at the end of the ROM compared to I_FULL. I_VAR outperformed both I_FULL and I_ZERO in terms of force and power. Coaches and therapists are advised to consider variable inertial profiles as a key parameter when designing exercise programs for athletes or patients.