Variable-cam resistance training machines: do they match the angle - torque relationship in humans?

Area of Science:

• Biomechanics
• Exercise Physiology
• Sports Engineering

Background:

• Variable-cam resistance training machines are widely used in strength training.
• Understanding how machine resistance matches human biomechanics is crucial for effective training.
• Previous research has not fully quantified the discrepancy between machine resistance and joint torque capabilities.

Purpose of the Study:

• To compare the resistance profiles of variable-cam knee extension machines with the in-vivo torque capabilities of human knee extensor muscles.
• To assess the degree of mismatch between machine-generated torque and joint torque across various knee angles.

Main Methods:

• Eight variable-cam knee extension machines from different manufacturers were tested.
• Resistive torque was measured at five knee joint angles for each machine.
• Knee extensor muscle torque was measured isometrically and dynamically in 10 healthy young men at the same angles.
• The angle-torque relationships of the machines and the participants were normalized and compared.

Main Results:

• Human knee extensor torque followed an inverted 'U' shape, with significant dynamic changes (+40% ascending, -60% descending limb).
• Machine resistance profiles were highly variable and consistently less curvilinear than human capabilities.
• Torque changes in machines ranged from +2.5% to +22.2% (ascending) and +37.6% to -20.5% (descending limb).

Conclusions:

• Variable-cam resistance machines do not accurately replicate the natural angle-torque relationship of knee extensor muscles.
• The design of resistance training machines should consider the joint's angle-torque profile and exercise dynamics.
• Optimizing machine design could enhance training efficacy and reduce potential for suboptimal loading patterns.