Exploring the Biomechanics of Human Force in Pushing
A new paper from MechE’s Newman Lab explores the limiting factors in human force generation
Most human physical interaction tasks require the production of force, but there are factors, including the mechanical stability of the given task and the person’s body configuration, that can compromise force production. Consider actions like turning a screwdriver or lifting weights at the gym where, “the generation of force also produces a destabilizing action which has the tendency to push the limbs away from their nominal posture.”
In these examples, which are drawn from a paper by Federico Tessari, research scientist in the MIT Department of Mechanical Engineering’s Eric P. and Evelyn E. Newman Laboratory for Biomechanics and Human Rehabilitation and Neville Hogan, the Sun Jae Professor in Mechanical Engineering at MIT, the researchers note that small deviation around the direction of force generation creates a de-stabilizing force. If not compensated, the deviation will tend to increase, leading to task failure.
The pair devised an experiment to answer the question: what are the factors limiting force generation? Their new paper, published in the journal Nature Communications, shows that the ability to generate mechanical stabilizing stiffness, not raw muscle strength, is the primary bottleneck in how hard humans can push.
“Using a custom gimbal apparatus that allowed us to systematically control wrist torque transmission, we measured maximum voluntary pushing forces in healthy participants across two distinct arm configurations,” says Tessari. “The results were surprising: reducing the ability to generate stabilizing stiffness caused a substantial drop in maximum force output, while changes in arm configuration had minimal effect.”
The researchers say the work reframes the understanding of human physical performance, showing that force generation is not just a muscle strength problem but a mechanical stability problem. The findings can indirectly inform therapy protocols for stroke survivors, spinal cord injury patients, and individuals with neuromuscular disorders, and may also reshape thinking about safe and effective physical coupling between humans and machines.
“The work provides key principles for assistive and rehabilitation devices that need to replicate or augment human force generation and offers new criteria for assessing posture and tool use in physically demanding tasks,” says Tessari.
The paper, Stabilizing stiffness is the most limiting factor in human force exertion, is available now via the journal Nature Communications. Hogan and Tessari also recently received the IEEE International Conference on Cyborg and Bionic Systems Best Paper Award for the robotic application of their findings in a related conference paper titled "Force Control Requires Stiffness Modulation During Unstable Contact Tasks."
