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Interaction of leg stiffness and surfaces stiffness during human hopping

PainSci » bibliography » Ferris et al 1997
Tags: running, movement, classics, exercise, self-treatment, treatment

Six articles on PainSci cite Ferris 1997: 1. Is Running on Pavement Risky?2. The Complete Guide to IT Band Syndrome3. The Complete Guide to Patellofemoral Pain Syndrome4. Complete Guide to Plantar Fasciitis5. Shin Splints Treatment, The Complete Guide6. Are Orthotics Worth It?

PainSci commentary on Ferris 1997: ?This page is one of thousands in the bibliography. It is not a general article: it is focused on a single scientific paper, and it may provide only just enough context for the summary to make sense. Links to other papers and more general information are provided wherever possible.

For this classic leg springiness experiment, subjects hopped in place at different frequencies on different surfaces. The purpose of this exercise was “to determine whether leg stiffness is adjusted to accommodate surfaces with different properties,” because “we know very little about the biomechanics of locomotion on substrates other than hard and smooth laboratory floors.”

The effect of different surfaces was dramatic: “The stiffness of the leg spring is increased by as much as 3.6-fold to accommodate decreases in surface stiffness.” In other words, when we’re hopping on harder surfaces, we bend our legs a lot more, like a softer spring. We’re so good at it that “many aspects of the hopping mechanics remained remarkably similar despite a > 1,000-fold change in [surface hardness].”

~ Paul Ingraham

original abstract Abstracts here may not perfectly match originals, for a variety of technical and practical reasons. Some abstacts are truncated for my purposes here, if they are particularly long-winded and unhelpful. I occasionally add clarifying notes. And I make some minor corrections.

When mammals run, the overall musculoskeletal system behaves as a single linear “leg spring”. We used force platform and kinematic measurements to determine whether leg spring stiffness (k(leg)) is adjusted to accommodate changes in surface stiffness (ksurf) when humans hoop in place, a good experimental model for examining adjustments to k(leg) in bouncing gaits. We found that k(leg) was greatly increased to accommodate surfaces of lower stiffnesses. The series combination of k(leg) and ksurf [total stiffness (ktot)] was independent of ksurf at a given hopping frequency. For example, when humans hopped at a frequency of 2 Hz, they tripled their k(leg) on the least stiff surface (ksurf = 26.1 kN/m; k(leg) = 53.3 kN/m) compared with the most stiff surface (ksurf = 35,000 kN/m; k(leg) = 17.8 kN/m). Values for ktot were not significantly different on the least stiff surface (16.7 kN/m) and the most stiff surface (17.8 kN/m). Because of the k(leg) adjustment, many aspects of the hopping mechanics (e.g., ground-contact time and center of mass vertical displacement) remained remarkably similar despite a> 1,000-fold change in ksurf. This study provides insight into how k(leg) adjustments can allow similar locomotion mechanics on the variety of terrains encountered by runners in the natural world.

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