Interaction of leg stiffness and surfaces stiffness during human hopping
PainSci summary of Ferris 1997?This page is one of thousands in the PainScience.com 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 at the bottom of the page, as often as possible. ★★★★☆?4-star ratings are for bigger/better studies and reviews published in more prestigious journals, with only quibbles. Ratings are a highly subjective opinion, and subject to revision at any time. If you think this paper has been incorrectly rated, please let me know.
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].”
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.
related content
- “Runners adjust leg stiffness for their first step on a new running surface,” an article in J Biomech, 1999.
These six articles on PainScience.com cite Ferris 1997 as a source:
- PS Is Running on Pavement Risky? — Hard-surface running may be risk factor for running injuries like patellofemoral pain, IT band syndrome, shin splints, and plantar fasciitis
- PS Save Yourself from IT Band Syndrome! — All your treatment options for Iliotibial Band Syndrome reviewed in great detail, with clear explanations of recent scientific research supporting every key point
- PS Save Yourself from Patellofemoral Pain Syndrome! — Patellofemoral pain syndrome (aka runner’s knee) explained and discussed in great detail, including every imaginable self-treatment option and all the available scientific evidence
- PS Save Yourself from Plantar Fasciitis! — Plantar fasciitis explained in great detail, including every possible treatment option, and all supported by recent scientific research
- PS Save Yourself from Shin Splints! — Causes and treatment options for shin splints explained and discussed in great detail, especially shin pain caused by myofascial trigger points, compartment syndrome, medial tibial stress syndrome, and stress fracture
- PS Are Orthotics Worth It? — A consumer’s guide to the science and controversies of orthotics, special shoes, and other allegedly corrective foot devices
