Worn out shoes do change the biomechanics of running, but not much

PainSci » bibliography » Kong et al 2009
updated Aug 26, 2025

Tags: running, structuralism, IT band pain, patellofemoral pain, plantar fasciitis, shin pain, devices, shoes & orthoses, foot, fun, exercise, self-treatment, treatment, sports, biomechanical vulnerability, risks, etiology, pro, knee, anatomy, leg, limbs, thigh, pain problems, overuse injury, injury, runners knee, tendinitis, arthritis, aging, knee pain, patellofemoral joint, vibe

Five pages on PainSci cite Kong 2009: 1. Is Running on Pavement Risky? 2. The Complete Guide to IT Band Syndrome 3. The Complete Guide to Patellofemoral Pain Syndrome 4. The Complete Guide to Plantar Fasciitis 5. Shin Splints Treatment, The Complete Guide

Alan Levine

PainSci commentary on Kong 2009: ?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 wherever possible.

When shoes wear out, the biomechanics of running do change. Just not much. Kong et al tested 24 runners before and after 200 miles of road-running in the same pair of shoes. There were a few minor changes: longer stance phase, less forward leaning, and less ankle flexion. However, hip and knee angles were unchanged.

It’s significant that the effects on gait were generally minor, of course, but note the lack of difference in knee angles particularly. Knees are the site of two of the most common runner’s knee injuries, both iliotibial band and patellofemoral syndrome. In general, forces and injury risks increase together with more movement — more bending puts more torque on joints. A deeper knee bend is more stressful than a shallower knee bend. If worn out shoes have no effect on how far knees bend as you run, they aren’t much cause for concern. Although there are surely other biomechanical factors involved in these injuries, it’s unlikely that they are affected much by the condition of your shoes, and it’s reassuring that the most significant factor in overall knee stress — degree of flexion — is probably entirely unaffected.

Also interesting: they compared kinds of shoes — air, gel, spring — and concluded “the adaptation strategies to shoe degradation were unaffected by different cushioning technologies.” So it makes no difference what kind of common cushioning method you have in your shoes — they all degrade and affect running about equally. A natural enemy of the salesman is science proving that there is no difference between products!

Reader T.R. (among several others) pointed out that “200 miles is pretty low on the replacement spectrum. Usual rule of thumb out there is to replace every 300-400 miles (500-700km).” That’s a great point — and it’s probably a serious weakness of this study. It may be that they simply didn’t go far enough. The biomechanical picture could be totally different after 400 miles, particularly given that there’s a good chance that shoe degradation starts slowly and then accelerates:

See, when it starts to fall apart, man it really falls apart
Like boots or hearts oh when they start, they really fall apart.
The Tragically Hip, Boots or Hearts

Nevertheless, the precautionary principle applies, and I do still recommend replacing your shoes when they begin to show obvious signs of wear. The risk of running in decrepit shoes may be small, but there’s not much reason to take that risk — just the modest cost of buying shoes somewhat more often. It’s not like you weren’t going to buy new shoes eventually! On the other hand, this data makes it pretty clear that replacing shoes while they still look fine isn’t really going to make much of a difference.

~ 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.

OBJECTIVES: In this study, the effect of shoe degradation on running biomechanics by comparing the kinetics and kinematics of running in new and worn shoes was investigated. Three types of footwear using different cushioning technologies were compared.

DESIGN: Longitudinal study.

SETTING: Pre- and post-tests on overground running at 4.5 m s(-1) on a 20-m laboratory runway; performance measured using a force platform and a motion capture system.

PARTICIPANTS: 24 runners (14 men and 10 women)

INTERVENTIONS: 200 miles of road running in the same pair of shoes. Within-group factor: shoe condition (new/worn); between-group factor: footwear type (air/gel/spring).

MAIN OUTCOME MEASUREMENTS: Stance time was calculated from force data. External loads were measured by maximum vertical force and loading rate. Kinematic changes were indicated by sagittal plane angles of the torso, hip, knee and ankle at critical events during the stance phase.

RESULTS: Stance time increased (p=0.035) in worn shoes. The torso displayed less maximum forward lean (p<0.001) and less forward lean at toe-off (p<0.001), while the ankle displayed reduced maximum dorsiflexion (p=0.013) and increased plantar flexion at toe-off (p<0.001) in worn shoes. No changes in the hip and knee angles. No between-group difference among the three footwear groups or condition by type interaction was found in any measured variables.

CONCLUSIONS: As shoe cushioning capability decreases, runners modify their patterns to maintain constant external loads. The adaptation strategies to shoe degradation were unaffected by different cushioning technologies, suggesting runners should choose shoes for reasons other than cushioning technology.

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.

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