Stability and neutral running shoes and their effect on knee joint mechanics
There is an ongoing debate about exactly how a running shoe should be structured, and whether or not certain "types" of footwear can influence running related injury (RRI).
Very recently I was involved in a debate with Professor Benno Nigg and Dr Kevin Kirby at the 25th Biomechanics Summer School in Manchester, U.K.
The struggle between Kevin and I has been titanic (and very entertaining), because he believes "motion control shoes", particularly those with dual density midsoles provide stability to the foot, and are a mainstay for injured runners. I do not believe this (and nor does Prof Nigg), preferring to support the theory that less structured, more flexible running footwear that has a focus on sagittal plane guidance rather than frontal plane "control' is far more likely to be effective.
Very soon you will be able to listen to a podcast series with my great mate Kevin and I on this topic and others. Stay tuned!
More importantly, a brand new paper has just been published, which I believe is a bit of a sleeper, and may well prove a benchmark for future research on this tricky topic.
It is an open access article, and I encourage you to read the full paper here.
If you are too busy, this is the gist of what has been published.
The paper is from a distinguished research faculty including Rana Hinman, Adam Bryant, Kim Bennell and Kade Paterson.
The paper examines higher peak external knee flexion moments (KFM) during running which have been observed in healthy people wearing athletic footwear compared to barefoot, which may increase risk of knee pathologies such as patellofemoral pain.
Footwear features such as pitch (i.e., heel to toe offset and euphemistcally termed "drop' in the footwear industry) and midsole thickness, typically higher in stability compared to neutral footwear, likely influence sagittal plane knee moments. Mechanistically, greater footwear pitch reduces peak ankle dorsiflexion angle and increases peak knee flexion angle, while increased midsole thickness appears to increase knee flexion excursion compared to barefoot.
So, it is plausible that these footwear-related kinematic changes contribute to an elevation in running-related peak KFM
The primary aim of this study was to examine differences in running-related peak KFM between barefoot, stability and neutral footwear in adolescent girls and young women spanning early to post-puberty.
A secondary aim was to determine whether the knee-GRF lever arm, sagittal plane resultant GRF magnitude and sagittal-plane kinematics (i.e., hip flexion angle, knee flexion angle, ankle dorsiflexion angle all at time of peak KFM, knee flexion excursion and knee flexion angle at initial contact) were associated with the change in peak KFM between footwear conditions.
The primary hypothesis was that both the stability and neutral shoes would increase peak KFM compared to barefoot, but that the increase in peak KFM would be less with the neutral shoes.
The study included the 60 early/mid- and late/post-pubertal girls, which is relevant in context of PFP given that this population is generally at higher risk of the condition compared to pre-pubertal girls.
I will not go into the details of the methods for the study, but they are interesting because thay factor in menstrual history and the use or otherwise of the monophasic oral conraceptive pill (MOCP), because these factors influence lower limb biomechanics in this cohort.
I encourage you to read the full paper for the methods!
The actual running task involved the participants completing three successful overground running trials at 2.8–3.2 m/s in each condition: i) barefoot, ii) neutral shoes and, iii) stability shoes, with the order of the trials pre-determined via block randomization.
There is currently no agreed definition for classification of shoes into stability and neutral shoes.
Clearly this is hugely problematic given running footwear recommendation by both retail and medical continues to rely on a system that has no validity or meaning.
Therefore, criteria were developed based on features outlined in the Footwear Assessment Tool (FAT), a protocol published in 2009 which may be read here.
The criteria include increased medial compared to lateral midsole density, and greater stiffness of the heel counter, midfoot and longitudinal aspects of the shoe.
Specifically, stability shoes were deemed to possess:
(i) a midsole that was denser medially than laterally (i.e., medial post),
(ii) < 10° midfoot frontal plane (torsional) stiffness,
(iii) < 10° heel counter stiffness and
(iv) < 45° midfoot sagittal plane (longitudinal) stability.
Based on these criteria, the stability shoes tested in the present study had a score of 9 on the motion control properties scale of the FAT.
In contrast, the neutral shoes were deemed to possess:
(i) a uniform midsole density,
(ii) 10–45° heel counter stiffness
(iii) 10–45° torsional stiffness and
(iv) > 45° midfoot longitudinal stiffness.
In combination, these features contribute to a score of 3 on the motion control properties scale of the FAT . As a result, the Asics Kayano-GS was selected as the stability shoe and the Asics Zaraca 3 as the neutral shoe.
OK.. with that out of the way, what did the study find?
This study found a higher peak KFM during running whilst wearing both stability and neutral shoes compared to barefoot, with no strong evidence of between-shoe differences in this sample. Furthermore, a novel finding of this study was a change in the knee-GRF lever arm is associated with a change in peak KFM wearing shoes compared to barefoot.
There is an incredibly important little red herring in the findings of this study, this being that:
the lack of between-shoe differences in peak KFM suggests that the relevant shoe design features such as footwear pitch did not influence peak KFM
Now THIS, is in contrast to just about every other paper published on this topic since interest was piqued in relation to shoe "drop".
Then there is the real sting in the tail in relation to the implications for footwear design of the future..
The regression analysis revealed an association between a change in peak KFM and a change in the knee-GRF lever arm, rather than lower limb kinematics or the sagittal plane resultant GRF magnitude wearing shoes compared to barefoot.
This novel finding suggests that future footwear modifications aiming to reduce peak KFM should consider shoe design features that have the potential to reduce the knee-GRF lever arm.
Specifically, footwear pitch (i.e., the heel to toe offset,) and midsole density/compliance may be important features contributing to difference in the knee-GRF lever arm.
At the moment, we do not know what the mechanism by which footwear changed the knee-GRF lever arm is, and running-related spatiotemporal variables may be important to include in future studies.
Specifically, examining the association between changes in stride length and knee-GRF lever arm distance between footwear conditions is suggested based on recent evidence demonstrating footwear-related effects on stride length and peak KFM.
So, more evidence that looking at sagittal plane effects of footwear are likely much more important than frontal plane characteristics.
And.. this paper has me bubbling with ideas for new footwear design characteristics!
Director of Bartold Clinical