The mid-stance or mid-support has been defined as much as a phase or event within the race cycle. However, it did not identify a specific definition or a biomechanical indicator that would allow it to be identified objectively.
Ciacci, S. et al (2010) identifies this event as the transition between braking and propulsion (BP-PPTRANS), concluding that a standardization of kinematic criteria is required for the evaluation of BP-PPTRANS.
Ciacci, S., Di Michele, R., & Merni, F. (2010). Kinematic analysis of the braking and propulsion phases during the support time in sprint running. Gait & posture, 31(2), 209–212. https://doi.org/10.1016/j.gaitpost.2009.10.007
Thanks for your interesting question, Felipe. I guess that midstance is a phase during running where the lower limb's function is to support the body as it moves from the loading phase in early stance to the unloading phase in late stance. There are several gait events that occur during the midstance phase, and which could be used as a single, specific descriptor of midstance. These would typically include, for example, a peak vertical GRF (although some athletes have a higher vGRF at impact), the knee at its most flexed, a 'flattened' foot position and, as you mention, the transition between braking and propulsion.
I think that all of these have merit as objective measures of 'midstance' given that which one is most useful depends on the measurement system being used (force plate, videography, pressure pad, optoelectronics), or the aim of the study. For example, I have previously used the braking-propulsion transition to define midstance in race walking, which is particularly useful in that event given the abnormal movements used (Article Ground reaction forces of Olympic and World Championship race walkers
) although I have also used 'a visually chosen position where the athlete’s foot was directly below the hip, used to determine the “vertical upright position"' in a paper on race walk judging because, although more subjective, this was felt the most ecologically valid regarding what the human eye would consider to be midstance (Article Assessment of IAAF Racewalk Judges' Ability to Detect Legal ...
).In terms of analysing running in competitive situations where force plate data are unavailable, I have used horizontal coordinate values via manual digitizing to determine midstance (defined as 'the instant during stance where the athlete’s foot center of mass was directly below the CM' in both road running: Article Men's and Women's World Championship Marathon Performances a...
and track runningArticle Changes in running biomechanics during the 2017 IAAF World C...
).These gait events are unlikely to describe exactly the same instant during midstance (it is more difficult to be precise in any case with video data) but are likely to be very close in timing during running and reflect some of the varied methods used in gait analysis. I hope this is helpful.
Thank you very much for your answer Brian Hanley , I think it is crucial what you say about the measuring instrument that is available and the context in which it is evaluated. We had come to a similar idea with a colleague, based on identifying mid-stance based on kinetic, kinematic or EMG variables. So I will continue in that search and be able to have greater clarity for the definition of this event.