Recreational running: the pronation-fatigue connection

The problem with over-pronation

While a degree of pronation is a normal and functional part of a healthy running gait, too much pronation – or over-pronation – can significantly increase the risk of injury in runners. Indeed, research has documented that over-pronation in runners can lead to injuries of the foot, ankle, knee, or lower back^(6-9). This is perhaps unsurprising; the excessive torsional forces that occur during an over-pronated running gait are readily transmitted up through the body to the knees, hips and lower back.

What is at the root of an over-pronated running gait? There are a number of reasons but the most common causes are a foot that is inherently flat (ie low arch), weak foot muscles that are less able to maintain arch height and toe strength, and excessively mobility (hypermobility) in the ankle region^(10,11). Given that many runners exhibit one or more of these morphological traits, it’s perhaps not surprising that over-pronation is a common trait among runners (especially recreational or novice runners). Indeed, you only have to visit any running shoe store and see the number of ‘pronation control’ shoes on sale to know that this is the case!

Pronation and fatigue

Because the muscles of the foot are crucial for ensuring the correct foot movement and controlling pronation following foot strike, it stands to reason that muscle fatigue – for example, accumulated fatigue during a longer run – can affect pronation. In short, foot muscle fatigue can be correlated with excessive pronation when running.

To date, a number of studies have confirmed that muscle fatigue in the foot following a period of running can modify foot pronation - because when fatigued, these muscles are not able to provide the same level of foot support^(12-15). What’s far less clear however, whether an extended period of running can induce a tendency to over-pronate and how much running duration is needed to bring this about? Also, can running-induced muscle fatigue induce over pronation in runners who do not show excessive pronation when running on fresh legs? The answers to these questions are important because they can inform injury-prone runners, or those seeking to avoid injury how to best structure or modify their training.

New research

To try and answer these questions, a team of Spanish researchers have carried out an investigation into the relationship between fatigue and pronation in novice runners during running and walking⁽¹⁶⁾. Published last month in the journal ‘Life’, this study studied a sample group of healthy recreational runners to see if there was a tendency for the foot to pronate after a period of running activity spanning 30, 45, and 60 minutes. It also compared the running results with similar durations of walking.

What they did

Thirty six healthy, injury-free recreational runners running at least three times a week for at least one hour were recruited for this study. Importantly, all of these runners were assessed and screened to ensure none of them had an over-pronated running gait (ie their running gait was not compromised by an over-pronation problem). Following a familiarization trial, the runners were asked to carry out two activities on two separate occasions:

·       Running for one hour at a comfortable self-selected speed, wearing their usual running shoes.

·       Walking for one hour at a comfortable self-selected pace (the control condition).

An experienced podiatrist attended all trials and conducted biomechanical examinations on the runners. In particular, two reliable measures of pronation were carried out on both the left and right feet immediately before, and after 30, 45 and 60 minutes of running/walking in each trial.

The first of these measures was the ‘Foot Pronation Index (FPI), which looks at six different foot criteria while the participant is in a comfortable barefoot standing position where they support their feet as they usually do naturally. The FPI is a well validated tool for accurately determining the tendency to pronate⁽¹⁷⁾. The second was the Navicular Drop Test (NDT), which measures the change in height of the tuberosity of the navicular bone on the inner side of the ankle - measured with a tape measure in millimetres (see figure 2). The NDT is an indicator of the height of the middle of the foot arch of the foot when it is flat on the floor, and is a simple, low-cost reliable pronation measurement protocol used in several fields of sport⁽¹⁸⁾.

Figure 2: Measuring navicular drop

The findings from the study

The key finding was that measures of pronation increased very significantly after 45 minutes of running while the increases observed when walking were non-significant. When running, the average change in FPI scores for the right feet rose from 2.6 at the start to 6.2 after 45 minutes and to 7.9 after 60 minutes. For the left feet, the change was similar, being 2.6, 5.1 and 7.4 respectively. Bear in mind that a score over 6.0 in the FPI indicates problematic levels of pronation. The NDT scores while running followed a similar trend, ranging from 4.4 at the start, up to 7.8 after 60 minutes. The NDT running scores became potentially problematical slightly later than the FPI scores - ie after 60 minutes of running. The NDT walking scores meanwhile only rose from 4.4 to a maximum of 5.7 in the walking trial.

At this point, you might be wondering why running gait changed from neutral to hyper-pronated when running (but not when walking)? The answer of course is fatigue; as foot muscle fatigue accumulates, the ability to maintain the optimum foot posture and function steadily declines. Particularly impacted is the ability of the muscles to maintain arch height (ie avoid navicular drop).

Implications for runners

In their conclusions, the authors of the above study stated that recreational non-pronating runners showed a tendency to problematical levels of foot pronation (ie at levels that are known to increase injury risk) after just 45 minutes of running activity. By contrast, while the same duration of walking activity produced an increase in foot pronation, this did not result in hyper-pronation (ie injury risk was not significantly increased).

What does this mean for recreational runners who are trying to stay injury free? The first and most obvious point to make is that even in runners with fairly neutral (non-pronating) gait, run durations of 45 minutes or more will likely induce enough additional pronation to raise the risk of injury. Therefore, if you are a recreational runner who runs just for fitness or weight loss, you might be best to keep your run duration under 45 minutes, and add in additional aerobic activities such as cycling, swimming, rowing etc if you wish to train more.

Secondly, recreational runners who are training for longer distances such as the half and full marathon should consider using a ‘pronation control’ running shoe - even if they have a fairly neutral ‘non-pronating’ running gait. That’s because the longer runs will greatly exceed 45 minutes’ duration, which means a lot of accumulated time will be spent running with an excessively pronated gait.

The third point is that needless to say, recreational runners who have a relatively flat arch to begin with (ie over pronate even when running on fresh legs) are particularly vulnerable to the accumulated fatigue effect. This means that top quality pronation control shoes are recommended, even for shorter duration runs. In addition, pronating runners with a previous history of running related injuries may also want to consider using orthotic inserts – either custom made or ‘off the shelf’ (see this article).

In the longer term, there are benefits to be had from a program of foot muscle strengthening exercises. The muscles of the feet are essentially no different to any other skeletal muscles in the body, which means they will respond to strength training. Stronger feet muscles can not only help to develop a higher arch (ie reduce flat footedness), but will also make the structure more resistant and resilient to the effects of fatigue. A quick Google search will reveal many such exercises online, but this excellent video presentation by Dr Jared Beckstrand is an excellent place to start! - www.youtube.com/watch?v=MHqMLLjmDPI

References

1.       Am J Sports Med. 2018 Jul;46(9):2211-2221

2.       PLoS One. 2023 Aug 17;18(8):e0288814. doi: 10.1371/journal.pone.0288814. eCollection 2023

3.       Foot Ankle Orthop. 2022;7:24730114221125455

4.       Gait Posture. 2022;93:20–25

5.       Gait Posture. 2018;65:65–72

6.       Acta Bioeng. Biomech. 2022;24:21–30

7.       Gait Posture. 2018;65:65–72

8.       J. Am. Podiatr. Med. Assoc. 2016;106:109–115

9.       Knee Surg. Sports Traumatol. Arthrosc. 2016;24:786–796

10.     Gait Posture. 2022 Mar:93:20-25

11.     BMC Musculoskelet Disord. 2014 May 23:15:171

12.     Gait Posture. 2021;88:132–137

13.     Foot Ankle Orthop. 2022;7:24730114221125455

14.     J. Electromyogr. Kinesiol. 2008;18:420–425

15.     J. Sports Med. Phys. Fitness. 2017;57:887–899

16.     Life (Basel). 2023 Nov 12;13(11):2202. doi: 10.3390/life13112202

17.     The Foot Posture Index. Clin. Biomech. 2006;21:89–98

18.     J. Foot Ankle Res. 2018;11:15