Fatigue-Related Biomechanical Changes During a Half-Marathon Under Field Conditions Assessed Using Inertial Measurement Units

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The introduction section needs to do a better job of justifying the importance of the study outcome measures to overuse injury in runners. Only two sentences (lines 46-51) justify importance of three outcomes with little substance. Provide detail of why these measures are important from the studies referenced. The authors provide nine biomechanical outcome measures in this study which is a bit overwhelming. I suggest reducing to the 2-4 most important measures which address a specific gap in knowledge.

I am not a fan of the abbreviations used. REA and evVel are very different abbreviations for very similar measures (eversion angle and eversion velocity). Why do you even need abbreviations, simply saying eversion angle does not take up much space.

The authors need to provide a more thorough review of what is known about running changes with fatigue. They provide a few studies but do not discuss them fully or clearly. Line 57-58 mentions PTA decreases across time whereas later down line 64 says it increased in another study. Reorganize this paragraph for clarity and communicate to the reader what is known and what is unknown. If the true gap in knowledge is inconsistency in literature, why will this study help clear things up and not just be another study which contrasts prior lit. In other words, why do you think there is inconsistency and what improvements in testing methods do you provide to address it to better answer the question “does running injury risk increase with time during a fatiguing run” and “can we assess fatigue-related changes in injury risk using wearables in real-world settings”. I suggest talking about peak GRF and loading rate along with PTA, which in my opinion is the more accurate measure of tibial loading (albeit more expensive to measure).

Tied to my last comment, provide justification for hypothesis two. You mentioned some studies found PTA to decrease, why do you expect it to increase?

In my opinion, hypothesis 3 is not supported by the introduction. Why would muscle fatigue cause peak ankle velocity in the frontal and sagittal planes to increase? Why would foot strike angle decrease? Why are any of these measures important? This study would be much stronger if the authors meticulously chose ~3 outcomes which quantify specific qualities of running mechanics.

Line 95 and other hypothesis statements – suggest saying “increase with time during a half marathon” instead of “running duration”, which is not specific.

The IMU used seems to be the sensor alone, did you develop a custom device to interact with the sensor and log data? If so what did that consist of (microcontroller etc.)?

I don’t understand why you applied a low-pass filter (line 151) and high-pass filter (line 156) with the same cutoff frequency to the accelerometer data for identifying ICs.

In figure 1, it looks like the onset of the first peak was the IC, but this sentence (line 156) makes it seem like the peak was IC. Which is it? How valid is this method? How accurate is your method of toe off detection (line 161)?

Would eversion be the angular difference between the tibia and foot? It seems like you are calling eversion just the angle of the foot. Why does peak sagittal plane velocity of the foot sensor quantify roll over (which I thought was related to eversion/inversion)? How was foot strike angle calculated? Obviously static assumptions would not be possible so that you cannot use the IMU as an inclinometer like is done with balance testing.

There is a lot of statistics presented. All statistics should test a specific hypothesis (which could be reduced). Analysis like lines 197-200 do not relate to a hypothesis and should be removed.

These figures are very nice.

Did running speed change between laps? That might explain differences in ground contact time and duty factor.

Some sentences in the discussion are overstatements. You cannot say based on the data collected that there was a reduction in performance and increased risk for injury (line 256). Your data suggests muscle fatigue, but it does not indicate muscle fatigue as that was not measured.

Potential typo in lines 291-293; stating increase in PTA but baseline is higher than end of run value. Which is correct?

The discussion section is way too long. If you shorten it to concise points the reader will engage and comprehend better. I suggest 1-2 paragraphs that discuss the findings of each hypothesis/quality of movement, compares with prior literature, discusses why contradicting findings may exist, and identify areas of future work.

Author Response

Response to Reviewer 1 Comments

Comments and Suggestions for Authors

The introduction section needs to do a better job of justifying the importance of the study outcome measures to overuse injury in runners. Only two sentences (lines 46-51) justify importance of three outcomes with little substance. Provide detail of why these measures are important from the studies referenced. The authors provide nine biomechanical outcome measures in this study which is a bit overwhelming. I suggest reducing to the 2-4 most important measures which address a specific gap in knowledge.

Response: We appreciate this important comment. The primary aim of our study was not to establish direct associations between the measured parameters and running injuries, but rather to demonstrate the feasibility of using IMUs in the field - even over longer periods - to capture a broad range of biomechanical data. For this reason, the manuscript was submitted to the Special Issue “Inertial Sensor Assessment of Human Movement”. In addition, we aimed to show how running biomechanics change due to fatigue and that IMUs are sensitive enough to detect these fatigue-related alterations. The inclusion of multiple parameters was therefore intentional, as it highlights the potential of IMUs to collect measures that have already proven valuable in controlled laboratory settings. The two sentences in lines 46–51 referring to overuse injuries were intended only as illustrative examples of potential applications of IMUs, rather than as the primary focus of this study. By showing that these parameters can also be reliably obtained during a half-marathon in the field, our study contributes to bridging the gap between laboratory research and real-world running conditions.

To make the aims of the study clearer and to avoid any misunderstandings, we have revised the rationale section accordingly.

I am not a fan of the abbreviations used. REA and evVel are very different abbreviations for very similar measures (eversion angle and eversion velocity). Why do you even need abbreviations, simply saying eversion angle does not take up much space.

Response: Thank you very much for this helpful suggestion. You are right that the abbreviations may not be entirely consistent, and we agree that simplification improves readability. Although REA is a commonly used abbreviation, we have revised it to evA (eversion angle of the foot) to be more intuitive and aligned with evVel (eversion velocity).

The authors need to provide a more thorough review of what is known about running changes with fatigue. They provide a few studies but do not discuss them fully or clearly. Line 57-58 mentions PTA decreases across time whereas later down line 64 says it increased in another study. Reorganize this paragraph for clarity and communicate to the reader what is known and what is unknown. If the true gap in knowledge is inconsistency in literature, why will this study help clear things up and not just be another study which contrasts prior lit. In other words, why do you think there is inconsistency and what improvements in testing methods do you provide to address it to better answer the question “does running injury risk increase with time during a fatiguing run” and “can we assess fatigue-related changes in injury risk using wearables in real-world settings”. I suggest talking about peak GRF and loading rate along with PTA, which in my opinion is the more accurate measure of tibial loading (albeit more expensive to measure).

Response: Thank you for your comment. As mentioned before, the focus of our work was not on investigating injuries but on methodological aspects. At the beginning of this section, we highlight that only a few field studies exist that have examined parameters such as PTA, and that their findings are inconsistent. We then point out that laboratory studies have also investigated these parameters (e.g., PTA), likewise reporting inconsistent results. From line 67 onward, we describe the different methodologies applied in these studies, which may explain the varying outcomes (e.g., differences between laboratory and field measurements).

A detailed discussion of why the results for specific parameters are inconsistent is provided later in the Discussion, also in relation to our own findings. Otherwise, this would have led to duplication and an overly long Introduction.

To provide additional clarity, we have also revised the aims of the study.

Tied to my last comment, provide justification for hypothesis two. You mentioned some studies found PTA to decrease, why do you expect it to increase?

Response: Thank you for your comment. It is correct that the current literature reports different and sometimes contradictory findings for several parameters under fatigue (e.g., PTA has been reported to remain unchanged, to decrease, or to increase). When formulating our hypotheses, we based our expectations on what we considered most likely to occur and on findings from studies that also observed fatigue effects in distance runners [e.g., 12].

[12] Derrick, T.R.; Dereu, D.; McLean, S.P. Impacts and kinematic adjustments during an exhaustive run. Medicine & Science in Sports & Exercise 2002, 34, 998–1002.

In my opinion, hypothesis 3 is not supported by the introduction. Why would muscle fatigue cause peak ankle velocity in the frontal and sagittal planes to increase? Why would foot strike angle decrease? Why are any of these measures important? This study would be much stronger if the authors meticulously chose ~3 outcomes which quantify specific qualities of running mechanics.

Response: Thank you for your comment. For example, it has been shown that eversion velocity can significantly increase with progressive muscular fatigue (line 64) [e.g., 12]. Based on this fatigue-related mechanism, we also expected an increase in the peak angular velocity of the foot in the sagittal plane, since, for instance, the tibialis anterior may lack the ability to actively decelerate plantarflexion after initial ground contact. This point is also addressed later in the Discussion (line 347). As mentioned before, the aim of this study was to demonstrate that such biomechanical parameters during a prolonged run can be used to identify fatigue-related effects. However, several of these parameters (e.g., peak angular velocity of the foot in the sagittal plane) have not been studied during prolonged endurance runs. Therefore, we formulated our hypotheses based on what would be expected in the context of muscular fatigue.

[12] Derrick, T.R.; Dereu, D.; McLean, S.P. Impacts and kinematic adjustments during an exhaustive run. Medicine & Science in Sports & Exercise 2002, 34, 998–1002.

Line 95 and other hypothesis statements – suggest saying “increase with time during a half marathon” instead of “running duration”, which is not specific.

Response: Thank you for pointing this out. We have adjusted the sentence for clarity.

The IMU used seems to be the sensor alone, did you develop a custom device to interact with the sensor and log data? If so what did that consist of (microcontroller etc.)?

Response: Thank you for your comment. To clarify, we used a custom-built measurement system consisting of four commercial IMUs (ICM-20601, InvenSense). The sensors were connected via cables to a storage unit worn on a belt, where the data were recorded for later processing. The IMUs were connected via serial peripheral interface (SPI) to a microcontroller (PIC18F47J13, Microchip). The microcontroller read the raw data from each IMU and stored them, also via SPI, directly on a micro-SD card for later processing. This setup was specifically designed for our running studies but did not require the development of new sensor hardware beyond the integration of the IMUs with the data logger.

I don’t understand why you applied a low-pass filter (line 151) and high-pass filter (line 156) with the same cutoff frequency to the accelerometer data for identifying ICs.

Response: Thank you for pointing this out. To clarify, in the first step (line 151), all signals were low-pass filtered to reduce minor noise in the acceleration (80 Hz) and angular velocity (50 Hz) data, which is a standard procedure in IMU data analysis. Subsequently, only the vertical axis of the accelerometer placed at the heel cap was high-pass filtered at 80 Hz (line 156). This filtering step was applied exclusively for the detection of initial ground contact and was not used for any further calculations.

In figure 1, it looks like the onset of the first peak was the IC, but this sentence (line 156) makes it seem like the peak was IC. Which is it? How valid is this method? How accurate is your method of toe off detection (line 161)?

Response: Thank you for this valuable comment and the opportunity to clarify. After applying the 80 Hz high-pass filter to the vertical acceleration signal of the heel cap mounted IMU, the signal characteristic is substantially altered and therefore not directly comparable to Figure 1b (black line). The peak in the high-pass filtered signal represents the initial ground contact, which corresponds to the turning point (beginning of the increase) in the unfiltered heel acceleration signal (see: [36]). For this reason, we indicated this time point in Figure 1b. The accuracy of this approach was investigated in a previous study [36], which demonstrated very high agreement with a reference system (force plate). In addition, the method for detecting toe-off was examined in another study [37] and validated against foot switches. The results showed that it can be considered acceptable and reliable, particularly in the context of gait analysis.

[36] Mitschke, C.; Heß, T.; Milani, T. Which Method Detects Foot Strike in Rearfoot and Forefoot Runners Accurately when Using an Inertial Measurement Unit? Applied Sciences 2017, 7, 959, doi:10.3390/app7090959

[37] Sabatini, A.M.; Martelloni, C.; Scapellato, S.; Cavallo, F. Assessment of walking features from foot inertial sensing. IEEE Trans. Biomed. Eng. 2005, 52, 486–494, doi:10.1109/TBME.2004.840727.

Would eversion be the angular difference between the tibia and foot? It seems like you are calling eversion just the angle of the foot. Why does peak sagittal plane velocity of the foot sensor quantify roll over (which I thought was related to eversion/inversion)? How was foot strike angle calculated? Obviously static assumptions would not be possible so that you cannot use the IMU as an inclinometer like is done with balance testing.

Response: Thank you very much for your comment. Regarding the first point: Eversion velocity was derived solely from the angular velocity of the IMU attached to the heel cap, i.e., the angular velocity of the foot/shoe in space. A previous study has shown that this angular velocity is highly consistent with the motion between the shank and the foot/shoe [39].

Regarding the second point: This needs to be considered in a differentiated manner. Eversion angle and eversion velocity occur in the frontal plane and are part of the damping mechanism (pronation/supination). In contrast, angular velocity in the sagittal plane is independent of this and primarily describes the forward rolling motion of the foot, which has also been associated with the perception of “ride” [33].

Regarding the third point: The methodology for calculating the foot strike angle was described in detail in a previous study [40], to which we refer here in order to avoid exceeding the scope of this manuscript. Briefly, the foot is in the plantar phase for a short period during stance, and this time point is used for drift correction.

[33] Bräuer, S.; Kiesewetter, P.; Milani, T.L.; Mitschke, C. The 'Ride' Feeling during Running under Field Conditions-Objectified with a Single Inertial Measurement Unit. Sensors (Basel) 2021, 21, doi:10.3390/s21155010.

[39] Mitschke, C.; Öhmichen, M.; Milani, T. A Single Gyroscope Can Be Used to Accurately Determine Peak Eversion Velocity during Locomotion at Different Speeds and in Various Shoes. Applied Sciences 2017, 7, 659, doi:10.3390/app7070659.

[40] Mitschke, C.; Zaumseil, F.; Milani, T.L. The influence of inertial sensor sampling frequency on the accuracy of measurement parameters in rearfoot running. Comput. Methods Biomech. Biomed. Engin. 2017, 20, 1502–1511, doi:10.1080/10255842.2017.1382482.

There is a lot of statistics presented. All statistics should test a specific hypothesis (which could be reduced). Analysis like lines 197-200 do not relate to a hypothesis and should be removed.

Response: Thank you very much for this helpful comment. Due to the large number of parameters, we grouped them into broader categories for hypothesis testing, with each group containing related parameters. Specifically, we formulated one hypothesis for the two exertion parameters (heart rate and perceived exertion), one for the impact parameter (PTA), one for the kinematic parameters (e.g., evVel, FSA), and one for the spatiotemporal parameters (e.g., stride length and duty factor). This grouping is also stated in line 265.

Thank you also for your comment regarding correlations. We have reconsidered this point and removed the part concerning the relationship with weekly running distance. However, we have added a hypothesis addressing the relationship between exertion parameters and biomechanical measures, as this aspect is revisited in the Discussion and compared with findings from other studies.

These figures are very nice.

Response: Thank you very much for your positive feedback on the figures.

Did running speed change between laps? That might explain differences in ground contact time and duty factor.

Response: Thank you for this comment. Running speed was kept constant throughout the entire run for all participants. As stated in lines 120–122, “Participants were instructed to choose a running pace they could maintain continuously over the entire half-marathon distance without the need to stop; however, a minimum speed of 10 km/h was required.” In addition, the data show that stride length and stride frequency remained very similar across laps, further supporting that running speed did not change (stated in line 425).

Some sentences in the discussion are overstatements. You cannot say based on the data collected that there was a reduction in performance and increased risk for injury (line 256). Your data suggests muscle fatigue, but it does not indicate muscle fatigue as that was not measured.

Response: Thank you for this important remark. The statement in lines 256ff. refers to findings from a previous study [42] and was not intended as a direct conclusion from our own data. To avoid any misunderstanding, we have revised the text to make clearer that this refers to prior literature and not to the results of the present study.

[42] Olaya-Cuartero, J.; Lopez-Arbues, B.; Jimenezolmedo, J.M.; Villalon-Gasch, L. Influence of Fatigue on the Modification of Biomechanical Parameters in Endurance Running: A Systematic Review. Int. J. Exerc. Sci. 2024, 17, 1377–1391, doi:10.70252/LLLT3293.

Potential typo in lines 291-293; stating increase in PTA but baseline is higher than end of run value. Which is correct?

Response: We appreciate your careful observation. You are correct — this was a typo, and we have corrected the values in the revised manuscript.

The discussion section is way too long. If you shorten it to concise points the reader will engage and comprehend better. I suggest 1-2 paragraphs that discuss the findings of each hypothesis/quality of movement, compares with prior literature, discusses why contradicting findings may exist, and identify areas of future work.

Response: Thank you very much for this helpful suggestion. As mentioned earlier, the primary aim of our study was to demonstrate the feasibility of using IMUs to record biomechanical data during a half-marathon. This explains the large number of parameters included. In a second step, these measures were compared with existing literature. Due to the number of parameters and their interrelationships, the discussion has necessarily become somewhat longer and more complex.

 

We sincerely thank the reviewer for the time and effort in carefully evaluating our manuscript and for the valuable suggestions, which have helped us to improve the quality and clarity of the paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Line 138, is adhesive tape the most recommended for these tests?

In Figure 1, image A), is everything placed in the sagittal plane?

Line 180, what would it mean if there were two legs?

Could the explanation in Table 2 be clearer? It is a little difficult to understand.

Line 275 Could the data from the references mentioned be noted for comparison with your results? As in the impact loading section. For example, in line 283, it mentions “muscular fatigue results in a diminished or delayed shock-attenuation response.” This is qualitative, and later the authors provide numerical data, implying that it is to ensure that the information in the reference is true, but is there numerical data that can be compared to know how close they are to the actual biomechanics?

Line 313: Is the analysis of your results or of the authors referenced in this paragraph?

 

 

Author Response

Response to Reviewer 2 Comments

Comments and Suggestions for Authors

Line 138, is adhesive tape the most recommended for these tests?

Response: Thank you for your comment. Yes, this adhesive tape is suitable for this type of study to ensure a stable attachment of the IMU to the leg or shoe and to prevent any displacement during running. In addition, this procedure was approved by the ethics committee and has already applied in previous studies [30, 33, 36].

[30] Hill, M.; Kiesewetter, P.; Milani, T.L.; Mitschke, C. An Investigation of Running Kinematics with Recovered Anterior Cruciate Ligament Reconstruction on a Treadmill and In-Field Using Inertial Measurement Units: A Preliminary Study. Bioengineering (Basel) 2024, 11, doi:10.3390/bioengineering11040404.

[33] Bräuer, S.; Kiesewetter, P.; Milani, T.L.; Mitschke, C. The 'Ride' Feeling during Running under Field Conditions-Objectified with a Single Inertial Measurement Unit. Sensors (Basel) 2021, 21, doi:10.3390/s21155010.

[36] Do Carbon-Plated Running Shoes with Different Characteristics Influence Physiological and Biomechanical Variables during a 10 km Treadmill Run? Applied Sciences 2022, 12, 7949, doi:10.3390/app12157949.

In Figure 1, image A), is everything placed in the sagittal plane?

Response: Thank you for raising this point. Figure 1 is intended solely as a schematic illustration of the approximate sensor positioning. The exact placement of the sensors is described in detail in the text. To improve clarity, we have revised the figure legend accordingly.

Line 180, what would it mean if there were two legs?

Response: If both legs had been affected by sensor issues, the respective participants would have been excluded entirely from the analysis. Fortunately, this was not the case.

Could the explanation in Table 2 be clearer? It is a little difficult to understand.

Response: Thank you very much for this valuable remark. We have revised the legends of all tables and added an additional symbol (*) to make the results clearer.

Line 275 Could the data from the references mentioned be noted for comparison with your results? As in the impact loading section. For example, in line 283, it mentions “muscular fatigue results in a diminished or delayed shock-attenuation response.” This is qualitative, and later the authors provide numerical data, implying that it is to ensure that the information in the reference is true, but is there numerical data that can be compared to know how close they are to the actual biomechanics?

Response: Thank you for your comment. The sentence in line 275 is intended as an introductory statement for this section, illustrating that high forces can occur during the initial ground contact, exceeding multiple times the body weight (i.e., compared to standing). The impact peak of vertical force and PTA cannot be expressed in a direct ratio, as they do not necessarily correlate. To provide context, we have added a few comparative values for PTA at the respective point in the manuscript.

Line 313: Is the analysis of your results or of the authors referenced in this paragraph?

Response: Thank you for this helpful remark. We have revised this section slightly to improve clarity.

 

We sincerely thank the reviewer for the time and effort in carefully evaluating our manuscript and for the valuable suggestions, which have helped us to improve the quality and clarity of the paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

While the authors made some minor changes to the text, they did not make the significant revisions requested. The introduction section still fails to identify a novel gap in literature. The authors discuss how there have been inconsistent findings when studying the effects of fatigue on running mechanics, then say that this could be due to the testing setting (treadmill vs real-world) and that the solution is to use IMUs (lines 75-82). The section also states that the study aims to demonstrate the potential of IMU-based systems for monitoring running biomechanics (lines 100-102). This is not novel. IMUs have been used for multiple decades – upon a quick search in google scholar there are several studies that have assessed running mechanics with IMUs in real-world settings and even systematic reviews on the topic.

I do think there are novel aspects of this study, but they are not articulated. I suggest revisiting your literature search and critically reviewing the studies that match yours the closest – that is the literature which should be reviewed in the introduction section. There are bound to be gaps identified through limitations or methodological differences in prior studies – for example, running distance evaluated, population (trained vs professional vs novice), outcome measure assessed. While PTA is very common I am less familiar with using IMUs to measure ankle kinematics.

Tied to that, the authors need to establish their outcome measures more thoroughly. I respect their response saying “The primary aim of our study was not to establish direct associations between the measured parameters and running injuries”, however the bottom line is that the motivation for this research is injury prevention. Having one sentence which broadly states that the three outcomes (PTA, evAng, and evVel) are related to tibial strain is insufficient. Reviewing the evidence for why these outcomes matter will strengthen your paper.

Additionally, the authors need to establish the validity of measuring ankle kinematics using two inertial measurement units. This is traditionally done in a laboratory setting using marker-based motion capture. How valid is measuring these parameters using inertial measurement units? How sensitive is eversion velocity to sensor alignment? I respect that foot strike angle was described previously, but it should be described in the manuscript as well. I see you are using the plantar phase for drift correction – are you backwards integrating velocity to get position at contact? How valid is that relative to motion capture?

While the introduction section stages the study for eversion angle and velocity, eversion angle was not computed. Instead foot strike angle, sagittal plane velocity, and other spatiotemporal measures were computed. Please justify the importance of these measures in the introduction section.

I suggest using “evAng” instead of “evA” to be descriptive and match “evVel”.

Author Response

Response to Reviewer 1 Comments

Comments and Suggestions for Authors

While the authors made some minor changes to the text, they did not make the significant revisions requested. The introduction section still fails to identify a novel gap in literature. The authors discuss how there have been inconsistent findings when studying the effects of fatigue on running mechanics, then say that this could be due to the testing setting (treadmill vs real-world) and that the solution is to use IMUs (lines 75-82). The section also states that the study aims to demonstrate the potential of IMU-based systems for monitoring running biomechanics (lines 100-102). This is not novel. IMUs have been used for multiple decades – upon a quick search in google scholar there are several studies that have assessed running mechanics with IMUs in real-world settings and even systematic reviews on the topic.

Response: Thank you very much for your valuable comments. We have thoroughly revised the introduction and addressed your suggestions. Among other changes, we have emphasized more clearly that, to date, no studies have investigated all relevant biomechanical parameters within a holistic approach under field conditions. In our view, this represents the novel aspect of our work.

 

I do think there are novel aspects of this study, but they are not articulated. I suggest revisiting your literature search and critically reviewing the studies that match yours the closest – that is the literature which should be reviewed in the introduction section. There are bound to be gaps identified through limitations or methodological differences in prior studies – for example, running distance evaluated, population (trained vs professional vs novice), outcome measure assessed. While PTA is very common I am less familiar with using IMUs to measure ankle kinematics.

Response: Thank you for your comment. As mentioned, we have substantially revised the introduction. We also agree with your observation that there are numerous studies on PTA, but far fewer on other parameters – particularly under field conditions. We have highlighted this more explicitly in the revised version.

 

Tied to that, the authors need to establish their outcome measures more thoroughly. I respect their response saying “The primary aim of our study was not to establish direct associations between the measured parameters and running injuries”, however the bottom line is that the motivation for this research is injury prevention. Having one sentence which broadly states that the three outcomes (PTA, evAng, and evVel) are related to tibial strain is insufficient. Reviewing the evidence for why these outcomes matter will strengthen your paper.

Response: Thank you very much for your comment. This section has also been thoroughly revised.

 

Additionally, the authors need to establish the validity of measuring ankle kinematics using two inertial measurement units. This is traditionally done in a laboratory setting using marker-based motion capture. How valid is measuring these parameters using inertial measurement units? How sensitive is eversion velocity to sensor alignment? I respect that foot strike angle was described previously, but it should be described in the manuscript as well. I see you are using the plantar phase for drift correction – are you backwards integrating velocity to get position at contact? How valid is that relative to motion capture?

Response: Thank you for this comment. As described in the manuscript (line 208), we did not use two sensors to calculate eversion velocity, but only a single IMU placed on the heel cap of each running shoe. This procedure has been validated against an electrogoniometer and showed very high agreement [39].

Additional information on the calculation of the foot strike angle has been included in the manuscript. The approach was validated in internal studies and demonstrated very high accuracy.

[39] Mitschke, C.; Öhmichen, M.; Milani, T. A Single Gyroscope Can Be Used to Accurately Determine Peak Eversion Velocity during Locomotion at Different Speeds and in Various Shoes. Applied Sciences 2017, 7, 659, doi:10.3390/app7070659.

 

While the introduction section stages the study for eversion angle and velocity, eversion angle was not computed. Instead foot strike angle, sagittal plane velocity, and other spatiotemporal measures were computed. Please justify the importance of these measures in the introduction section.

Response: Thank you for your comment. We calculated evAng; however, since the plantar phase of the foot in the frontal plane could not be clearly identified, neither offset nor drift correction could be applied. Consequently, this parameter could not be reliably evaluated.

The other parameters have now been introduced and described in the introduction.

 

I suggest using “evAng” instead of “evA” to be descriptive and match “evVel”.

Response: Thank you for your suggestion. We have removed this parameter from the manuscript to avoid any confusion, as it was not investigated in our study.

 

We sincerely thank the reviewer for the time and effort in carefully evaluating our manuscript and for the valuable suggestions, which have helped us to improve the quality and clarity of the paper.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript, in my opinion, does do a much better job of justifying the outcome measures in the introduction section as requested. I still believe this section is failing to identify a crucial gap in knowledge. You have made the point that prior investigations of the effects of fatigue on biomechanics outcomes are inconsistent. Why do you believe they are inconsistent and what does this study do to address this unknown? Is this just another study that will tip the scales in the direction of ‘fatigue increases tibial loading’? Or does this overcome limitations in prior literature to understand on more fundamental level the effects of fatigue on specific half marathon training in a natural setting?

I noticed that you put effort in to keep the pace consistent throughout the trial. I presume this might be novel to the experimental protocols of prior literature – maybe you could use that to help justify the gap which this study fills. In prior studies, participants are allowed to decrease effort (pace) when fatigued. As runners are less likely to decrease pace during a half marathon event, are these studies valid? I’m not sure exactly what the narrative is here but I urge the authors to consider this point upon their final round of revisions.

Author Response

Response to Reviewer 1 Comments

Comments and Suggestions for Authors

The revised manuscript, in my opinion, does do a much better job of justifying the outcome measures in the introduction section as requested. I still believe this section is failing to identify a crucial gap in knowledge. You have made the point that prior investigations of the effects of fatigue on biomechanics outcomes are inconsistent. Why do you believe they are inconsistent and what does this study do to address this unknown? Is this just another study that will tip the scales in the direction of ‘fatigue increases tibial loading’? Or does this overcome limitations in prior literature to understand on more fundamental level the effects of fatigue on specific half marathon training in a natural setting?

I noticed that you put effort in to keep the pace consistent throughout the trial. I presume this might be novel to the experimental protocols of prior literature – maybe you could use that to help justify the gap which this study fills. In prior studies, participants are allowed to decrease effort (pace) when fatigued. As runners are less likely to decrease pace during a half marathon event, are these studies valid? I’m not sure exactly what the narrative is here but I urge the authors to consider this point upon their final round of revisions.

Response: Thank you very much for this helpful comment. We recognize that runners may adopt different pacing strategies during a half-marathon, such as even pacing, negative splits, or fast starts depending on their goals and experience. In research, most studies have either used constant-pace protocols or fatiguing runs, a distinction that we also address in the discussion. The crucial gap our study addresses, however, lies in investigating a broad range of biomechanical parameters under field conditions, which we have emphasized more clearly in the revised introduction.

 

We sincerely thank the reviewer for the time and effort in carefully evaluating our manuscript and for the valuable suggestions, which have helped us to improve the quality and clarity of the paper.

Author Response File: Author Response.pdf