Sensor-Based Assessment of Task-Dependent Visual–Postural–Muscular Responses to Smartphone Holder Use During a Simulated Riding-Posture Task

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript addresses a relevant and timely topic concerning the ergonomic and sensor-based assessment of smartphone-holder use in a motorcycle-related context. The integration of kinematic and electromyographic measurements is appropriate for the scope of Sensors, and the study provides potentially useful preliminary data regarding task-dependent visual–postural–muscular responses.

However, several methodological and interpretative issues require substantial revision before the manuscript can be considered for publication.
1-Experimental realism and external validity
The experimental setup described in Section 2.5 (pp. 5–6) uses a stationary electric scooter positioned in front of an LCD display showing prerecorded riding footage. While this provides a controlled laboratory environment, it does not reproduce key real-world riding factors such as vibration, active steering corrections, braking, traffic interaction, environmental hazards, or true balance control. Therefore, the phrase “simulated motorcycle riding” in the title, abstract, and conclusions may overstate ecological validity. The manuscript would benefit from more cautious terminology, such as controlled riding-posture simulation.
2-Fixed task sequence and possible order bias
The riding-related tasks were performed in a fixed sequence (dynamic viewing → static viewing → texting), as described in Section 2.5 (p. 6, lines 206–214). Since no counterbalancing or randomization was applied, the observed differences may be partially influenced by learning, adaptation, anticipation, or fatigue effects. This represents a significant methodological limitation that should be more critically addressed.
3-Task-condition confounding
The task definitions in Section 2.2 (pp. 3–4) combine differences in smartphone interaction type with differences in environmental context. For example, texting was performed while stationary, whereas dynamic viewing occurred during moving footage. As a result, it is difficult to isolate whether the observed differences are attributable to task demand, simulated motion context, or both. This design confound should be explicitly acknowledged.
4-Insufficient sensor acquisition detail
Although posture measurement is described in Section 2.3 and EMG acquisition in Section 2.4, essential methodological details are missing for reproducibility. Specifically, the manuscript does not clearly report motion-capture sampling frequency, calibration procedures, camera specifications, synchronization method between EMG and kinematic acquisition, or the exact duration of analyzed segments. The phrase “representative stable periods” (Section 2.4, lines 174–175) is insufficiently defined.
5-EMG processing requires clarification
Section 2.4 (p. 4) states that IEMG values were calculated and normalized to MVC, but the exact processing workflow remains unclear. The authors should specify the analysis window duration, whether RMS was considered, artifact rejection procedures, and the rationale for using IEMG rather than other standard normalization metrics. This is important because EMG processing choices significantly affect interpretation.
6-Statistical reporting lacks completeness
The statistical methods described in Section 2.6 (pp. 6–7) are appropriate in principle; however, reporting is incomplete. The manuscript should provide actual assumption test outcomes (normality, homogeneity, sphericity), corrected degrees of freedom where Greenhouse–Geisser adjustments were used, exact p-values when feasible, and confidence intervals for repeated-measures correlations reported in Table 3.
7-Interpretation of sex differences may be overstated
The authors report higher CES and UTZ activity in females (Section 3.2, p. 7), and the discussion attributes this to anthropometric or physiological differences. However, no direct measurements of arm length, neck strength, muscle cross-sectional area, riding experience intensity, or habitual smartphone use were collected. These explanations should therefore be framed as plausible hypotheses rather than evidence-based conclusions.
8-Safety-related interpretations exceed measured outcomes
The discussion in Section 4.4 (pp. 12–13) introduces safety implications such as reduced situational awareness and possible riding risk. However, the study did not directly measure hazard perception, glance duration, reaction time, cognitive workload, or vehicle control performance. These interpretations should be substantially moderated.
9-Novelty should be articulated more clearly
The manuscript presents a combination of visual, postural, and muscular measurements, but the specific novelty relative to existing smartphone ergonomics, driver distraction, and wearable sensor literature should be better defined in the Introduction (pp. 1–3). At present, the contribution appears incremental rather than clearly differentiated.
10-Figures require improvement
Figures 3–5 (pp. 8–9) contain relevant information but could be improved in readability and presentation quality. Figure 4, in particular, combines multiple outcome domains in a visually dense format, making interpretation difficult. Clearer formatting and potentially separating biomechanical and EMG outcomes into distinct figures would improve clarity.

Comments on the Quality of English Language

The manuscript is generally understandable and written in academically appropriate English; however, the language requires moderate revision to improve clarity, precision, and consistency. Several sentences are overly long and conceptually dense, which reduces readability, particularly in the Introduction and Discussion sections. Some terminology is used inconsistently (e.g., “smartphone-holder use,” “riding-related tasks,” “simulated riding tasks,” and “baseline conditions”), and standardization would improve coherence. In addition, certain interpretations are phrased too strongly relative to the experimental design, and the language should be adjusted to better reflect the limitations of a controlled simulation study. A careful professional English revision is recommended to improve conciseness, scientific precision, and overall readability.

Author Response

Please see the acttachment. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript addresses an interesting and timely topic regarding the ergonomic and sensor-based assessment of smartphone-holder use during simulated motorcycle riding. The integration of postural, visual, and EMG measures represents a valuable multidimensional approach, and the manuscript is generally well organized.

The study has merit and presents potentially publishable findings; however, several methodological and interpretative issues should be addressed before the manuscript can be considered for publication.

Major comments

1. Ecological validity and interpretation of “motorcycle riding”

The main limitation of the study is the discrepancy between the simulated laboratory setup and actual motorcycle riding conditions. Although the authors acknowledge this limitation in Section 4.6, the manuscript repeatedly refers to “motorcycle riding” in a manner that may overstate the ecological validity of the findings. The experimental setup did not include: real vehicle motion, vibration, steering perturbations, balance control, acceleration/deceleration, traffic interaction, or real hazard perception. Consequently, the study primarily evaluates smartphone-use posture while seated on a stationary scooter rather than actual riding behavior. The authors should substantially moderate statements implying riding-performance or safety implications. Terms such as “during motorcycle riding” should be carefully revised throughout the manuscript, especially in the title, abstract, discussion, and conclusions. A title emphasizing “simulated riding posture” or “scooter-mounted smartphone use” may better reflect the actual experimental conditions.

2. Fixed task order and possible order effects

The riding-related tasks were performed in a fixed sequence (dynamic viewing → static viewing → texting). This is a significant methodological concern because posture adaptation, familiarization, fatigue, or attentional adjustment may have influenced the results. Counterbalancing should ideally have been used. At minimum, the authors should discuss more explicitly how order effects may have influenced task comparisons. The current limitation statement is too brief relative to the importance of this issue.

3. Lack of task standardization details

The manuscript does not clearly describe several important procedural details, including:

• smartphone size and weight,
• screen brightness,
• font size,
• notification content,
• texting task complexity,
• handedness during texting,
• texting duration,
• duration of each trial,
• exact timing of analyzed segments,
• and whether participants used their own phones or a standardized device.

These variables may substantially influence posture, gaze behavior, and muscle activity. More methodological detail is required to improve reproducibility.

 

4. Interpretation of “visual–postural–muscular coupling”

The concept of a “visual–postural–muscular coupling mechanism” is interesting and potentially valuable. However, the current wording occasionally suggests a mechanistic or causal interpretation that is not fully supported by the data. The study demonstrates correlations, not causal propagation mechanisms. For example, statements such as “visual adjustments may propagate through the musculoskeletal system” should be softened unless supported by experimental manipulation or modeling approaches. The authors should frame this more conservatively as an observed association pattern.

5. Statistical considerations

The statistical approach is generally appropriate, but some clarifications are needed: a) Effect size interpretation: The manuscript reports ηp² values but does not provide interpretation criteria. The authors should indicate whether conventional thresholds were used. b) Assumption reporting: The manuscript states that normality and sphericity were tested, but actual outcomes are not reported. Please specify: which variables violated assumptions, where Greenhouse–Geisser correction was applied, and whether residual inspection was performed. c) Multiple comparisons: The manuscript includes multiple ANOVAs and repeated-measures correlations. Please clarify whether any correction for multiplicity beyond Bonferroni pairwise adjustment was considered. 

6. Interpretation of sex differences

The explanation attributing higher female EMG activity to anthropometric differences and lower muscle strength is plausible but speculative. No direct anthropometric or strength measurements were collected. Therefore, the discussion should clearly state that these are hypothetical explanations rather than demonstrated mechanisms.

7. Absence of fatigue-related analysis

The authors discuss fatigue implications extensively, particularly for delivery riders. However, the tasks appear to be short-duration laboratory trials. No fatigue outcomes were measured. Therefore, statements regarding fatigue accumulation should be presented more cautiously.

8. Clarification of “dynamic viewing”

The “dynamic viewing” condition remains somewhat ambiguous. Was the participant actively riding cognitively, or simply watching riding footage? How frequently did smartphone notifications appear? Were participants instructed to glance at the device for a fixed duration?How was compliance verified? This condition is central to the manuscript and requires more detailed operational definition.

9. Need for stronger justification of task selection

The manuscript states that the five tasks represent combinations of visual demand and manual interaction. However, no theoretical framework or prior taxonomy is provided to justify the selection. The authors should better explain: why these specific tasks were selected,how they relate to real-world rider behavior, and whether observational or epidemiological evidence supports their ecological relevance.

10. Potential overinterpretation of safety implications

The manuscript repeatedly discusses hazard perception, situational awareness, crash risk, and safety. However, none of these variables were directly measured. The authors should reduce speculative safety-related language or explicitly separate biomechanical findings from hypothetical safety implications.

 

Minor comments

11. English revision

Several sections of the Discussion contain long and repetitive sentences. Examples include Sections 4.1 and 4.2. The manuscript would benefit from professional English editing for conciseness and readability.

12. Figure improvements

Figure 4 is informative but visually crowded.

Suggestions: increase font size, separate visual/postural/muscle panels, improve contrast between conditions, and consider plotting males/females separately if sex effects are important.

13. Clarify EMG normalization

The manuscript states that peak EMG amplitude from a 0.5-s moving window during MVC was used. Please clarify whether RMS or integrated EMG was normalized, whether EMG was time-normalized, and how stable segments were selected.

14. Terminology consistency

The manuscript alternates between: motorcycle,scooter,riding,riding-related,scooter-mounted,and motorcycle-use. More consistent terminology would improve clarity.

15. Ethical and practical considerations

Because texting while riding is dangerous and illegal in many regions, the manuscript may benefit from a brief ethical framing emphasizing that the study does not endorse smartphone interaction during riding.

Recommendation

The manuscript has scientific merit and addresses an important topic using an interesting multimodal sensor-based approach. However, substantial revisions are necessary to improve methodological transparency, moderate interpretative claims, and strengthen the discussion of ecological validity and causal inference.

I therefore recommend: Reconsider after major revision

Comments on the Quality of English Language

Overall understandable and technically adequate, but several sentences are overly long and repetitive, particularly in the Discussion.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript examines the ergonomic effects of smartphone-holder use during simulated motorcycling. Forty participants completed five tasks, including dynamic viewing, static viewing, texting, and seated and standing baseline conditions. The study measured neck flexion, upper thoracic angle, gaze angle, viewing distance, and EMG activity of the cervical erector spinae and upper trapezius muscles. The results suggest that texting created the highest ergonomic demand, females tended to adopt shorter viewing distances and showed higher muscle activation than males, and viewing distance was strongly negatively correlated with neck flexion. Based on these findings, the authors propose a “visual-postural-muscular coupling mechanism” and recommend sensor-based ergonomic assessment approaches.

1. The task order was fixed for all participants. Why was task order not counterbalanced? A fixed sequence can introduce fatigue, learning, or posture adaptation effects that may influence the results independently of the task itself. For example, participants may gradually adopt a more flexed posture over time regardless of the condition being tested. The authors should clarify how potential order effects were addressed.
2. In the texting condition, participants used both hands while the smartphone remained attached to the handlebar mount. This setup does not fully resemble typical real-world texting behavior during motorcycling, where users may hold the phone in one hand, remove it from the mount, or interact through brief glances. The authors should clarify what specific real-world scenario this experimental condition is intended to model and discuss its ecological validity.
3. EMG normalization was performed using MVC measurements collected before the experimental tasks. Were MVC measurements repeated after the experiment or checked for consistency? Repeated tasks and prolonged sitting could alter muscle activation or introduce fatigue, which may influence normalized EMG values, especially if the texting condition was always performed later in the sequence.
4. The effect sizes for EMG activity appear smaller than those observed for visual and postural variables. Does this suggest that muscle activity is less sensitive to task differences than posture or visual behavior? If so, the authors should discuss whether this affects the interpretation of a tightly coupled “visual-postural-muscular” mechanism.
5. The reported correlation between viewing distance and neck flexion (r = −0.815) is very strong. However, repeated-measures correlations can sometimes be influenced by between-subject differences. The manuscript does not clearly separate within-subject and between-subject contributions to this relationship. Providing this analysis would help clarify how robust the correlation actually is.
6. The manuscript attributes the higher muscle activation in female participants to anthropometric factors such as smaller stature or shorter arm length. However, variables such as arm length, shoulder width, and sitting height were not directly measured. Without these measurements, the observed sex differences could also reflect different task strategies or interaction habits rather than purely biomechanical factors.
7. A hole was cut into the helmet near the tragus region to allow marker placement. This modification could potentially alter helmet fit, weight distribution, or natural head movement. The manuscript would benefit from a control or validation showing that the helmet modification itself did not influence participant posture or movement behavior.
8. The discussion frequently refers to situational awareness and riding safety, but no direct safety-related outcomes were measured. Metrics such as off-road glance duration, task completion time, or responses to unexpected events would provide stronger support for claims related to situational awareness and real-world riding safety.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been substantially improved and the authors have adequately addressed the major concerns raised during the review process. The revised version demonstrates stronger methodological clarity, more balanced interpretation of the findings, appropriate acknowledgment of limitations, and improved statistical reporting. I recommend acceptance of the manuscript in its current form.

Author Response

We sincerely thank the reviewer for the positive evaluation of our revised manuscript and for the constructive comments throughout the review process. We are pleased that the reviewer found the manuscript substantially improved and that the major concerns were adequately addressed. We greatly appreciate the reviewer’s valuable time and feedback.

Reviewer 2 Report

Comments and Suggestions for Authors

Overall, the response adequately addresses the reviewer’s concerns and the manuscript has clearly improved in terms of methodological transparency, interpretive caution, and terminology consistency. However, a few minor issue still deserve attention before final submission. Although the response to Comment 4 states that causal or propagation language was removed, the manuscript still includes the sentence “visual adjustments may propagate through the musculoskeletal system as an integrated visual–postural–muscular coupling mechanism” in the Discussion section. This wording still implies causality and should be revised to more associative language consistent with the reviewer response.   try “visual adjustments were accompanied by changes in head–neck posture and muscular loading.”

Author Response

We thank the reviewer for this careful observation and agree that the previous wording could still imply causal interpretation. The sentence has been revised to more appropriately reflect an associative relationship. Specifically, “visual adjustments may propagate through the musculoskeletal system as an integrated visual–postural–muscular coupling mechanism” has been revised to “visual adjustments were accompanied by changes in head–neck posture and muscular loading.” [Revised in Lines 456-457]