The efficacy of treatment strategies for clinical decision-making and research purposes in patients with IC is often evaluated on the basis of changes in walking distances (3-5). Such distances may be estimated by the patient or measured using a treadmill. The value of both assessments is debatable as these parameters merely provide insight into walking capacity, which not necessarily corresponds with patient perceived disability 6,11. Self-reported walking capacity appeared a poor reflection of objectively measured MWD 5-7,10. The present study confirms these findings.

 

 Our results may indicate that treadmill testing provides a reliable reflection of the outside walking distance in IC patients. Differences between MWD after either a treadmill or outside walking were not significant. However, only moderate ICCs and substantial COVs were found regarding these parameters. MWD variation in a single patient, when repeatedly measured, probably contributes to these large COV values, which were also found by others 15. Furthermore, others judged that differences in MWD following treadmill walking and off treadmill corridor were due to the inclination in treadmill protocols 6. Although this explanation may seem adequate, the present study challenges this assumption as worse results regarding variability, reproducibility and reliability of a NGTP compared to a GSP with regard to outside walking were found. Another possible explanation for this difference is the difference in walking speed. Walking at a faster pace might give a patient the impression of longer distance coverage, while a higher walking speed causes a higher metabolic demand leading to lower walking distances. In other words, the incremental inclination in the GSP may compensate for the increased outside walking speed. Surprisingly, patients did not walk faster during NGTP testing, possibly causing the poor results in variability, reproducibility and reliability values. On the other hand, walking speed during outside walking might have been slightly influenced by our testing conditions as a researcher was escorting the patient during the test. In conclusion, a single MWD assessment is not

a proper reflection of walking impairment in IC patients and may not necessarily correspond with daily life walking.

 

 The findings of our study have implications for the evaluation of outcomes in clinical practice and future research. Researchers should realize that the frequently used MWD outcome shows substantial variability. Additionally, our study confirms results of other studies indicating that a functional impairment questionnaire such as the WIQ may be an adequate instrument for monitoring walking capacity in IC 6,16. However, one should realize that the Spearman rank correlation coefficients to analyze WIQ data may have been overestimated compared to ICC values in this study, since Spearman's coefficients does not correct systematic measurement error. Nevertheless, future research should be focused more on patient-reported outcomes of perceived disability and burden of disease, both can be of more importance than the determination of a patients' walking capacity. In addition walking (exercise) behavior should be monitored over prolonged periods of time to provide a more reliable reflection of a patients' walking impairment (outside walking). A 6-minute walking test, a shuttle walking-test or GPS-based accelerometers (physical activity monitors) may be alternatives to treadmill testing. The present data also indicate that a dedicated application on a smartphone is a valid alternative for a measuring wheel. Future novel applications for measuring walking behavior in combination with an assessment of disease burden in daily life could contribute to a better understanding of the impact of walking limitations in IC patients.

Study Limitations

 The present study potentially harbors some methodological shortcomings. Although the test order was randomized, a potential 'training effect' could have biased our results. Furthermore, walking distances and speed were assessed as a one-time measurement whereas a multiple assessment might have strengthened our conclusion. Patients were not blinded to distance and time while walking the treadmill tests. In addition, a relatively small population was studied although its size is similar or even larger compared to most previous studies 7,10,13,17.