Yonago Acta medica 2014;57:61–63

Short Communication

Modified Six-Minute Walk Test: Number of Steps per Second
Naoto Burioka,* Akari Imada,* Akiko Kiyohiro,* Fumika Sugitani,* Takenori Fujii,* Akari Hosaka,* Sachiko
Nakamoto,* Takashi Amisaki† and Eiji Shimizu‡*Department of Pathobiological Science and Technology, School of Health Science, Tottori University Faculty of Medicine, Yonago
683-8503, Japan, †Department of Biological Regulation, School of Health Science, Tottori University Faculty of Medicine, Yonago
683-8503, Japan and ‡Division of Medical Oncology and Molecular Respirology, Department of Multidisciplinary Internal Medicine,
School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8504, Japan

ABSTRACTThe 6-min walk test (6MWT) has been used to examine subjective dyspnea, predict mortality and measureclinical outcomes in studies of patients with chronic
pulmonary or heart disease. Although the 6MWT is
useful in assessing the general ability for patients toperform daily physical activities, it is difficult to evaluate time-dependent responses. To improve the 6MWT,
we devised a new index–the number of steps walked per
second (NSPS). Eleven healthy subjects and 7 patientswith chronic obstructive pulmonary disease (COPD)
performed the 6MWT. Then, we calculated the NSPS.
The mean NSPS was significantly higher in the healthy
subjects than in the COPD patients, while the coefficientof variation of the NSPS was significantly smaller inhealthy subjects compared with COPD patients. Calculation of the NSPS was useful in evaluating walking patterns. This modified 6MWT may be helpful for assessing
the efficacy of rehabilitation and drug therapy for COPD.

difficult to evaluate in terms of time-dependent parameters such as the instantaneous walking speed. Since
oxygen desaturation can improve after resting, the resting time and walking pattern may influence the resultsof this test in COPD patients. We developed a modified
6MWT in which the walking pattern was examined bydetermining the number of steps per second (NSPS).
SUBJECTS AND METHODS
Subjects and patients

Eleven healthy subjects (6 males and 5 females) and 7
clinically stable male patients with COPD participatedin this study (Table 1). Exclusion criteria were unstablecardiovascular, orthopedic or neurologic conditions.
This study was approved by Tottori University and the
Hitachi Memorial Hospital ethical board (approval number 1845) and all participants gave written informed
consent.
6MWT

Key words

6-min walk test; chronic obstructive pulmonary disease; dyspnea; walking pattern

The test was conducted in flat corridors with lengths of
54 or 74 m at Tottori University or Hitachi Memorial
Hospital, respectively, according to the published guideline.1 Oxygen saturation of arterial blood was measured
by pulse oximetry (SpO2) with a finger sensor (Pulsox
300i, Konica-Minolta, Tokyo, Japan) and was continuously recorded during the test. Then pulse oximetry
variables were analyzed by DS-5 Pulsox analysis software (Konica-Minolta). The severity of dyspnea was
subjectively assessed before and after the 6MWT usinga modified Borg scale.3 It has been reported that at least
2 practice walks should be performed before the 6MWT
because training influences the results.4, 5 Healthy subjects did 2 practice walks before the 6MWT, which was
performed at the 3rd trial. COPD patients performed atleast 2 practice walks on another day before the actual
test because dyspnea and fatigue could influence theresults of the 6MWT if patients practiced on the sameday. The patients were allowed to sit on a chair to rest if
needed, and resumed walking when they had recovered
sufficiently. The duration of every rest was measured
with a stopwatch. All of the walking tests were recordedby digital video and the 6MWD was measured.

The 6-min walk test (6MWT) has been used to assess
functional status and measure clinical outcomes in studies of patients with chronic respiratory disease. This test
assesses the farthest distance that a patient can walk in
6 min.1 It was recently reported that the 6-min walking distance (6MWD) is useful for predicting mortality
in patients with chronic obstructive pulmonary disease(COPD) but the efficacy of exercise-related oxygen
desaturation is less clear.2 However, patients with severe COPD frequently cannot walk for 6 min without
stopping and may have to rest several times during the
6MWT due to dyspnea. Moreover, some patients cannot
walk at a constant pace, and the conventional 6MWT is
Corresponding author: Naoto Burioka, MD, PhD
burioka@med.tottori-u.ac.jp
Received 2013 October 28
Accepted 2013 November 12Abbreviations: 6MWD, 6-min walking distance; 6MWT, 6-minwalk test; COPD, chronic obstructive pulmonary disease; CV, coefficient of variation; NSPS, number of steps walked per second

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follows: [SD/mean of NSPS] × 100 (%). The mean and
CV of NSPS were determined in each subject.

Table 1. Profile of the subjects and results of the
6MWT: average NSPS
Healthysubjects

Patients with
COPD

Pvalue*Number of subjects
11		
7
Age (yr)
68.6 (5.4)
70.7 (4.8)
Height (m)1.58 (0.07)
1.60 (0.06)
Weight (kg)61.0 (8.3)
48.7 (7.9)
FVC (L)2.98 (0.72)
2.78 (0.92)
FEV1 (L)2.48 (0.62)
1.34 (0.75)
FEV1, % predicted (%) 103.2 (14.2) 55.2 (27.1)
6MWD (m)486.9 (35.0) 289.5 (123.7)
Total number of steps 711.9 (47.1) 511.0 (153.5)
Mean NSPS (step/s)
1.98 (0.13)
1.42 (0.43)
CV of NSPS (%)
7.4 (1.3)
44.7 (45.3)
Total resting time (s)0		
65.6 (83.8)
Delta Borg scale
0.1 (0.2)
1.6 (1.1)
94.4 (1.2)
88.0 (5.7)
Lowest SpO2 (%)
95.5 (0.9)
90.9 (4.1)
Mean SpO2 (%)

0.316
0.934
0.012
0.696
0.008
0.00180.005
0.009
0.00090.00970.00620.002
0.00520.0043

Statistical analyses

The values are presented as the mean (SD). We used
Mann-Whitney’s U test for the assessment of any difference between healthy subjects and COPD patients(StatFlex, ViewFlex, Tokyo, Japan). Differences wereconsidered to be statistically significant at P < 0.05.
RESULTS
Figure 1 shows examples of the walking pattern determined from the NSPS in a healthy subject and 2 COPD
patients with and without rest. NSPS increased as thewalking pace became faster. While the healthy subject
walked without resting, the patients rested for a total of
83 s (Fig. 1B). Table 1 shows the characteristics of all
subjects and the results of the 6MWT with NSPS. The
6MWD of the healthy subject [486.9 (35.0) m] was significantly longer than that of the patients [289.5 (123.7)m]. Four of the 7 patients had to rest on a chair [1.0 (1.2)
times] during the 6MWT, and the total resting time was
65.6 (83.8) s. In contrast, the healthy subject did not stop
during the test. The mean NSPS of the healthy subject[1.98 (0.13) step/s] was significantly larger than that ofValues, mean (SD).
* Mann-Whitney’s U test.
6MWD, 6-min walking distance; 6MWT, 6-min walk test;
COPD, chronic obstructive pulmonary disease; CV, coefficientof variation; FVC, Forced vital capacity, FEV1, Forced expiratory volume in 1 s; NSPS, number of steps walked per second.
The NSPS was calculated as [steps walked in 5 s] 0.2, and was
calculated 72 times (360 s ÷ 5 s) in each subject.
The CV of NSPS was calculated as [SD/mean NSPS] 100 (%).
Delta Borg scale: modified Borg scale after walking – modified
Borg scale before walking.

Calculating the NSPS

The number of steps walked by each subject every
5 s was counted manually with a hand counter
while viewing a video monitor. The walking speed(m/s) can be considered as the average step length(m/step) × NSPS (step/s). We defined that NSPS
was calculated as the number of steps walked ina 5-s period divided by 5, and this was consecutively done 72 times (360 s ÷ 5 s) in each subject.
Because NSPS is proportional to the walking
speed, it usually decreases when a patient walks
more slowly and falls to zero if the patient stops
walking. The mean value of NSPS was defined as
NSPS(5k)] ÷ 72, where NSPS(5k) is the value[of NSPS at 5k s (k = 1, 2, 3, . . . , 72). The coefficient of variation (CV) of NSPS was calculated as

Fig. 1. Walking patterns of a healthy subject and COPD
patients based on the NSPS. The healthy subject (A: a66-year-old woman) and the COPD patient with rest (B:a 64-year-old man) and the COPD patient without rest (C: a 68-year-old man) walked 504.5 m, 224.5 m and 356.5 m, respectively. Although the healthy could walk without stopping, the patient (B) rested for 83 s. When the walking pace is slower, the NSPS is smaller, andbecomes zero if the patient stops walking. COPD, chronic obstructive pulmonary disease; NSPS, number of steps per second.

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the patients [1.42 (0.43) step/s]. The CV of NSPS, whichassesses both the walking pace and the resting time, also
showed a significant difference between the healthy subjects [7.4 (1.3) %] and the COPD patients [44.7 (45.3) %](Table1). The 6MWD of the patients without rest [405.0(42.9) m] was significantly longer than that with rest
[202.9 (77.7) m] (P < 0.05). The mean NSPS of patients
without rest [1.80 (0.10) step/s] was significantly larger
than that with rest [1.20 (0.37) step/s] (P < 0.05). The CV
of NSPS of patients without rest [8.0 (1.4) %] was significantly smaller than that with rest [72.2 (41.9) %] (P <0.05). The lowest SpO2 of patients without rest [92.7 (2.1)%] was significantly higher than that with rest [84.5 (4.9)%] (P < 0.05).

Consideration of walking steps during 6MWT may
contribute new analyses of physiological condition in patients with COPD. Moreover, it has been reported that a
3-dimensional accelerometer can accurately detect steps
while a subject is walking.7 Use of an accelerometer
would allow the NSPS to be computed automatically.
The present study has several limitations with the mostobvious being its relatively small sample size. Also, we
did not investigate a random sample of COPD patientswith a spectrum of disease severity. We could not compare the clinical efficacy between 6MWD and NSPS.
Further studies are needed to clarify the clinical usefulness and estimate whether the prognosis and mortality
in COPD patients can be predicted by our new indicesfor the 6MWT.

DISCUSSION
To improve the 6MWT, we determined NSPS as a newindex. When we performed this modified 6MWT in
healthy subjects and COPD patients, we found that the
NSPS was useful for examining the walking property inthe 6MWT.
The 6MWT has been reported to predict mortality
in COPD patients2, 6 and this test has been used in manystudies. Golpe et al.2 recently reported that exerciserelated oxygen desaturation during the 6MWT was
not an independent predictor according to multivariate
analysis. However, some severe COPD patients cannot
walk for 6 min without resting, and their walking pace isslower than that of healthy subjects. Some of our COPD
patients rested during the 6MWT, and exercise-related
oxygen desaturation could improve after resting. Therefore, if exercise-related oxygen desaturation is used as
an index for predicting mortality, the resting time needs
to be considered and previous reports are not clear
about this point. In the present preliminary study, we
examined the NSPS as a new index for the 6MWT that
could be calculated easily. The mean and CV of NSPS
in COPD patients were significantly different with andwithout rest. In particular, the CV of NSPS is an index
of both the walking pace and the resting time. A graphof NSPS versus time displays the walking pattern of apatient during the 6MWT, and the results can be easilyinterpreted.

The authors declare no conflict of interest.
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