Results
Seventy-six MHD patients and 40 normal controls were recruited and studied. Five participants were excluded because of prosthetic leg (one MHD patient) or not wearing the activity monitor for at least 7 days (three MHD patients, one normal). We obtained sufficient data for analysis on 72 MHD patients and 39 normal controls. Their characteristics are shown in Table 1. MHD patients and normal controls were 52.3 ± 12.9 (SD) (range, 24–85), and 51.0 ± 12.7 (range, 20–75) years old, respectively. Dialysis vintage was 53.9 ± 45.0 months. Only 31.6 % of the MHD patients had diabetes mellitus, which may reflect the fact that the inclusion/exclusion criteria selected a healthier subset of MHD patients and, hence, a disproportionately large number of diabetic patients were excluded from the study. There were no significant differences between MHD patients and normals with regard to age, weight, gender distribution, educational level, and racial/ethnic distribution. MHD patients and normals each had normal serum albumin levels, averaging 4.1 g/dL. Fat-free, edema-free (lean) body mass, and percent body fat were not different, and their BMI averaged in the overweight range (Table 1). The Charlson Comorbidity Index in the MHD patients averaged 5.7 ± 2.7 (median value, 5; range of values, 2–13).
Compared with normals, MHD patients had significantly worse scores in all measures of daily physical activity (Table 2). The 7-day mean daily physical activity of MHD patients, measured by the ActiGraph activity monitor and expressed as the VM, averaged 60.8 % of the daily physical activity of normals. MHD patients, in comparison to normals, showed a significantly greater percent of their time in sleep or marked physical inactivity and a significantly smaller percent of their time performing either light daily activity or moderate or greater daily activity. Daily physical activity in the MHD patients varied according to their hemodialysis (HD) schedule (Table 2). Daily physical activity was significantly lower on the day of HD (day 0) than on the day following dialysis (1-day-post-HD). Daily physical activity 2-days-post-HD tended to be greater than daily physical activity on the day of HD and less than daily physical activity on day-1-post-HD but was not statistically different from either of these days. On each of these 3 days of the HD schedule, daily physical activity was significantly lower than the daily physical activity of the normals.
Both human activity profile scores, the maximal activity score and adjusted activity score, were reduced (Table 2). The 6-MWT, sit-to-stand, and stair-climbing measurements in MHD patients were 71.5, 60.7, and 161.2 %, respectively, of the values obtained from normal controls (p < 0.001 for each comparison, Table 2). The reduced distance walked during the 6-MWT occurred even though no patient stopped walking.
In either MHD patients and normal adults, the daily physical activity, human activity profile Maximum Activity Score and Adjusted Activity Score values were not significantly different in women as compared to men (Table 3). In MHD patients, the 6-MWT and the stair-climbing speed were significantly reduced in women. In normal adults, performance in the sit-to-stand test was significantly lower in women than in men. Moreover, male and female MHD patients, analyzed separately, each displayed significantly lower daily physical activity and human activity profile maximum activity and adjusted activity scores and reductions in all three physical performance tests as compared to normal men and women, respectively.
Table 3 Physical activity and physical performance by gender group
MHD patients N = 72 MHD:male vs. female Normal controls N = 39 Normals: male vs. female Males: MHD vs. normals Females: MHD vs. normals
Males N = 49 Females N = 23  p Males N = 23 Females N = 16  p  p  p
Age (years) 51.8 ± 13.0 53.8 ± 13.0 0.545 51.3 ± 14.7 50.6 ± 9.5 0.868 0.884 0.407
Vintage (months) 46.9 (46.7) 51.4 (72.4) 0.018 – – – – –
Physical activity
 Human activity profile
  Maximum activity score 84 (10) 82 (18) 0.155 92 (5) 89.5 (4) 0.472 <0.0001 <0.0001
  Adjusted activity score 76 (21) 69 (26) 0.319 91 (4) 88 (4) 0.285 <0.0001 <0.0001
 Time spent by monitor (%)
  Sleep or inactive 82.0 ± 5.8 81.7 ± 6.0 0.840 72.9 ± 6.1 72.7 ± 6.0 0.920 <0.0001 <0.0001
  Light activity 14.2 ± 4.4 14.9 ± 4.5 0.534 19.3 ± 4.3 21.0 ±4.6 0.245 <0.0001 0.0002
  ≥ Moderate activity 3.8 ± 2.0 3.3 ± 2.2 0.341 7.8 ± 3.3 6.4 ± 2.3 0.151 <0.0001 <0.0001
Average daily physical activity (vector magnitude)
  Daily physical activity (mean of 7 days) 421,539 (260,044) 365,050 (184,522) 0.398 644,556 (305,427) 596,642 (244,376) 0.435 <0.0001 <0.0001
  Day of HD 324,559 (203,547) 329,942 (135,789) 0.315 – – – – –
  1-day-post-HD 443,085 (261,536) 442,355 (211,902) 0.438 – – – – –
  2-days-post HD 398,185 (268,775) 360,273 (273,908) 0.887 – – – – –
Physical performance
 6-min walk distance (meters) 467.6 ± 98.9 385.3 ± 147.6 0.007 632.9 ± 60.7 595.5 ± 61.9 0.068 <0.0001 <0.0001
 Sit-to-stand test (cycles per 30 s) 16.5 ± 5.6 14.7 ± 4.2 0.175 28.1 ± 3.9 23.6 ± 6.3 0.009 <0.0001 <0.0001
 Stair-climbing test (seconds per 22 stairs) 13.5 ± 5.0 20.8 ± 17.2 0.008 9.6 ± 1.2 10.2 ± 1.0 0.109 <0.0001 0.008
Data expressed as mean ± standard deviation. Non-normally distributed data are expressed as median values with the interquartile range (IQR. i.e., the arithmetic difference between the 25th percentile value and the 75th percentile value) shown in parentheses
Weekly averaged daily physical activity was negatively associated with age in MHD patients in unadjusted analyses (Fig. 1a), as reported by Johansen et al. [22], and after adjustment for gender and diabetes mellitus (r = −0.342, p < 0.004). In normal controls, weekly averaged daily physical activity was not correlated with age, whether unadjusted (Fig. 1b) or adjusted for gender (r = −0.224, p = 0.177). Daily physical activity correlated directly with MHD vintage in unadjusted analyses (Fig. 2) and with adjustment for age and gender or age, gender, and diabetes (Table 4). In unadjusted analyses, weekly averaged daily physical activity correlated with 6-MWT (Fig. 2), sit-to-stand, and stair-climbing time in MHD patients. In MHD patients, DPA correlated only with 6-MWT but not with sit-to-stand or stair-climbing after adjustment for age and gender or age, gender, and diabetes (Table 4). In normal subjects in unadjusted analyses, daily physical activity correlated directly with the 6-MWT (r = 0.345, p = 0.032) but not with adjustment for age and gender. In normals, daily physical activity did not correlate with either sit-to-stand or stair-climbing measurements in either unadjusted or adjusted analyses. As might be expected, in both unadjusted and adjusted analyses, averaged weekly daily physical activity was significantly associated with the daily physical activity analyzed for each day of the HD cycle as well as for percent time in sleep or marked physical inactivity or in moderate or greater physical activity (Table 4).
Fig. 1 Unadjusted correlation between weekly averaged daily physical activity (DPA) and age in MHD patients (a) and normal controls (b)
Fig. 2 Unadjusted correlation between weekly averaged daily physical activity (DPA) and 6-min walk test in MHD patients (a) and normal controls (b)
Table 4 Correlations in MHD patients with weekly averaged accelerometer-measured daily physical activity and with the human activity profile (HAP) with no adjustments and with adjustment for covariates
Unadjusted Age, gender adjusted Age, gender, diabetes adjusted
R  p Value  R  p Value  R  p Value
Correlations with accelerometer-measured daily physical activity
Vintage (months) 0.266 0.024 0.252 0.035 0.257 0.033
Human activity profile
 Maximum activity score 0.277 0.019 0.102 0.403 0.101 0.410
 Adjusted activity score 0.366 0.002 0.206 0.087 0.207 0.087
Time spent by monitor readings (%)
 Sleep or marked physical inactivity −0.868 <0.0001 −0.881 <0.0001 −0.885 <0.0001
 Light activity 0.745 <0.0001 0.794 <0.0001 0.796 <0.0001
 ≥ Moderate activity 0.843 <0.0001 0.816 <0.0001 0.820 <0.0001
Day of HD 0.898 <0.0001 0.881 <0.0001 0.881 <0.0001
1-day-post-HD 0.948 <0.0001 0.942 <0.0001 0.942 <0.0001
2-days-post-HD 0.815 <0.0001 0.819 <0.0001 0.819 <0.0001
6-min walk distance (meters) 0.387 <0.0001 0.258 0.031 0.263 0.029
Sit-to-stand test (no. cycles in 30 s) 0.268 0.023 0.148 0.221 0.148 0.226
Stair-climbing test (seconds per 22 stairs) −0.283 0.016 −0.112 0.358 −0.111 0.363
Correlations with human activity profile
Maximum activity score vs.
 6-min walk distance (meters) 0.628 <0.0001 0.496 <0.0001 0.480 <0.0001
 Sit-to-stand test (no. cycles in 30 s) 0.623 <0.0001 0.534 <0.0001 0.533 <0.0001
 Stair-climbing test (seconds per 22 stairs) −0.707 <0.0001 −0.595 <0.0001 −0.584 <0.0001
Adjusted activity score vs.
 6-min walk distance (meters) 0.642 <0.0001 0.523 <0.0001 0.504 <0.0001
 Sit-to-stand test (no. cycles in 30 s) 0.581 <0.0001 0.481 <0.0001 0.479 <0.0001
 Stair-climbing test (seconds per 22 stairs) −0.608 <0.0001 −0.454 =0.0001 −0.437 =0.0002
Average weekly daily physical activity was significantly associated with the maximum activity and adjusted activity scores of the human activity profile in unadjusted analyses but not when the analyses were adjusted for age and gender or age, gender, and diabetes. Interestingly, the maximum activity scores and the adjusted activity human activity profile scores each strongly correlated with 6-MWT and sit-to-stand and stair-climbing tests in both unadjusted and the age, gender, and diabetes adjusted analyses (Table 4, Figs. 3 and 4).
Fig. 3 Unadjusted correlations between the human activity profile maximum activity score and 6-min walk test (a), sit-to-stand test (b), and stair-climbing test (c) in MHD patients
Fig. 4 Unadjusted correlations between the human activity profile-adjusted activity score and 6-min walk test (a), sit-to-stand test (b), and stair-climbing test (c) in MHD patients