The overall pattern of correlations between GRF and accelerations for activities considered separately and activities combined was largely similar irrespective of monitor and, to a lesser extent, irrespective of wear site. Considering all activities together, peak accelerations at the hip and wrist were positively correlated with peak vertical GRF (r > 0.8 and r > 0.7, respectively, both P < 0.001), irrespective of monitor (final column, Table 2). Correlations between peak acceleration and peak loading rate were lower, both at the hip (r > 0.7, P < 0.001) and at the wrist (r > 0.57, P < 0.001). The inclusion of multiple data points from each subject for these correlations violates the assumption of independence of observations. Thus, a series of linear mixed effect models was run to assess whether subject had a significant effect on the relations shown in Table 2. There was no significant effect of subject on any of the relations observed.
Considering activities separately, peak accelerations were positively and significantly correlated with peak vertical GRF for low jumps, high jumps, and box drops irrespective of monitor or wear site (r = 0.32–0.68, P < 0.01–0.05) (Table 2). Significant positive correlations were also generally observed for running and fast walking for peak vertical acceleration at the hip and resultant acceleration at the wrist from either accelerometer (r = 0.3/0.4, P < 0.01–0.05). Peak resultant acceleration at the hip did not correlate significantly with peak vertical GRF for running. Peak accelerations were positively and significantly associated with peak loading rate during walking (r = 0.2–0.6, P < 0.01–0.05) and jumping (r = 0.3–0.5, P < 0.01–0.05), but not running. Patterns of association between peak acceleration and peak loading rate were largely independent of monitor or wear site.
Resultant and vertical peak accelerations from both the GENEActiv and the ActiGraph GT3X+ accelerometers discriminated between peak loading rates greater and less than 43 BW·s−1 with high levels of accuracy, irrespective of wear site (AUC >0.92, P < 0.001). The ROC statistics and the cut-points that maximized sensitivity and specificity are presented in Table 3. Sensitivity of the optimal cut-point was high, >92%, with specificity >89% at the hip and >83% at the wrist.
The cut-points shown in Table 3 were cross-validated in the remaining 10 participants. The sensitivity and specificity for each of the cut-points were >85% (Table 4), with the exception of the specificity for the GENEActiv peak resultant acceleration at the wrist (75%). Overall agreement was >85% for each accelerometer cut-point for the cross-validation sample and for leave-one-out cross-validation.
Figure 2a–f depicts the relations between peak loading rate and peak resultant acceleration at the hip (1a = GENEActiv, 1b = ActiGraph), peak vertical acceleration at the hip (1c = GENEActiv, 1d = ActiGraph), and peak resultant acceleration at the wrist (1e = GENEActiv, 1f = ActiGraph) for the calibration and cross-validation sample. The horizontal lines represent the 43 BW·s−1 loading rate threshold, and the vertical lines represent the optimal cut-point for each accelerometer variable. The bottom left and top right quadrants represent correct classification. Data points in the top left quadrant are false negatives, and data points in the bottom right quadrant are false positives.
Peak loading rate as a noninvasive estimate of strain rate provides an important measure relevant to bone, but it is not possible to measure peak loading rate during habitual physical activity. The positive relations between peak vertical GRF, peak loading rate, and peak accelerations shown herein demonstrate that these accelerometers worn at the hip or wrist provide a measure that reflects the short, dynamic bursts of activity known to benefit bone (2,5,13,14,26,32) and can be used during normal habitual activity.
Indeed, the frequency of daily acceleration peaks >3.9g recorded by an accelerometer worn at the hip has previously been shown to be associated with changes in peak BMD in premenopausal women (32). The current study has determined peak acceleration cut-points specific to the commercially available GENEActiv and ActiGraph GT3X+ accelerometers that can be used to capture the occurrence of peak accelerations that exceed the loading rate of 43 BW·s−1 shown to be beneficial to bone by Bassey et al. (2). Furthermore, the data indicate that this is possible when the accelerometer is worn at the wrist — this is very significant because the wrist is a wear site that is more acceptable to participants and thus results in greater wear time (33).
The optimal cut-points for the hip were greater than those for the wrist, indicating that cut-points need to be specific to the wear site. This supports findings from EE calibration studies (8). At the hip site, there appeared to be no consistent advantage of a triaxial measure of peak acceleration (resultant) over vertical peak acceleration. Some, but not all, studies focusing on relations between EE and acceleration have reported stronger relations when using triaxial accelerometer output (7,9,15,22,27). The lack of an advantage of three axes of measurement in the current study probably reflects the criterion measures of peak vertical GRF, peak loading rate, and the nature of the activities. It is possible that in “real world” or sports activities, resultant accelerations may better reflect the strains on the bone because of movements involving sudden changes of direction and movement in different planes (6). Inclusion of such movements was beyond the scope of the present study partly because of difficulties in a range of participants being able to sufficiently control and safely replicate unfamiliar movements. When an accelerometer is worn at the wrist, it is necessary to have a measure of resultant acceleration because there is no dominant plane of movement. Similar accuracy (>85%) at the two sites for classifying accelerations equivalent to peak loading rates above and below the threshold of 43 BW·s−1 supports the use of the wrist-worn accelerometer to assess activity beneficial to bone. This is very encouraging because of the greater acceptability of wrist-wear and thus greater wear time achieved (33). However, the correlations between peak vertical GRF, peak loading rate, and peak accelerations at the wrist were slightly lower than that of the corresponding peak accelerations assessed at the hip. Further research is needed to establish whether relations between bone health and the frequency of peak accelerations above the relevant cut-point during habitual physical activity are similar between wrist- and hip-worn accelerometers.
Correlations between peak vertical GRF, peak loading rate, and peak accelerations were similar between the two brands of accelerometer. Classification accuracy during cross-validation was also similar, irrespective of monitor brand or wear site (>85%). This was expected because both monitors are measuring raw accelerations in the same planes and were worn taped together at the respective sites. However, there was a discrepancy evident in the peak acceleration values from the GENEActiv and ActiGraph GT3X+ accelerometers, particularly during higher intensity activities, with the ActiGraph GT3X+ recording lower values than the GENEActiv. The higher dynamic range of the GENEActiv (±8g compared with ±6g for the ActiGraph) may contribute to this discrepancy. This highlights the importance of monitor-specific calibrations, despite a common output measure.
Vainionpää et al. (32) have previously shown that the number of vertical peak accelerations exceeding 3.9g recorded by an accelerometer worn at the hip is associated with changes in BMD in the proximal femur of premenopausal women. These accelerations equate to values of 4.9g in the current study because Vainionpää et al. (32) subtracted 1g from values to account for the acceleration of gravity. This cut-point was not related to mechanical loading or strain but to the outcome of a positive change in BMD. The criterion loading rate used in this study was also related to the outcome of a positive change in BMD (2). However, the acceleration value reported by Vainionpää et al. is considerably higher than the values in the current study. Whether this discrepancy is a real difference in peak accelerations determined by the two different methods or whether it is confounded by differences between the accelerometers themselves is not clear. The vertical acceleration peaks reported by Vainionpää et al. during walking are similar to those derived from the GENEActiv and ActiGraph in our study, but at higher intensities (e.g., box drops), the peak accelerations recorded by the GENEActiv are slightly lower (equivalent to 4.7g ± 1.2g) and those recorded by the ActiGraph much lower (equivalent to 2.8g ± 0.7g) compared with approximately 5.6g presented by Vainionpää et al. (32). This may be because the drops described previously by Vainionpää et al. (31) ranged from the height of a one-step bench (10 cm) up to the height of three benches (30 cm), depending on participant progression through the 12-month exercise intervention (Vainionpää et al. (31)). Therefore, it is not possible to determine the exact magnitude of peak gravity from the study of Vainionpää et al. (32) associated with a drop height of 20 cm as used in this study. These discrepancies in acceleration output again underline the importance of determining monitor-specific cut-points.
Peak accelerations in this study were significantly correlated with peak loading rate across the range of activity intensities and, when looking at individual activities, walking as well as jumping was positively correlated with peak loading rate. This is important because it means that peak acceleration can be used as a proxy measure of strain rate across the intensity range and not just for higher strain activities. Generally, it is reported that high-impact exercise, particularly jumping, is needed for beneficial effects on bone mass (22,23). Consequently, the criterion peak loading rate used in this study to develop the cut-points was based on the association between the mean peak rate of loading experienced by premenopausal women during a jumping intervention that resulted in significant increases in BMD postintervention (2).
However, there have also been several studies that have reported that walking has associations with fracture risk or bone health. For example, postmenopausal women who walked 4 h·wk−1 had a 41% reduced hip fracture risk compared with those who walked less than 1 h·wk−1, and those who walked fastest had a 65% lower risk of hip fracture than slower paced walkers (10). Walking has also been found to attenuate bone loss in older men (3), and brisk walking (60 min three times per week at 67%–70% HRmax) has been reported to increase BMD in human immunodeficiency virus-infected patients receiving combination antiretroviral therapy (4). Peak loading rate, or strain, is relatively low during walking, but although strain rate has been found to be a clear determinant of cellular activity governing bone adaptation (5), it has also been suggested that the mechanosensory system of bone is sensitive to a variety of mechanical stimulations (11,19). In particular, it has been indicated that low-level stimulations normally “ignored” by bone may become highly anabolic if the temporal pattern of activity increases to higher frequencies (19). Thus, it is possible that relatively low peak accelerations may also be osteogenic if repeated often enough; however, the specific frequency-related mechanisms that may lead to beneficial effects on bone from low-intensity activities are not yet defined. If this is the case, the presence of a correlation between peak loading rate and peak acceleration at the hip and wrist across the whole range of intensities, and not only above a certain threshold, means that these frequency-related mechanisms can be identified. Thus, it is recommended that future studies examining relations between habitual physical activity and bone outcomes not only assess the frequency and duration of time spent above the peak acceleration cut-points presented but also investigate the effect of differing combinations of magnitude and frequency of peak accelerations on bone health.
A limitation in our previous study was the lack of activities where arm movement was different to leg movement. During normal daily life, many activities take place that involve arm movement that is disassociated from lower body movement. The current study included two such activities: sweeping and walking while carrying bags. Wrist accelerations were elevated in the absence of any increase in loading rate during sweeping. However, they remained below the cut-point associated with the loading rate of 43 BW·s−1. It is possible that other confounding activities may lead to more errors in classification when using a wrist-worn accelerometer; e.g., wrist accelerations will be high during arm swinging while standing still and during certain occupational activities, e.g., hammering. Examination of the time spent in different acceleration thresholds as determined by hip and wrist accelerometers worn contemporaneously alongside an independent measure of activity (e.g., observation, doubly labeled water, and use of time instrument) will help determine how much of a confounder this may be during habitual activity measurement.
We based our acceleration cut-points on a peak loading rate of 43 BW·s−1. This was the mean peak loading rate experienced by the premenopausal women during the jumping intervention that led to increases in BMD at the trochanter (P < 0.05) and at the femoral neck (P = 0.06) (2). However, it should be noted that there were no increases in BMD at the lumbar spine in premenopausal women and no effect on BMD at any site in postmenopausal women. Thus, the cut-points identified in this study are based on a single intervention. Furthermore, they are applicable only to premenopausal women and may be only applicable to hip BMD and not to other clinically relevant skeletal regions.
In conclusion, this study has presented cut-points for the GENEActiv and ActiGraph accelerometers worn at the wrist or hip, which can be used to estimate the occurrence of peak loading rates previously shown to be beneficial to bone in premenopausal women. These accelerometry-based activity monitors can be used by researchers on premenopausal women during habitual activity to unobtrusively measure the duration and frequency of periods spent at and above peak acceleration cut-points that reflect loading rates known to benefit bone. Cut-points offer an efficient and simple way to analyze accelerometer data, but we believe it is important not to lose sight of the richness of the data provided. Because peak accelerations are related to peak loading rate during low- as well as high-intensity activities, they may also be used to investigate possible frequency-related mechanisms that may lead to osteogenic benefits from lower intensity activities. Data suggest that a wrist-worn accelerometer performs similarly to a hip-worn accelerometer. Further research is needed to confirm whether this is the case in free-living individuals. The ability to unobtrusively quantify osteogenic components of habitual activity patterns from hip- or wrist-worn monitors may directly inform physical activity interventions aiming to prevent osteoporosis in later life.
The authors would like to thank the Peninsula Research Bank Database, Exeter Clinical Research Facility, for assisting in participant recruitment and Isabel Moore for assistance with analysis of the force plate data. V. H. Stiles and A. V. Rowlands received an Innovative Award from the National Osteoporosis Society, UK (reference number 117/208), to carry out this work. P. J. Griew was employed on the project.
There are no conflicts of interest. The views in this article do not necessarily reflect the views of the National Osteoporosis Society.
The results of this study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:© 2013 American College of Sports Medicine
ACTIVITY MONITORS; GENEACTIV; ACTIGRAPH; LOADING RATE; PHYSICAL ACTIVITY