Validation of Cut-Points for Evaluating the Intensity of Physical Activity with Accelerometry-Based Mean Amplitude Deviation (MAD)

Abstract

Purpose: Our recent study of three accelerometer brands in various ambulatory activities showed that the mean amplitude deviation (MAD) of the resultant acceleration signal performed best in separating different intensity levels and provided excellent agreement between the three devices. The objective of this study was to derive a regression model that estimates oxygen consumption (VO2) from MAD values and validate the MAD-based cut-points for light, moderate and vigorous locomotion against VO2 within a wide range of speeds.

Methods: 29 participants performed a pace-conducted non-stop test on a 200 m long indoor track. The initial speed was 0.6 m/s and it was increased by 0.4 m/s every 2.5 minutes until volitional exhaustion. The participants could freely decide whether they preferred to walk or run. During the test they carried a hip-mounted tri-axial accelerometer and mobile metabolic analyzer. The MAD was calculated from the raw acceleration data and compared to directly measured incident VO2. Cut-point between light and moderate activity was set to 3.0 metabolic equivalent (MET, 1 MET = 3.5 ml · kg-1 · min-1) and between moderate and vigorous activity to 6.0 MET as per standard use.

Results: The MAD and VO2 showed a very strong association. Within individuals, the range of r values was from 0.927 to 0.991 providing the mean r = 0.969. The optimal MAD cut-point for 3.0 MET was 91 mg (milligravity) and 414 mg for 6.0 MET.

Conclusion: The present study showed that the MAD is a valid method in terms of the VO2 within a wide range of ambulatory activities from slow walking to fast running. Being a device-independent trait, the MAD facilitates directly comparable, accurate results on the intensity of physical activity with all accelerometers providing tri-axial raw data.

Fig 1. Illustration of pace conducted test performance.

The lower graph describes the required speed and the upper curves the measured VO₂ and MAD values during the whole test. The unit mg denotes milligravity.

Fig 2. Preferred gait types in different speeds.

The preferred gait of the fully completed stages is shown for each speed.

Fig 3. Oxygen cost and consumption in different speeds.

The oxygen cost of the locomotion (black circles) describes the economy and the oxygen consumption (open circles) the intensity of the movement.

Fig 4. Sensor placement and MAD in different speeds.

The measured mean MAD value with the three sensor positions is shown only for speeds performed by all 29 participants. The whiskers denote the 95% confidence intervals of the mean value. The unit mg denotes milligravity.

Fig 5. Relationship between VO₂ and MAD.

Stages containing only walking have open circle and other stages black circle. The dotted lines denote 3 MET and 6 MET thresholds and the unit mg denotes milligravity.

Fig 6. Optimal cut-points.

The dotted lines represent the optimal MAD cut-points for 3 and 6 MET limits. The MAD values are shown up to 600 mg. The unit mg denotes milligravity.

Fig 7. ROC curves and AUC for cut-points.

ROC curve and AUC (mean and 95% confidence interval (95% CI)) for 3.0 MET limit (left) and for 6.0 MET limit (right).