Pros and cons

Pros
Many of the advantages of the accelerometry and heart rate monitoring are summated in combined heart rate and motion measurement.

Provides an objective measure of total physical activity
Provides an accurate estimate of energy expenditure during physical activity
Combined measurement has been shown to be reliable and valid at both individual and group level
Gives a detailed description of activity patterns
Combined devices have storage capacity of at least 7 days
Able to detect change in activity
Unlikely to induce a large change in behaviour
Single-piece devices involve a reasonably low individual burden
Suitable for use in the young and old
Easy and quick for data collection and analysis
Non wearing time easily identified
Waterproof
Reasonably low level calibration can yield accuracy suitable for epidemiological studies
Using the combined approach means that many of the drawbacks when the methods are used in isolation are overcome

Cons

Combined devices are expensive
The data produced from either combined devices or the single methods used simultaneously are complex and requires careful interpretation and guidance from others experienced in the method. This is particularly true for dealing with measurement noise in HR recordings
Combining a heart rate monitor with a separate accelerometer imposes a reasonable individual burden
Compliance may be difficult in some populations especially adolescents
If using electrodes individuals may experience an adverse skin reaction to these