Measuring Physical Function and Mobility in Real World Settings

Physical function and mobility are relevant to virtually all clinical indications and are significant determinants of an individual’s quality of life. Because of this, function and mobility are often assessed in clinical trials using subjective questionnaires or in-clinic performance tests, but these assessments may not reflect a patient’s lived experience. Wearable sensors provide an opportunity to move assessments of function into the real world to measure how patients feel and function in their everyday environments.

In this blog, we examine the importance of physical function and mobility, review the drawbacks of current assessment methods and discuss the promise of wearable technology to provide comprehensive real-world measurements of these outcomes.

Impairments in Physical Function and Mobility Increase the Risk for Poor Outcomes

Physical function is defined as the ability to perform activities of daily living (e.g., washing dishes, getting out of chairs, climbing stairs). In contrast, mobility generally refers to the ability to move with ease¹.

Both physical function and mobility are central to the maintenance of independence and essential for quality of life. Chronic disease, treatment side effects, or acute illness or injury are just a few ways physical functioning can become impaired. Regardless of the cause, physical function and mobility impairments increase the risk of falls, disability, and result in poor survival outcomes.

For these reasons, physical function and mobility are commonly assessed in clinical trials to demonstrate treatment effects.

Traditional Assessments of Physical Function and Mobility Have Limitations

To capture a patient’s real-world physical functioning, we must measure not only how a patient moves but how they move in their own environment. The World Health Organization (WHO) acknowledges that function is a result of the interactions between biological, environmental, and personal factors. In other words, the context in which the person exists cannot be ignored when assessing function.

One of the most frequently used objective assessments of physical function is the 6-minute walk test (6MWT). During this test, researchers measure the distance a patient can travel in 6-minutes down a long, wide, and unoccupied hallway at their fastest walking pace. The 6-minute walk test has been used for decades in numerous clinical populations.

It is generally agreed there are two significant problems with 6MWT:

1. The 6-minute walk test eliminates the challenges of navigating through everyday life that influence the speed and ease at which an individual can move from one point to the next. During even short walks, patients may encounter obstacles in their walking path requiring them to stop short or cover surfaces with varying levels of friction, like tile or carpet. The controlled setting of the 6-minute walk test eliminates these factors and may yield a misleading picture of a patient’s actual physical function.
2. Clinic-based tests of physical function, like the 6-minute walk test, are assessed relatively infrequently. These sporadic assessments are of particular concern for patients with relapsing and remitting conditions. They may be tested at the beginning of a study on a “good” day and at the end on a “bad” day. This day-to-day variability may result in potentially missing a treatment effect because the patient felt differently during the assessment days.

Outcome Measures that Can Be Assessed

We think of real-world measures derived from wearable accelerometers in two domains:

Participation Measures

• Steps per day
• Time spent in moderate physical activity
• Time spent in sedentary behaviors (e.g., sitting/lying)

Function Measures

• Best 6-minute effort (B6ME)
• Time to rise from a seated position
• Preferred cadence
• Stride length and

Like any measure derived from a wearable sensor in the real world, accuracy and precision depend on the sensor modality, patient population, and measurement environment. Many algorithms used to extract physical function and mobility measures have been developed in healthy populations, but clinical populations often move differently from the average healthy adult. Unfortunately, algorithms are not “one-size-fits-all” compatible – meaning they are designed to work well on the data they were trained on and often diminish in accuracy when applied to data from populations of varying characteristics.

A New Class of Outcome Measures Derived from Wearable Sensors

Wearable kinematic sensors (e.g., accelerometers, gyroscopes) can collect real-world data remotely for weeks to months at a time and provide a solution to the challenges posed by in-clinic assessments. In general, they have been used to capture a patient’s participation in physical activity using metrics like total steps per day or time in moderate-intensity physical activity. With the advancements in technology, there are opportunities to derive novel metrics of real-world physical function and mobility.

The chair rise test is another commonly used measure of physical function and mobility. In this test, researchers measure how long it takes for patients to get up from a chair. Like the 6-minute walk test, rising from a stiff chair in a clinic setting does not entirely reflect a patient’s usual environment, which likely includes chairs with varying levels of support and cushioning. Using an accelerometer placed on the thigh, we can identify when patients are seated and then measure how long it takes them to move from a seated position to an upright position in their real-world environment.

Ultimately, we want treatments to improve how patients feel, function, and survive in their own environments. Wearable sensors allow researchers to understand whether a treatment translates to a meaningful change in the way patients function in their own homes, where it matters most.

Wearables Offer Complementary Measures to Improve Clinical Decisions

We must recognize that legacy physical function and mobility measures, like the 6-minute walk test and time to get up and go test, have been clinically validated and are mainstays in clinical trials. To suggest eliminating these tools would be premature. There are, however, limitations to these assessments. Functional and mobility measures acquired by wearable sensors, implemented in conjunction with legacy measures, could complement and fill in the gaps where clinic visits are infrequent. Ultimately, combining these approaches will provide a complete picture of patient health and function which better inform clinical decisions.

Visit our digital endpoints page to learn more about measuring components of physical function and mobility in real-world settings. We’ll help you select the best continuous monitoring wearable sensor and obtain the most clinically meaningful outcome measures.

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