Inclinometer

An inclinometer measures how far an object tilts from a horizontal or vertical reference. It works on a principle similar to a spirit level used in construction: a bubble in liquid or a sensor inside uses the direction of gravity as its reference. Applied to the body, this principle allows the tilt of a particular area, such as the head, trunk, or limbs, to be read in degrees. This is why it is used to record range of motion.

There are several types.

• Mechanical (bubble) inclinometer — A bubble or needle within a graduated dial indicates the tilt.
• Electronic or digital inclinometer — A sensor reads the angle and displays it as a number.
• Smartphone app — Uses the phone's built-in tilt sensor.

Using the neck as an example, the process goes like this.

1. Sit or stand comfortably and take a starting position with your eyes looking straight ahead. 2. Place the inclinometer against the head (at the forehead or crown) and set it to zero. 3. Bend your head forward, backward, or to the side until you cannot move any farther. 4. Read and record the angle shown on the inclinometer.

There is also a method that uses two inclinometers. One is placed on the moving area and the other on a lower reference area (for example, near the hips when measuring the lower back), and the difference between the two readings is calculated. This can help filter out some of the movement contributed by surrounding areas. The two-inclinometer method has long been used to measure lower back range of motion.

Unlike the elbow or knee, the neck and lower back have joint centers that are difficult to locate precisely from the outside. An inclinometer, which takes its readings relative to gravity, is especially convenient in these areas. Comparing it with the goniometer below makes the angle-measurement methods easier to understand.

A goniometer is a joint angle measurement tool with two arms and an angle scale. One arm is aligned with the reference body segment and the other with the moving segment, and the angle between the two arms is read. It is well suited to measuring joints whose positions are readily visible from the outside, such as the elbow, knee, and fingers. General-purpose models made of plastic or metal are the most common, and electronic goniometers and smartphone tools are also used for the same purpose.

The difference from an inclinometer is the reference used for the reading. An inclinometer reads tilt relative to gravity, while a goniometer directly reads the relative angle between two body segments. This makes a goniometer easier to use where the joint center is clearly identifiable, such as the elbow, and an inclinometer easier to use where the center is difficult to locate and an entire segment tilts as a unit, such as the neck or lower back.

What a goniometer measures is the angle observed in a specified position using defined reference points. It does not directly measure pain intensity, muscle strength, or tissue condition, so it helps to consider how movement feels and how the person functions in daily life alongside the angle. A review of studies measuring adult elbows with goniometers found that agreement between repeated measurements by the same examiner (ICC) ranged widely, from 0.45–0.99, depending on study conditions. Higher agreement means that the same values were obtained more consistently. When precise comparisons are needed, it is important to use the same tool, position, and reference points.

Angles can vary with the following factors, so it is important to keep conditions consistent when comparing changes.

• Use the same tool. Mechanical devices, digital devices, and phone apps give slightly different readings.
• Measure in the same position. Switching from sitting to standing produces different readings.
• Move in the same way. Active range of motion, in which you move on your own, differs from passive range of motion, in which a practitioner moves you.
• Keep your trunk from moving along with the measured area. If your shoulders or trunk turn when you turn your neck, the angle will appear larger than it actually is.
• Mark the reference points. Placing the sensor in the same location each time makes comparisons easier.
• Have the same person take the measurements if possible. Different examiners place sensors differently and have different reading habits.

• Rehabilitation and physical therapy — Comparing initial and later range-of-motion measurements of the neck, lower back, or shoulder to track changes.
• Sports and exercise instruction — Checking athletes' joint flexibility and developing training plans.
• Self-checks at home — Taking weekly measurements in the same position with a smartphone app lets you see changes in your body for yourself.

• Keep a measurement log. Recording the date, position, tool, and angle on one line makes it easier to compare results a month later.
• Do not get too caught up in the numbers. A difference of a few degrees over several days may fall within the range of measurement error, so look at trends over several weeks.
• Measure at the same time of day. A body that feels stiff in the morning may loosen up by evening, producing different readings.