Segmental Body Water Analysis

• Derived from impedance measured in each segment separately. InBody 970 measures eight frequencies across five segments, that is 40 impedance measurements per examination.
• The split into water inside and outside cells is made possible by the different frequencies: low ones do not pass well through the membrane and reflect mainly extracellular water, high ones pass through and also capture intracellular water.
• The manufacturer presents these values as measured rather than estimated. It does not publish the equation converting impedance into a volume of water, however.
• A reference range is shown for each segment; the manual does not publish how it is calculated. The bar length in the chart relates to ideal weight, not to current weight.
• Eight touch electrodes provide the same starting point for every segment, which according to the manufacturer improves the reproducibility of repeated measurements.

• Do not compare absolute litres between segments. The trunk is the largest by volume and the manual notes that its muscle makes up almost half of the total — a higher number there is expected.
• Compare side against side (right arm to left, right leg to left) and each segment against its own reference range on the sheet.
• Asymmetry between limbs can relate to a one-sided sport or to the aftermath of an injury — the manual gives these causes for segmental lean mass, and the same logic applies to water, because water and muscle are closely linked.
• A high value in a segment is not in itself good news. The manual recommends checking the segmental ECW Ratio for segments with high water, because lean mass can be overstated by that water.
• Breaking the value down into intracellular and extracellular water sharpens the picture: water bound in cells relates to cell mass, water outside cells to fluid distribution. Neither value on its own establishes a cause.

• Compare only measurements with the same preparation. The manual notes that water shifts towards the lower half of the body during the day, so an afternoon measurement of the legs is not comparable with a morning one.
• After training, water in the loaded segments tends to be temporarily higher because of increased perfusion. The manual therefore recommends not exercising before the measurement.
• Read a simultaneous rise in water across all five segments differently from a rise in one. The first points attention to overall water balance and to measurement conditions, the second to what happened to a particular limb.
• A sustained rise in water in a segment together with rising lean mass and a stable ECW Ratio corresponds to tissue development. A rise in water with a rising ECW Ratio means the gain falls more to the extracellular compartment.
• The manufacturer Comparison Result Sheet shows the difference between the current and previous measurement for the whole body and for each segment, so it is visible where the change concentrated.

General principles: how to read change between measurements →

• The segmental ECW Ratio – a mandatory check for any segment with high water.
• Segmental intracellular and extracellular water – split the total into the compartment bound in cells and the compartment outside them.
• Segmental lean mass – the value water can overstate; the two sections are read together.
• Total body water and the whole-body ECW Ratio – place the segmental picture in overall context.
• Segmental phase angle – another segmentally measured quantity derived directly from the electrical properties of tissue.

• More water in a segment does not mean more muscle. The manual explicitly warns that in people who do not maintain stable water balance, muscle mass can be overstated.
• A difference between the right and left side is not automatically a problem. It commonly relates to the dominant side, a sport or an old injury.
• High water in one segment does not establish oedema. Oedema is a clinical finding assessed by examination.
• Absolute litres are not compared between segments. The trunk naturally has the highest value because it is the largest.
• Right and left are given from the point of view of the person being measured. Right arm on the result sheet means your right arm.

• Time of day — the manual notes that water shifts towards the lower half of the body during the day.
• Body position before the measurement; the manual recommends standing for at least 5 minutes, because standing lets gravity draw blood into the lower limbs.
• Training shortly beforehand and one-sided loading of a single limb.
• Sauna, showering and room temperature outside the recommended 20 to 25 °C.
• Electrode contact: dry or unwiped skin, a poor grip on the handle, arms touching the trunk or thighs touching each other.
• Food and drink shortly before the measurement and a full bladder.
• Menstruation, during which body water fluctuates.

• Compare sides first, then each segment against its reference range. Leave the absolute numbers between segments alone.
• For every segment with high water, open the segmental ECW Ratio. This is the procedure the manual itself recommends.
• When muscle development interests you, read segmental water alongside segmental lean mass, not instead of it.
• Judge asymmetry as a trend. A one-off difference after one-sided loading is not the same as a difference that holds across several measurements.
• Swelling, pain or the sudden enlargement of one limb belong with a doctor. Segmental water neither confirms nor excludes such a state.

• It does not diagnose oedema, inflammation, injury or disease.
• It cannot establish whether a difference between sides comes from muscle, fat or fluid.
• The manufacturer displays a reference range for each segment but does not publish how it is calculated.
• Segmental water values are not covered by the manufacturer internal validation study against DEXA; that study concerns lean mass.
• Values are sensitive to time of day, body position and the quality of electrode contact.