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Body Talk: Defining Volume, A Technical Dissertation on Hair Body and Volume

Contact Author Trefor A. Evans, Ph.D., TRI-Princeton, Princeton, New Jersey
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An aspect of my job that I especially enjoy is the opportunity to regularly talk hair science with a variety of others. My position brings me into frequent contact with many other researchers in our field with whom I relish such discussions; at the same time, teaching and consulting keeps me on my toes in terms of countless interesting questions. This said, my stomach sinks every time the phone rings and the person on the other end wants to talk about hair body and volume.

Such nebulous and sometimes spurious language used by consumers has been a reoccurring theme of past columns, but the terms volume and body are perhaps the most indefinable. To a scientist, the term volume has a clear definition—Webster’s dictionary defines it as “the amount of space occupied by a three-dimensional (3D) object.” But, to quote Chris Gummer of Cider Solutions—who reviews hair claims for Clearcast, the British claims clearance agency—“warning lights start flashing when I see the dictionary being used as a scientific reference.” Figure 1 and Figure 2 show individuals whose hair occupy a great deal of space—yet neither of these represent desirable states. The hair in Figure 1 would more likely be termed frizzy; while Figure 2 shows an extreme case of static flyaway. Clearly the consumer parameter is considerably more complex than this simple technical definition.

Consumers know volume and body when they see it but pushing for further description generally results in other similarly intangible terms—hair is full of life, vibrance, bounce, etc. The same is true for the antithesis of this state, where language such as flat, limp and lifeless may be encountered.

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It would be nice to report that rigorous scientific attention has unlocked these mysteries and provided lucidity—but no. These complex inter-related attributes have no simple definition and therefore no simple means of measurement. This is an important point as, to paraphrase the measurement scientist’s axiom, attributable to the illustrious Lord Kelvin (1824–1907), “When you cannot express it in numbers your knowledge is of a meager and unsatisfactory nature.” That pretty much sums up our understanding of hair volume and body.

This article nonetheless attempts to wade through the muddiness surrounding these parameters and shares views acquired from years of personal experimental endeavors, reading scientific literature and participation in end-user panel discussions and focus groups. It is hoped that this discourse defines our current comprehension—while perhaps encouraging others to pick up the torch and move forward.

Scientifically Speaking

It is difficult, and possibly even dangerous, to tackle consumer-speak from a scientific perspective but a few studies have attempted to do so for our two inextricably-linked consumer attributes. Some of these have been published1, 2 while others I am also aware of have not. The good news, at least, is that all pretty much come to the same conclusions: namely, a number of single fiber properties seem to associate.

Fiber dimensions, for example, are believed important; where body and volume are often difficult to attain for those with fine, thin hair. Lower fiber density on the scalp is also thought a hindrance. Fiber stiffness was similarly identified in all studies, where less rigidity is believed an impediment. These ideas/parameters sound reasonable and possibly even instinctive—but at the same time they are not factors that appear readily altered by cosmetic treatments (although more on this later).

Fiber curvature is thought to be another key parameter—and one that can be cosmetically manipulated either through temporary heat styling or a long-lasting permanent wave. However, the one parameter identified in all studies that is most easy altered is inter-fiber friction.

Consumers know volume and body when they see it; but pushing for further description generally results in other similarly intangible terms.

A degree of inter-fiber friction is thought desirable to “prop-up” voluminous styles and help to prevent them from relaxing or settling under their own weight. This concept has a number of interesting implications. On one hand, increasing hair curvature could be argued to produce more tenacious points of contact between adjacent fibers and so the effect of curvature might itself be a corollary of the friction. Perhaps we also think about the popular (though ill-advised) practice of back-combing to achieve volume and body. That is, hair is combed from tip to root—and against the grain of the cuticle scales—which creates friction due to irreversible uplifting of the tile-like structures. A simpler (and immensely less damaging) approach might be to use a hair spray, which achieves this goal by introducing polymeric spot-welds and seam-welds between adjacent fibers.

The lubricating effect of traditional conditioner products has been heralded in many earlier articles in this column; but here a downside manifests. Specifically, this lubrication reduces inter-fiber friction which, by the above argument, should hinder the ability to maintain voluminous styles. The additional weight of surface deposits is also often cited as a negative. To this end, conditioners are generally thought to have a negative effect on body and volume—where “extra-body” or “bodifying” variants in product lines are usually on the light end of the conditioning spectrum.

Hair Color for Body and Volume

A somewhat common belief is that there is an improvement in these parameters accompanying a permanent color treatment. The science behind these products has been touched upon previously in this series,3 while also highlighting the permanent changes to the hair structure4, 5 (i.e., damage) that can occur. One such manifestation of damage involves some degree of cuticle deterioration, thus increased inter-fiber friction might be theorized as the root-cause.

However, we are learning that a variety of treatments—including many traditionally thought to be damaging—can indeed increase individual fiber stiffness. This can be quantified in terms of the dry state Young’s modulus via conventional single fiber tensile testing6—and has been shown previously for bleaching,5 UV exposure and heat treatment7 of hair. Figure 3 shows the Young’s modulus for hair dyed with a commercial permanent dye relative to the untreated control state and similarly demonstrates a comparable effect.

The reason for this occurrence is still a puzzle since such treatments are known to deplete strength-supporting bonds within the hair, and would therefore be expected to lower all mechanical properties. However, despite the diminishment of other tensile properties, these treatments consistently raise the dry state Young’s modulus. Therefore, contrary to an earlier statement, there is the possibility for cosmetically increasing fiber stiffness—although we would like to learn how to accomplish this feat without causing other compromising issues associated with these treatments.

Skill and know-how, coupled with the right hair care products, can allow for a degree of “manufactured” volume and body in individuals not blessed with optimal natural properties.

Moment of Clarity

The above scientific discourse sheds light on how fundamental single fiber properties apparently relate to the perception of body and volume. But on the other hand, a professional stylist is able to generate these states in even the limpest hair. Clearly, there must also be a skill-related component to the creation of these desired states.

For me, a revelation in this area occurred while watching, from behind a one-way mirror, focus group participants styling their hair. Panelist after panelist created voluminous hair styles using a blow drier and their own hair care products in accord with their daily routine. But then, each was asked to re-wash their hair and again go through the same process—except this time the study moderator confiscated their mousse products.

The difficulty created by the absence of this product form was dramatic. The study participants were no longer able to adequately hold and manipulate their hair with the styling brush, as fibers flailed in an uncontrollable, chaotic manner under the influence of blow driers. Without the gripping ability provided by such a product, the participants had to fight against the lubrication delivered by their conditioners—and, at the end of the process, all participants ended up with notably less voluminous styles.

This experience illustrated to me the importance of the style creation process. Clearly, professional stylists are trained and skilled in this area to a degree above and beyond a typical individual. But the importance of so-called style creation products (i.e., mousses, gels, etc.) to facilitate this process becomes eminently apparent. I have also seen professionals use hair sprays during the styling process to the same end.

There may, indeed, be fundamental hair properties that predispose the wearer to a natural, innate state of body and volume; but there is also an ability to create a manufactured version of these states, with the extent of this acquired state having dependence on individual skill and know-how.

Measurement Attempts

Each of the aforementioned single fiber properties themselves (diameter, stiffness, curvature, fiber density and friction) can be quantified without too much difficulty but it is still likely their relationship to the desired end property is complex. For example, very thick, dense, curly hair may well be described as frizzy and unmanageable (see Figure 1) rather than voluminous. Moreover, only one of these properties would seem affected by traditional shampoo and conditioner products—and, in theory, conditioning-related lubrication would be considered a negative.

There is desire for quantification at the fiber array level. Most often this is performed by taking the consumer language literally and using the combination of high quality photography and image analysis techniques to calculate a spatial measure of hair volume. An example is shown in Figure 4. Of course, this strategy is subject to all the caveats already mentioned.

Another approach that is sometimes encountered involves measuring forces associated with pulling hair tresses through a series of rings with decreasing radii.8 Here, the belief is that more voluminous hair will produce higher forces as it squeezes through progressively smaller holes. In theory, curlier, stiffer and thicker hair would seemingly give rise to higher forces; however, per previous stipulations, it appears a stretch to directly equate this property to consumer body and volume. The same is seemingly true for the so-called radial compression method, which measures the force (or energy) to squeeze a fiber bundle.9

Presumably, having "just right" combinations of given single hair fiber properties predisposes the wearer to volume and body.

Hair Motion

Hair motion is a concept I propose to also relate to body and volume. At some point, every TV hair care commercial shows a model flipping her hair around in a beautiful, flowing manner. Clearly, this motion is characteristic of attractive, healthy hair, with the controlling variables presumably being very similar to those already described for body and volume. Hair dimensions, density, weight, stiffness, curvature and inter-fiber friction would all seem factors that will greatly impact hair motion.

This is a topic that has received surprisingly little attention in the scientific literature. Our own experimental efforts indeed suggest that different sources and states of hair do indeed move very differently (see Figure 5). Thick, heavy Asian hair moves very differently from fine Caucasian hair; curly hair moves differently from straight hair; chemically damaged hair moves differently from healthy hair. That being said, a means of quantifying these visually noticeable movements is not obvious or straightforward.


On the surface, it appears gratifying that independent studies have reached the same conclusions regarding the importance of certain single fiber technical properties to the consumer terms body and volume. But further poking produces some wrinkles. Namely, while parameters such as fiber thickness, curvature, stiffness, friction and density all appear reasonable contributors, it seems a Goldilocks scenario must exist where, for example, hair that is too thick, too dense and/or too curly will produce an undesirable, unmanageable and very frizzy state. Seemingly, fibers should not be too thick, or too thin; hair should have some curvature, but not too much, etc.

Herein, perhaps lies the key to the problem—whereby hair that possesses what consumers call body and volume appears to sit at a sweet spot in terms of actual technical volume. Hair should have some life and not hang limply from the head but at the same time, it should not be too expansive. Presumably, having “just right” combinations of the aforementioned single fiber properties predisposes the wearer to a high degree of innate volume and body. Yet, as highlighted, these are mostly unaffected by cosmetic treatments.

Nonetheless, skill and know-how—coupled with the right hair care products—can allow for a degree of “manufactured” volume and body in individuals not blessed with optimal natural properties. Specifically, products such as mousses and gels greatly facilitate the style creation process and allow those with relatively flat hair to go bigger (so-called up-volume). On the other hand, those with a very thick, curly natural state may desire a means to tame their hair to a more-desired state (down-volume). In this instance, heavy conditioning treatments may achieve this end through the action of additional weight—although such treatments would clearly be disastrous to those desiring volume-up.

This sweet spot complicates measurement approaches as there seemingly is not a linear relationship between parameters such as hair’s actual physical volume or compressive forces and the desired end result. Some increase within an appropriate hair type might indeed be desirable; but not too much, and not for all hair types.

Moreover, sample preparation takes on a still higher significance—where, for example, a given hair tress treated with the same treatment will give very different volume if it is air-dried under static conditions, blow-dried or dried with physical manipulation (i.e., brushing). Accordingly, the significance of the treatment itself might be muddled.

A means for differentiating between natural and manufactured volume might lie in hair’s motion. That is, while styling products can be used to produce highly voluminous styles, these are usually accompanied by a stiff feel and limited motion. Similarly, the use of heavy conditioning products and oils to tame overly-voluminous styles can also preclude a natural motion.

Clearly, there is much work for hair researchers going forward.


  1. PS Hough, JE Huey and WS Tolgyesi, Hair body, J Cos Sci 27 571–578 (1976)
  2. CR Robbins and GV Scott, Prediction of hair assembly characteristics from single fiber properties, J Cos Sci 29 783–792 (1978)
  3. TA Evans, Quantifying hair color fading, Cosm & Toil 130(1) 30–35 (2015)
  4. TA Evans, How damaged is hair? Part 1: Surface damage, Cosm & Toil 132(4) 38–48 (2017)
  5. TA Evans, How damaged is hair? Part 2: Internal damage, Cosm & Toil 132(6) 36–45 (2017)
  6. TA Evans, Measuring hair strength, part 1: Stress-strain curves, Cosm & Toil 128(8) 590–594 (2013)
  7. TA Evans, How damaged is Hair? Part 3: Better defining the problem, Cosm & Toil 132(7) 58–67 (2017)
  8. CR Robbins and J Crawford, A method to evaluate hair body, J Cos Sci 35 369–377 (1984)
  9. HD Weigmann, YK Kameth and H Mark, Radial compression for evaluation of hair body, Proc Int Wool Textile Res Conf, Pretoria, South Africa (1980)

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Figure 1. 'Voluminous' hair occupies considerable 3D space

Figure 1.

This hair would likely be described a frizzy rather than voluminous.

Figure 2. Static flyaway increases 3D space

Figure 2. Static flyaway increases 3D space

Static flyaway increases the 3D space of hair but not in a manner that consumers would call voluminous.

Figure 3. Young’s modulus for dyed hair

Figure 3. Young’s modulus for dyed hair

The use of permanent color products increases the dry state Young’s modulus, i.e., stiffness, of hair.

Figure 4. High quality photography to quantify literal volume

Figure 4. High quality photography to quantify literal volume

There is desire for quantification at the fiber array level. Most often this is performed by taking the consumer language literally and using the combination of high quality photography and image analysis techniques to calculate a spatial measure of hair volume.

Figure 5. Quantifying hair motion has gone largely unexplored.

Figure 5. Quantifying hair motion has gone largely unexplored.

Thick, heavy Asian hair moves very differently from fine Caucasian hair; curly hair moves differently from straight hair; chemically damaged hair moves differently from healthy hair.

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