Recently, I wrote a blog introducing research that my laboratory has been doing on what we call the soft tissue-to-bone ratio (SBR). The idea surrounding the SBR is that the weight of the soft tissue (lean muscle mass + fat mass) places stress on the bone. The SBR can be determined by dividing the bone mass (i.e., bone mineral content) by the amount of lean muscle mass plus fat mass divided by the bone mass from a dual X-ray absorptiometry (DXA) body composition scan. Using DXA it is possible to examine the mass on the total skeletal frame as well as various regions of the body.

To give you a little background, the data for this blog on SBR is based upon a recent scientific paper [Dengel et al., 2023] that came out of my laboratory on the muscle-to-bone ratio (MBR) in National League Football (NFL) players. I decided to re-examine the data from that paper, but instead of looking at the MBR in these NFL players I decided to look at the SBR in this group of NFL players (n=346) and compare it to a group of healthy aged-matched males (n=228).

Both groups had their fat, muscle and bone masses determined using DXA and these values were then used to calculate the total as well as regional SBR. The first blog in this two-part series compared total as well as regional SBR in NFL players to a healthy aged-matched male control group. Briefly, total SBR was lower in NFL players than in healthy aged-matched controls suggesting that NFL players have a higher amount of bone for a given amount of lean muscle and fat masses. This is obviously important considering the load the bone must support. 

In this second blog, we will compare the NFL players to each other by position. The NFL players are categorized into one of nine position categories: defensive backs (DB, n=64), defensive lineman (DL, n=47), linebackers (LB, n=48), offensive lineman (OL, n=38), quarterbacks (QB, n=21), running backs (RB, n=29), tight ends (TE, n=27) and wide receivers (WR, n=55). Punters and placekickers were combined into one category named punters/kickers (PK, n=17). Total as well as regional (i.e., arm, leg, and trunk) SBRs were calculated from DXA measures of fat, muscle, and bone masses. 

So, let us look at the positional total and regional SBR data. If you look at the figure below, you will notice that above each player position there is an image that sort of looks like a violin. These “violin plots” depict distributions of the data for each group using what are called density curves. The width of each curve corresponds with the approximate frequency (i.e., number) of data points or players in each region. Above each violin image there is a letter or series of letters. Violin plots (i.e., positions) that share a letter with another violin plot are not significantly different from each other. On the other hand, violin images that do not share the same letter are significantly different from each other.

For example, if you look at the total SBR panel at the bottom right corner of the figure below you will notice that OL and DL violin images have a different letter than the other positions. This indicates that both the OL and DL have a greater amount of soft tissue mass per unit of bone mass than in the other positions. Also, you will notice that DL and OL do not share the same letter. If you look a little closer, you will notice that the mean line is higher in the OL than DL positions. Indicating that the OL position group has a greater average total SBR than the DL. The greater amount of total SBR might indicate greater stress being placed on these players’ skeletal frames due to the greater SBR. Does that mean they are at greater risk for injury? Well at this point it is hard to say since this is the first research to look at SBR in NFL players. 

Now let us look at the regional SBRs. If you look at the arms, legs, and trunk panels of the figure below you will also notice that, like the total SBR panel, not all positions share the same letter. For example, in the arm SBR panel (upper left corner panel), DB and DL violin images do not share the same letter. Indicating that the arm SBR is significantly lower in DB than DL. This indicates that DL had a greater amount of soft tissue mass per unit of bone mass than in the DB. Similarly, the LB and the OL do not share the same letter. This indicates that the arm SBR is significantly lower in the LB than the OL. Looking at the other positions you can see that the DL and OL stand out from the other positions. If we look at the trunk SBR (lower left corner panel) you will see a similar picture with DL and OL having greater values than most of the other positions. Again, the higher SBR for the trunk region indicates that these two positions have greater amounts of soft tissue mass per unit of bone. Examining the leg MBR (upper right corner panel) the OL and DL again have the highest leg SBR. For most of the figures, you will notice that positions that mirror each other (i.e., DB and WR or OL and DL) have remarkably similar total SBR as well as regional SBR values. This is not too surprising since we have previously reported [Dengel et al., 2014; Bosch et al., 2019] that offensive and defensive positions that mirror each other have similar total as well as regional measures of fat, lean, and bone masses. In the present study, we also observed similar overall patterns of body compositions in individuals who played offensive or defensive that mirrored each other. Although OL had a higher percent body fat and total fat mass than DL, the two positions were similar in overall bone and soft tissue masses.

What does it all mean?
The SBR provides athletes, coaches, and dietitians with a way to assess the weight that is being placed on the total skeletal frame or region (i.e., legs, arms, etc.). Using DXA to determine body composition allows one to not only examine total SBR, but also regional (i.e., arm, leg, etc.) SBRs. This information may prove to be useful in return from injury or weight gain/loss. The figure in this blog also demonstrates differences in SBR by position. The idea that there is an SBR that fits all players is not a correct assumption. There might be a range for the SBR for each position. Finally, it is important to note that this is a new area of research. Future studies are needed to investigate the relationship between SBR and performance metrics (e.g., power, strength, and game performance), as well as determining if changes in SBR are related to changes in physiological and mechanical loading. In addition, future research is needed to examine the effect of training on SBR.

Bosch TA, Carbuhn A, Stanforth PR, Oliver JM, Keller KA, Dengel DR: Body composition and bone mineral density of division 1 collegiate football players: a consortium of college athlete research study. Journal of Strength and Conditioning Research 33(5):1339-1346, 2019. 

Dengel DR, Bosch TA, Burruss TP, Fielding KA, Engel BE, Weir NL, Weston TD: Body composition of National Football League players. Journal of Strength and Conditioning Research 28(1):1-6, 2014.

Dengel DR, Evanoff NG. Positional Differences in Muscle-to-bone ratio in National Football League Players. International Journal of Sports Medicine 44:720-727, 2023.

About the Author
Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.

Read Part One here
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