How was the "Flexibility" Value of the BioAge Developed?

With the introduction of the EGYM Fitness Hub, we also introduced the brand new "Flexibility" value for the EGYM BioAge. In this article, you will discover the background and history of the development of the flexibility analysis!

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Please note that the exact formulas are confidential to EGYM. This article describes the origins and basic ideas behind the calculations

The Challenge

The underlying goal of the EGYM BioAge is to reflect your physical state as accurately as possible and motivate the exerciser in the best possible way. Therefore, the following should always apply: If my performance is equal to the average of my benchmark group, my BioAge is equal to my chronological age. If I am better or worse than the benchmark, the BioAge goes down or up, respectively. 

The key here is to gain the trust of both the exerciser and trainer. Only when I have confidence in what I see – real motivation is achieved. BioAge is probably one of the motivational tools that create the greatest "wow-effect" on the client. Yet, it can also have a negative impact on motivation in the case of implausible results.

The Origin of the “Flexibility” Value of the BioAge

Like other developments at EGYM, the "flexibility" value started as an idea. Then we investigated whether the idea meets our scientific standards. So, before the feature "Flexibility" value was developed, the following questions had to be answered:

Does flexibility decrease with age? 

Yes: Numerous studies have demonstrated this. Between the ages of 30 and 70, the total range of motion decreases by 20-30% (Adams et al., 1999). For neck flexion (Lind et al., 1989; Youdas et al., 1992) and trunk side flexion (Fitzgerald et al., 1983), there is a relationship between age and decreasing mobility. 

Do mobility exercises improve flexibility? 

Yes: The primary mechanism for the reduction of range of motion with age is the reduction in the number of sarcomeres in series (Narici et al., 2003), i.e., it becomes "shorter." Muscle length training addresses precisely this issue. An optimal training stimulus is set, which triggers the build-up of sarcomeres in series (Morgan & Proske, 2004; Blazevich et al., 2007; Franchi et al., 2017).

The basic requirements for the BioAge are therefore met. Everything is now in place for the development of a new, scientifically based BioAge. But how do we get from a good idea to a reliable formula?

Target-flexibility - how good should I be?

Let's go back to the basic idea of BioAge: If I am as good as my reference group, my BioAge should be equal to my chronological age. 

Here, again, the scientific literature is the first source of information. For example, for the neck flexibility test, some studies have measured the relationship with age. Among others, sc using x-rays. The results were as follows:

As we can see, the angle decreases with age using the following formula: 
Neck flexibility (degrees) = 67.54 - 0.50 * chronological age.

Using this formula, we have an excellent basis for our BioAge. We now know that the neck flexibility for 0 years is 67.54° (y-axis intersection) and decreases with every successive year by about 0.50° (negative slope of the line). With the help of this formula, we can now calculate which degree we should reach which age in the lateral flexion of the neck. 

Of course, we must keep in mind that the measurement systems differ significantly. It was measured with X-ray in the study, while the EGYM Fitness Hub uses a system based on the latest in 3D camera technology. Also, the participant sample may not match, and there may have been some changes in the flexibility of the general population in the years since the publication of the study. Furthermore, the number of people measured in the above source, while sufficient for publication, is still far from satisfactory for our standards.

Consequently, we did our own research. First, we measured every EGYM employee and supplied some gyms with a very early version of the Fitness Hub with the mandate to measure as many customers as possible to help us improve accuracy. The inputs resulted in our final formulas for each of the planned flexibility tests, describing the relationship between age and flexibility. 

Please note that the calculation of target flexibility is only one part of the BioAge calculation. In the subsequent steps, the deviation of the achieved degree from the target norm is calculated, normalized, and converted into the BioAge. This article deals with the first step, i.e., the basis and the basic idea of the BioAge.

BioAge - Making Performance Tangible

The basic framework of the "flexibility" value of the BioAge is the scientific literature in combination with internal investigations and preliminary tests in well-chosen gyms. Consequently, we can ensure that there is already an excellent estimate of the BioAge when the Fitness Hub is released. However, due to the difficult situation caused by the coronavirus, the database for the first version is not yet sufficient in the long term. It is expected that shortly after the start of series deliveries of the Fitness Hub, we will have enough test results to make the "flexibility" value of the BioAge even more accurate.

Learn more about the scientific foundation of our BioAge values!


Adams, K., O'Shea, P., & O'Shea, K. L. (1999). Aging: its effects on strength, power, flexibility, and bone density. Strength and conditioning Journal, 21, 65-77.

Blazevich, A.J., Cannavan, D., Coleman, D.R., Horne, S. (2007). Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles. Journal of Applied Physiologie, 103(5), 1565-1575.

Fitzgerald, G. K., Wynveen, K. J., Rheault, W., & Rothschild, B. (1983). Objective assessment with establishment of normal values for lumbar spinal range of motion. Physical therapy, 63(11), 1776-1781.

Franchi, M.V., Reeves N.D., Narici, M.V. (2017) Skeletal Muscle Remodeling in Response to Eccentric vs. Concentric Loading: Morphological, Molecular, and Metabolic Adaptations. Front. Physiol. 8, 447.

Lind, B., Sihlbom, H., Nordwall, A., & Malchau, H. (1989). Normal range of motion of the cervical spine. Archives of physical medicine and rehabilitation, 70(9), 692-695.

Morgan, D. L., & Proske, U. (2004). Popping sarcomere hypothesis explains stretch-induced muscle damage. Clinical and Experimental Pharmacology and Physiology, 31(8), 541 - 545.

Narici, M. V., Maganaris, C. N., Reeves, N. D., & Capodaglio, P. (2003). Effect of aging on human muscle architecture. Journal of applied physiology, 95(6), 2229-2234.

Youdas, J. W., Garrett, T. R., Suman, V. J., Bogard, C. L., Hallman, H. O., & Carey, J. R. (1992). Normal range of motion of the cervical spine: an initial goniometric study. Physical therapy, 72(11), 770-780.