"Put your hand on a hot stove for a minute and it seems like an hour. Sit with a pretty girl for an hour and it seems like a minute. That's relativity.”
- Albert Einstein
Introduction
The theory of relativity, developed by Albert Einstein in the early 20th century, revolutionized our understanding of space, time, and gravity.
The special theory of relativity is based on two postulates: the laws of physics are the same for all observers in uniform motion, and the speed of light in a vacuum is constant for all observers, regardless of the motion of the light source or the observer. Another consequence of the special theory of relativity is the famous equation E=mc^2, which states that energy and mass are interchangeable and that a small amount of mass can be converted into a large amount of energy.
The general theory of relativity, on the other hand, extends the principles of the special theory to include gravity. According to general relativity, gravity is not a force as Newton believed but rather a curvature in spacetime caused by the presence of mass and energy. Massive objects like planets and stars warp the fabric of spacetime, causing other objects to move along curved paths.
From time dilation and black holes to the expansion of the universe, the theory of relativity continues to inspire scientists worldwide. However those of you reading this most likely are not pushing forward discoveries in astrophysics, yet the relationship between mass and energy is thought provoking. Could force production with respect to an individuals' body mass mean anything to you?
What even is relative force production?
Relative force production, also known as force relative to body mass, is a crucial concept in biomechanics and sports science that helps to normalize force values across individuals of different body sizes. Although there are also budget-friendly ways to utilize relative metrics, a common tool to investigate them are force plates. By expressing force in units of Newtons per kilogram (N/kg), practitioners can compare force production capability relative to body mass, beginning to provide insight into profile-specific strength, power, and performance outcomes.
Force plates are devices used to measure the ground reaction forces generated by an individual in contact with their surface. When an individual applies force to the plates, the plates generate an electrical signal proportional to the force applied. This can be accomplished with single- or multi-axial systems, which can display the production of vertical, anterior-posterior, and medial-lateral force vectors. All providing valuable information about how forces are distributed and how they contribute to overall movement or performance.
Most widely used is applied settings are single-axis (vertical) force plates that culminate the total force production via load cells. This force is typically associated with activities like jumping, landing, or squatting. By analyzing the vertical force production, users can assess how much force an individual is exerting against gravity and how this force changes over time.
There are pros and cons to using relative force metrics...
Figure 1: Pros and cons of relative force metrics.
How to interpret relative force production...
When discussing relative force production, it is important to understand the two key components independently as well as their dynamic relationship.
Force: measured in Newtons (N), represents the muscular effort applied by an individual to generate movement or resist external loads.
Body mass: measured in kilograms (kg), represents the amount of non-specific matter that makes up an individual and is a fundamental parameter in determining an individual's overall size and weight.
Force (N) / Mass (kg) = __ N/kg
Once each individual component is understood individually (E.g., FFM, body composition, absolute force production, rate of force development, etc...) the relationship between the two can be explored to the fullest. This allowing for interpretation as outlined in Figure 2 below.
Figure 2: An overview of what relative strength trends could indicate.
By expressing force in N/kg, as mentioned users can standardize force measurements across individuals with varying anthropometrics, allowing for more precise assessments, comparison, and enhancement of strength and power capabilities. Why though? Strength-to-mass ratios is the short answer, and how they can be advantageous or a negative determinant for various sports/careers.
For example, powerlifting is biased toward absolute force production outside of the weight classes. They to some degree place value on having a greater strength-to-mass ratio with respect to your opponents. A 100m sprinter however relies on force production in the window of milliseconds where mass can influence economy of effort. Further, with the tactical population, there are task specific demands layered into a longer career that could put relative metrics under a microscope. However, the further from absolute force production you get the closer you get to speculation and the question, what is strong enough?
Figure 3: Exploring what "strong enough" could look like for a population.
How to influence relative force production...
As seen in Figure 2, the relationship between force production and body mass can be influenced by many factors including muscle mass, muscle fiber type, neuromuscular coordination, and other training adaptations. Furthermore, neuromuscular factors, such as motor unit recruitment, rate coding, and muscle synchronization, play a crucial role in determining an individual's force production profile. When analyzing and enhancing relative force production, the confounding influence of said factors amongst others need to be considered. This is in addition to asking; what are the population specific norms? Are there certain archetypes that are successful in this sport/job and/or position? The test is conceptual, how does this actually translate? And more...
Programs aimed at improving relative force production often focus on quite obviously improving force production per unit of body mass (I.e., N/kg). It is rare when this is a bad thing. Avoiding the nuances of developing strength, power, and speed, regardless of what physical characteristic is of top priority, there is a constant emphasis placed on the relationship between the two components- force and mass. Where at times doesn't mean they are always trending parallel, but in a sequential fashion toward the desired end result.
Programming basics do not generally need to change when it comes to specific muscle groupings, movement patterns, and energy systems. Although in some cases muscle hypertrophy needs to be managed closely, and dietetic interventions can be a necessity. The constant emphasis becomes highlighted in intertwined assessments into the daily training environment as a measurement underpinning a KPPI overall. Examples include Isometric Mid-Thigh Pull relative force production, Relative Dynamic Strength Index, without tech even Sprint Momentum (mass × velocity) or Vertical Jump Work/Power calculations with respect to body mass.
Understanding of relative force production in the context of your specific population could tap into untouched physical development. At the very least it will being intent to your programming and overarching systems and structure.
Alexander J. Morgan, MSc., CSCS, RSCC, CEP
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