010: What is the Difference Between Muscular and Mechanical Strength?

biomechanics strength Aug 18, 2021

A reader of the blog wrote to me and asked if I could explain the difference between muscular strength and mechanical strength and if mechanical strength and stiffness are the same. I enjoy receiving your questions because they provide me with ideas for blog posts that you actually want to read, and I always enjoy geeking out on the biomechanical aspects of flexibility training, so please keep sending them in!

The short version is that muscular strength and mechanical strength are not the same, and also, no, mechanical strength and stiffness are not the same, although they are related concepts.

Muscular strength is the amount of force (active tension) the muscles can voluntarily overcome external resistance. Weight lifting enthusiasts typically define strength as the amount of weight that a person can move during one-repetition-maximum (1RM) efforts, but during research, strength is most often measured as the amount of active tension that muscles can produce during isometric conditions at a specific joint angle. Researchers opt for this method because it eliminates many mechanical factors affecting muscle strength that may act as compounding variables (Atha, 1981).

Before discussing mechanical strength, it is a good idea to first look at stiffness. Stiffness is a term that originated in the engineering sciences to describe the mechanical behaviour of a particular material when it is loaded with stress. "Mechanical behaviour" refers to how an object behaves physically when subjected to force. "Stress" is a force that is distributed across the internal components of an object, i.e., it is the "internal force," and it tells us how hard a given load works to change the shape of a material (a process called deformation, also known as "strain"). Stiffness is calculated by measuring the force and displacement of the material as it is deformed at different rates. It is the ratio of stress to strain (specifically, in the elastic region of a stress-strain curve), or in simpler terms, it is the force needed to deform (change the shape of) an object or material. In the context of flexibility, stiffness is how much a tissue (e.g., tendon) resists stretching.

An object's mechanical strength is the maximum force or total mechanical energy it can absorb before it fails. A stress-strain curve tells us how much energy was absorbed and how much mechanical work was performed on the object. Using a tendon as an example, the mechanical strength is the total force (mechanical energy) the tendon can absorb before it breaks (ruptures). Mechanical is a subject of particular interest to rehabilitation professionals because muscular strains and ligament sprains are examples of when absorbed forces surpassed the tissues' mechanical strength.

You may have already intuited that since muscular strength and mechanical strength are not the same, the methods for developing them will also not be the same. Many health and fitness professionals often advise clients to "get stronger" to safeguard against soft tissue injury, but regular (muscular) strength training (resistance greater than 80% 1RM for sets of 1-5 reps) is not the most effective way to improve the tissues' mechanical strength. Instead, local muscular endurance training (sets of 15 or more repetitions with light resistance) increases the mechanical strength of slow-twitch fibres (Gleim & McHugh, 1997), which are relatively stronger than fast-twitch fibres, and also increases the strength of the connective tissue elements within the muscle (Tipton et al., 1975). Hypertrophy training will increase the number of units of the cross-sectional area over which the internal load is applied, but it will not necessarily increase the structural strength of each unit of CSA as much as endurance training.

References

Atha, J. (1981) Strengthening Muscle. Exercise and Sport Sciences Reviews vol. 9, pp. 1-73.

Gleim G. W., & McHugh, M. P. (1997) Flexibility and Its Effects on Sports Injury and Performance. Sports Medicine vol. 24, no. 5, pp. 289-299.

Tipton, C. M., Matthes, R. D., Maynard, J. A., & Carey, R. A. (1975) The Influence of Physical Activity on Ligaments and Tendons. Medicine and Science in Sports vol. 7, no. 3, pp. 165-175.