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Quantifying Maximal, Voluntary Muscle Force

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Quantifying Maximal, Voluntary Muscle Force

The production of force (SI unit: [N]) is a fundamental muscle function. However, there's generally a need to clarify what "force" actually means:

  1. Muscle force cannot be measured directly in a living person. In the simplest case of an unpennated muscle, this would require severing at least one tendon end, connecting it in line with a force sensor, and activating it.
  2. Instead of direct muscle force, 2 kinds of surrogate indicators are usually used in humans: (a) muscle size (muscle volume, muscle (fibre) cross-sectional area, lean mass, or the muscle mass calculated from it) and/or (b) external torques or forces (e.g. ground reaction force, pedal force, etc.). The internal forces can be estimated from these external measurements using inverse dynamics.
  3. The correlation between muscle size and torque or force is not as good as one might expect. Studies in humans have shown that the size of the leg muscles explains only around 25–50% of the variability in torque or force. More size or mass therefore doesn't necessarily lead to an improvement in the physiologically relevant force function.
  4. Muscle fibres do not produce different, distinct force entities, as terms like "speed strength", "explosive strength" or "strength endurance" misleadingly suggest. Depending on the speed and direction of the length change, however, the force varies, and so does the power (the product of force and velocity, [W]). In fact, such misleading terms are really an attempt to describe movements from an external perspective, e.g. a "fast" or "explosive" movement. In measurement terms, a movement-specific speed of force development can be quantified via the so-called "rate of force development" (increase in force per unit of time) or, better, via the impulse (the integral of force over time for a given time interval).
  5. There's an important difference between the peak force that can be produced voluntarily during a functional or movement manoeuvre, and the maximal voluntary force. In the two-legged countermovement jump ("countermovement jump" [CMJ]), for example, the typical peak force acting per forefoot corresponds to around 1.2 times body weight. During repeated hopping on one leg with the knee extended and no heel contact, the typical peak force is around 3–3.5 times body weight, i.e. around 2.5–3× higher than in the CMJ. To determine the maximal voluntary force externally, the functional or movement manoeuvre with the highest typical peak force must therefore be selected.
  6. An often-neglected aspect when determining maximal force is the fact that, for any given level of muscle activation, the maximal force occurs at a negative contraction velocity (i.e. during eccentric contraction). It follows from these considerations that, to speak of a maximal voluntary force, the force must be captured using a movement manoeuvre that produces the highest typical peak force, and that it must also be measured with the greatest possible muscle activation during eccentric contraction. The jump manoeuvre that meets these conditions is multiple one-legged hopping ("multiple onelegged hopping" [m1LH]), i.e. repeated hopping on the forefoot with the knee extended and no heel contact. To measure the ground reaction force, the jump manoeuvre is performed on a force plate, which can be mobile or stationary. The peak force during m1LH occurs during the landing phase (i.e. during the eccentric contraction), and the typical peak force is higher than for the other known jump manoeuvres. Using m1LH in combination with pQCT, it's therefore possible to capture the maximal voluntary force (Fm1LH), relate it to bone strength, and thereby quantify the muscle-bone unit.

Source: Dr. M. Toigo - Journal für gynäkologische Endokrinologie