How does planking burn calories if there is no force * distance? In plank position, the body is not moving, but it still burns calories.
Work is defined as force multiplied by distance ($Fd$). There is force, but no distance, so the energy should be $0$ ($F \times 0 = 0$), as anything multiplied by $0$ is $0$. Someone please explain how planking burns any calories.
 A: I remember this confusing me in high school, and I didn't find out the answer until grad school. The answer is that the force is applied over a distance inside the muscles!
Muscles are made up by sections of thin filaments interwoven with each other. When the muscle wants to exert a force the filaments contract together a small amount. If the muscle needs to exert a force for a longer amount of time the filaments contract then expand and then contract again, over and over.

Each time the muscle filaments contract a small amount of work is done. The cycle of contraction and relaxation repeats quickly. It seems to depend highly on how much force is applied and the kind of muscle, but 10 - 100 Hz seems likely. These little bits of work add up over time, and that's where your calories go.
(Image citation: Richfield, David (2014). "Medical gallery of David Richfield". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 2002-4436. Obtained via https://en.wikipedia.org/wiki/Muscle_contraction)
A: *

*Single muscle consists of many muscle cells. And when a muscle contracts - it doesn't happen to all muscle cells at a time. The tension isn't static. Some of your muscle cells contract while others get relaxed, then they change (Frequency Summation). That's why muscles start shivering under the load (especially when tired - some of them fail to contract when it's their turn). And this is also how muscles can control the force - different number of muscle cells contract simultaneously (but one cell can't control how much it contracts, it's binary: just 1 or 0). In fact there's a particular order in which different types of muscle cells are activated when more strength is required (Henneman's size principle).

*On protein level chemical energy (ATP molecules) is spent on detaching (watch Cross bridge detachment). When muscles contract:

*

*Myosin protein heads are allowed to attach to actin protein

*Then they slide

*Then they need to detach in order to re-attach farther and slide again.



This cycle repeats many-many times during a cell contraction. And the detachment is where ATP molecules are consumed. This is why shortly after people die their muscles become stiff (rigor mortis) - myosin fails to break the bond because there's no more ATP available. After some time myosin/actin degrades and muscles "relax".
A: What you are not taking into account is that your body is a complex machine with many moving parts that are not obvious from simple observation. Your heart is continuously pumping blood throughout your body in order to supply oxygen to maintain your level of activity
When you exercise (even an isometric exercise such as planking) your heart beats faster to supply the extra oxygen required for the activity. Your lungs expand and contract faster to provide that extra oxygen. This burns extra calories. There is an F. dx  taking place but it is internal rather than external.
A: This is because you are generating a force that has to generate a normal force and as you use the parts of the body where muscles brings the stiffness you have to utilize your internal energy to make those muscles stiff.
Now you should also have seen that you get tired while standing?(Same logic works here)
Our bones and muscles of legs are so developed that they have to generate a very little force and rest is done by the tension generated in them. That is why you also get tired while standing but not that early.
You get tired because the enrgy in your body starts depleting.
