# Why are our muscles limited to 7 Hertz?

I have heard from a (usually very knowledgeable) friend before, that a human can only tap his fingers 7 times per second.

I generalized this to "our muscles are limited to 7 hertz"

When my wife heard this she was like "no way, I'm faster" and we recorded a video of her blinking as fast as she could (her eyes hurt afterwards) and over 5 seconds she averaged exactly 7 blinks per second.

I can't find any articles on the subject, but I did find this video of a drummer doing 1208 beats in 60 sec, setting him up at 20.13hz, meaning 10hz each hand. However if you look at his hand, his muscles are not even really tapping, they are just in a vibration (like a spasm), while holding the drumsticks, which than do all the tapping...

Ussain Bolt sprinted 100m in 9.6 seconds, with 41 steps, that's a little over 4 steps per second.

Both these are very impressive. The drummer sticks out for coordinating his 10 taps per second on each hand, alternating with the other hand, while the motion is certainly faster then our eyes can see.

So what's up with the 7 Hz speed limit? Is it a thing, or did the drummer disprove it? Could the 100m world record be beaten by another 25% when we can find a tall guy capable of doing 7 or even 10 steps per second?

• Could you imagine the activity where we need higher frequency for tapping? Touch screen games? Do we need higher frequency?
– Ilan
Jul 20, 2015 at 5:46
• I just tabbed a key on my keyboard and got to 182 characters in 20 seconds, so there's that. Anyways, it can either be neurologically limited (i.e. how fast you can switch between the motor unit recruitment for the muscles which antagonize each other for the given moment), as well as muscularly (how fast the muscle can actually contract and relax). This last thing varies widely across muscles, depending on the type of muscle fibers (i.e. which myosin heavy chain form they contain). Jul 24, 2015 at 13:44
• This is an interesting question. Is it limited by the speed, feedback mechanisms of nerve transmission, or the compliance-inertial limitations of the muscles? It turns out that the resonance of the human body is different depending on the parts of the body, however all the frequencies are near 7 Hz. Aug 7, 2015 at 14:33
• I have timed my personal max clicks/sec on a mouse and consistently get 10+- 1 click per second. Aug 7, 2015 at 23:41

Our muscles are not limited to 7 hertz.

You provide two examples yourself. I can provide a third: world's fastest clapper: 804 claps in 1 minute (video here), which is an average of 13.4 claps/sec -- though, you can see his pace changes so at times he's doing more than that.

What about a non world-record holding example? I can provide one of those as well: I can tap my finger 12x per second (or after 10 attempts, an average of 10.4x (SD=1.02) per second).

@Docscience's answer should help provide some background to better understand the science behind muscle limitations in oscillatory movement.

According to the book, Principles of Animal Locomotion by R. McNeil Alexander, oscillatory movements in animals are limited by an organism's nature in general to minimize metabolic energy costs. The energy capacity, inertial, compliant, and dissipative forces involved in the muscles, tendons, skeleton, and non-actuating load all enter into this cost equation.

Strictly speaking these structures are very difficult to model with any degree of fidelity, and so are often approximated by simple mechanical lumped parameter systems of masses, springs and dashpots. From these simple systems, differential equations can be written, and the frequency that minimizes cost turns out to be the natural frequency - since at the natural frequency the system has the greatest ability to trap, store and re-use energy for the movement.

Size and compliance are therefore the two parameters that matter most. Highest oscillatory speeds are able to occur in the smallest, stiffest muscles, assuming dissipative forces are small and input energy is available.

The human hand-brain reaction time is normally 100ms minimum - 300ms average, which aligns with the frequency that you talk about. There are different types of muscle movement though, and trembling type movements are very fast.

Most trembling in medicine is at 4-8 Hz from Parkinson's disease etc., but orthostatic tremor is characterized by a fast (>12 Hz) rate, which includes teeth chattering and trembling from cold.

Interestingly, your example of a finger movement isn't actually a direct muscular contraction — the muscle of the fingers sit in the forearm, and travel through a ligament and 4 joints, making the contraction indirect.

The jaw can voluntarily chatter at about 4-10 Hz, but it can tremble at well above 12 Hz.

Other muscular actions such as clapping are caused by two sets of independent and opposed strong muscles, flexing independently, which can be synchronized to act very fast at the same speed and up to almost twice the 7 Hz limit you mentioned.