Most muscles don't contract very fast. A few exceptional creatures have specialised muscles that register contraction frequencies of several hundred Hertz for sound production, but most muscles involved in walking don't come close to that. Jonathan Wright, working with Grace Wu and other colleagues from Pomona College and Harvey Mudd College, USA, explains that muscles trade off contraction frequency against force: fast contracting muscles cannot produce much force, while high force-producing weight-bearing muscles only contract slowly. So what about minute teneriffiid mites? Their leg muscles must be strong enough to carry their weight, but the smaller you are, the higher your stride frequency. Could mite leg muscles break the rule and contract super fast to produce a high speed stride while still carrying the arthropod's weight? Wright and his colleagues collected two new species of mite from the Californian sage scrub, weighed them and filmed the arthropods at 1000 frames s–1 as they scuttled around to see how fast they could run ().
The first species weighed in at 182 μg and hit colossal speeds up to 133 body lengths s–1 with leg stride frequencies of up to 90 Hz at 50°C. Meanwhile, the second smaller species, weighing in at 30 μg, got up to a similar relative speed (129 body lengths s–1) and stride frequencies of up to 112 Hz. So the mites' leg muscles can contact super fast while still carrying their body weight and the stride frequency is similar to the values that you would predict based on the arthropods' size relative to that of other creatures. Calculating that the muscle contraction phase of a stride could last as little as 4–5 ms and knowing that the mites only run in 4–5 s bursts, the team speculate that the muscles may be anaerobic, allowing them to pack more muscle fibres in at the expense of aerobic energy-producing mitochondria to increase force production despite their superfast contraction rate.
