Long-term activity in upper- and lower-limb muscles of humans

TLDR

Previous studies suggest a link between daily muscle activity duration and type‑I fiber proportion, though human data are scarce. The study quantified limb muscle activity in healthy adults during normal use and compared it to published fiber‑type proportions. Seven men and seven women completed two 10‑hour EMG recordings of four nondominant limb muscles using surface electrodes. Upper‑limb muscles were active 18% of the time versus 10% for lower limbs, with upper‑limb activation 67% more frequent but lower‑limb bursts having higher amplitude, temporal coupling was strong in lower limbs and weak in upper limbs, only biceps differed by sex, and no association was found between activity duration and type‑I fiber proportion.

Abstract

Despite limited data on humans, previous studies suggest that there is an association between the duration of daily muscle activity and the proportion of type I muscle fibers. We quantified the activity of limb muscles in healthy men and women during normal use and compared these measurements with published reports on fiber-type proportions. Seven men (age range = 21–28 yr) and seven women (age range = 18–26 yr) participated in two 10-h recording sessions. Electromyogram (EMG) activity of four muscles in nondominant upper (first dorsal interosseus and biceps brachii) and lower limbs (vastus medialis and vastus lateralis) was recorded with surface electrodes. Hand and arm muscles were active for 18% of the recording time, whereas leg muscles were active for only 10% of the recording time. On average, upper-limb muscles were activated 67% more often than lower-limb muscles. When lower-limb muscles were activated, however, the mean amplitude of each burst was greater in leg muscles [18 and 17% maximum voluntary contraction (MVC)] compared with hand (8% MVC) and arm (6% MVC) muscles. Temporal association in activity between pairs of muscles was high for the two lower-limb muscles ( r 2 = 0.7) and relatively weak for the two upper-limb muscles ( r 2 = 0.09). Long-term muscle activity was only different between men and women for the biceps brachii muscle. We found no relation between duration of muscle activity in 10-h recordings and the reported values of type I fibers in men and women.