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

Despite limited data on humans, previous studies suggest that there is an a ssociation between the duration of daily muscle activity and the proportion of type I muscle fibers. We quantified the activity of limb muscles in hea lthy men and women during normal use and compared these measurements with p ublished 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 upp er (first dorsal interosseus and biceps brachii) and lower limbs (vastus me dialis 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 mus cles were activated 67% more often than lower-limb muscles. When lower-limb muscles were activated, however, the mean amplitude of each burst was grea ter in leg muscles [18 and 17% maximum voluntary contraction (MVC)] compare d with hand (8% MVC) and arm (6% MVC) muscles. Temporal association in acti vity 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). L ong-term muscle activity was only different between men and women for the b iceps brachii muscle. We found no relation between duration of muscle activ ity in 10-h recordings and the reported values of type I fibers in men and women.