EFFECTS OF IN VIVO-LIKE ACTIVATION FREQUENCY ON THE LENGTH-DEPENDENT FORCE GENERATION OF SKELETAL-MUSCLE FIBER-BUNDLES

It is known that a range of firing frequencies can be observed during in vivo muscle activity, yet information is lacking as to how differen t in vivo-like frequencies may affect force generation of skeletal mus cle. This study examined the effects of constant (CSF, constant within one contraction) and decreasing stimulation frequencies (DSF) on mean sarcomere length-force characteristics of rat gastrocnemius medialis fibre bundles. The CSF resulted in an optimal mean sarcomere length (l (so)) of 2.30 (SEM 0.02), 2.46 (SEM 0.03), 2.76 (SEM 0.03) and more th an 2.99 (SEM 0.07) mu m, for 100, 50, 30 and 15 Hz, respectively. Comp ared to 100-Hz stimulation, both l(so) and the ascending limb of the r elationship significantly shifted to higher lengths with lower frequen cies. No shift was encountered for the initial part of the descending limb. The DSF reduced the frequency-induced shift to higher mean lengt hs [l(so) 2.33 (SEM 0.02), 2.52 (SEM 0.08) and more than 2.92 (SEM 0.1 0) mu m, respectively, for 50, 30 and 15 Hz]. No effect of activation time on length-force characteristics was observed. It was concluded fr om these studies that the frequency and history of stimulation is a ma jor determinant of the length-force characteristics of muscle fibre bu ndles, and should be taken into account when analysing animal and huma n locomotion. The previously observed frequency-induced shift in whole muscle length-force relationship resides mainly at the level of fibre bundles.