Soleus fiber force and maximal shortening velocity after non-weight bearing with intermittent activity

This study examined the effectiveness of intermittent weight bearing (IWB) as a countermeasure to non-weight-bearing (NWB)-induced alterations in soleus type I fiber force (in mN), tension (Po; force per fiber cross-sectional area in kN/m-2), and maximal unloaded shortening velocity (Vo, in fiber lengths/s). Adult rats were assigned to one of the following groups: normal weight bearing (WB), 14 days of hindlimb NWB (NWB group), and 14 days of hindlimb NWB with IWB treatments (IWB group). The IWB treatment consisted of four 10-min periods of standing WB each day. Single, chemically permeabilized soleus fiber segments were mounted between a force transducer and position motor and were studied at maximal Ca2+ activation, after which type I fiber myosin heavy-chain composition was confirmed by sodium dodecyl sufate-polyacrylamide gel electrophoresis. NWB resulted in a loss in relative soleus mass (-45%), with type I fibers displaying reductions in diameter (-28%) and peak isometric force (-55%) and an increase in Vo (+33%). In addition, NWB induced a 16% reduction in type I fiber Po, a 41% reduction in type I fiber peak elastic modulus [Eo, defined as (delta force/delta length) x (fiber length/fiber cross-sectional area] and a significant increase in the Po/Eo ratio. In contrast to NWB, IWB reduced the loss of relative soleus mass (by 22%) and attenuated alterations in type I fiber diameter (by 36%), peak force (by 29%), and Vo (by 48%) but had no significant effect on Po, Eo, or Po/Eo. These results indicate that a modest restoration of WB activity during 14 days of NWB is sufficient to attenuate type I fiber atrophy and to partially restore type I peak isometric force and Vo to WB levels. However, the NWB-induced reductions in Po and Eo, which we hypothesize to be due to a decline in the number and stiffness of cross bridges, respectively, are considerably less responsive to this countermeasure treatment.