Fig. 4.

Contributions of passive (SEE) and active (CE) elements to total force and work production, under each of the three simulation conditions. A: when old MTUs were cycled with the same conditions as young MTUs, the peak total force doubled because of a large increase in passive force and a reduction in active muscle force. When old MTUs were allowed to begin cycling at shorter lengths and with smaller strain excursions, both the absolute and relative passive and active peak force patterns resembled that of young MTUs. B: in young MTUs, CE and SEE both contribute 50% of the work in a work loop. The relative values are similar in old MTUs cycled under the same conditions, but the absolute value of work is more than doubled. However, when the peak muscle force is kept at realistic values for old MTUs (initial muscle length at 1L₀ and 12% MTU strain), the whole MTU performs less than a third of the work that a young MTU does under physiological conditions.