Fig. 4. Representative in vivo work loops (force versus length) performed by (A) the lateral (LG) and medial gastrocnemius (MG) and (B) the plantaris (PL) muscles of wallabies #2 and #3 during level versus incline hopping. The strain and activation patterns for gastrocnemius are based on measurements made of the LG head. The period of muscle EMG is shown in gray. The path of force relative to length change is shown by the arrows. The net work performed (area inside the loop) is also shown for each muscle cycle. Net work is positive when the loop is counterclockwise in nature (e.g. wallaby #2 LG) and negative when the loop moves in a clockwise direction (e.g. wallaby #3 LG). Although different patterns were observed among individual animals, work loop patterns were always consistent across level and incline hopping conditions within an individual animal. The patterns shown here for wallaby #2 versus wallaby #3 differed the most among the four animals. Gastrocnemius work by the two animals not shown was generally more similar to that of wallaby #3. Differences in force-length behavior were generally greater for the gastrocnemius than for the plantaris. In general, the PL did less net work (negative or positive) than the LG.