Fig. 4. Patterns of activation and strain in the biceps femoris (A) and vastus lateralis (B) during three strides of slow trotting. Data are from the same individual trotting at 2.0 m s^–1. Dark gray shading represents the stance phase; light gray shading represents the swing phase. The biceps remains fairly isometric during the interval in which electromyographic activity is present in late swing and early stance and then shortens rapidly and substantially later in stance. The vastus is generally stretched during much of its activation period early in stance before shortening late in stance.