## 5 Discussion  
### Power Production by Individual Muscles  
The integration of muscle work and power allows us to have an improved understanding of each muscle's role at each joint crossed. The mechanical behavior of the lower extremities in gait is like that of an inverted triple pendulum. That is, power (and motion) produced at an adjacent joint may affect the kinematics of the adjacent joint via transfer of energy across segments [18]. This is particularly evident during the pre-swing and early swing phases of gait, as the adjacent ankle and, to a lesser extent, hip joint muscle forces and power influence knee kinematics.  
While active knee flexor and extensor power is largely limited to late swing, it is instructive to examine joint power in pre-swing and early swing, where power is independent of activity by muscles crossing the knee. Pre-swing positive plantar flexion power initiates knee flexion [39]. Hip flexion continues into early swing to advance the ipsilateral, trailing limb. Here, hip flexion also coincides temporally with negative power by muscles and soft tissues crossing the hip [43]. Thus, the substantial power produced by the planter flexors may also initiate hip flexion. As ipsilateral stance transitions to swing, positive hip muscle power begins due to concentric contractions by both rectus femoris and ilipsoas, and positive hip power continues into mid-swing (approximately 75%GC [43]).  
Positive hip power advances the ipsilateral limb in early swing [42]. As noted, muscles that do not span a joint can contribute to forces and moments a that joint [42]. The bairticular hamstrings, via their effect on slowing hip flexion (via negative power), result in passive knee extension as energy is transferred across segments. Hamstrings' negative power continues through the swing. Later in the swing (\(\approx\)90%GC), positive power is produced by the vastus muscles to enhance the swing phase knee extension. This relatively small amount of positive power (Figure 5) slightly increases step length to increase walking velocity.  
The roles of both hamstrings and vastus muscles change abruptly at the transition from swing to stance. The hamstrings are lengthened throughout the swing phase, with their MTU length at \(\approx\)105% L0 at the transition from swing to stance (Figure 6). As stance commences, these muscles continue to produce force, yet now concentrically contracting (positive power). This initiates hip extension, which continues through stance as the body's center of mass horizontally translates over the fixed foot. The hamstrings' collective role at the hip is described in greater detail elsewhere [40], and they have no active role at the knee during the remainder of stance.  
The transition from swing to stance also changes the role of the vastus muscles. Positive power began in late swing to maximize step length and is likely to prepare for the mechanical demands of the subsequent stance phase. However, vastus power turns from positive to negative at the transition from swing to stance. All vastus muscles are lengthened in weight acceptance, and with force production produce negative power to regulate stance phase knee flexion.  
A slight amount of positive vastus power is produced in stance, progressing from initial double support to single limb support. After a brief period of positive power (18-30%GC), knee extension continues. The forward progression of the body's center of mass over the fixed-in-space foot extends the knee without the need for active muscle forces or power.