## 4 Results  
The EMG-to-Force Processing (EFP) and Kinematics plus Kinetics (KIN) power curves were highly correlated (r = 0.81; Figure 4), which satisfies Equation (2). EFP showed slightly greater positive power than KIN in early swing, nearly identical power magnitude and direction in late swing (net difference < 2 W), with EFP power estimates--both positive and negative--were more modest than KIN values in the weight acceptance and early single support phases.  
Terminal swing showed the highest values for negative power for knee muscles, demonstrated by the hamstring muscles. Each of the hamstring muscles (peak power SMEMB, \(-55\) W; BFLH, \(-24\) W; STEND, \(<\)\(-10\) W) showed negative power in swing to slow ipsilateral knee extension in preparation for the subsequent stance phase. STEND produced more positive power than negative power, in contrast with the other hamstring muscles, but its power production always had similar timing and direction (if not magnitude) as the other hamstring muscles. In each case, the negative power in swing was used to decelerate what is essentially passive knee extension, then positive power in weight acceptance (SMEMB \(>\) BFLH \(>\) STEND) to extend the hip [43] (Figure 5). The hamstrings do not appear to have a role at the knee during stance.  
Smaller magnitude positive power was produced by the quadriceps muscles in swing (all \(<13\) Watts, with VMO \(>\) VL \(>\) VI) (Figure 5). Knee extensor power transitioned from positive (late swing, above) to negative power in the subsequent ipsilateral stance phase. VMO and VL each produced peak power larger peak power (\(\approx\) -30 Watts) with VI producing slightly less negative power (\(-17\) W).  
Other muscles that cross the knee produce substantially lesser amounts of peak (positive) power, and their respective power production is essentially limited to early swing (during what is mostly passive knee flexion). Peak power for these muscles was \(<\)5 Watts, each (GRAC \(<4\) W: BFSH \(<3\) W, and SART slightly more than 2 W).  
Ankle plantar flexors produce a substantial amount of force and power and were included in the net EFP power estimates. Their greatest power production was in late single support and pre-swing (Figure 5). The positive power produced in pre-swing is related to their role at the ankle and is discussed in greater detail elsewhere [39; 42].  
Unlike muscles crossing the hip [40], each of the knee muscles has simpler architecture. That is, they all have a single innervation source, and distinct functional segments have not been reported for any of these muscles. However, one knee muscle--the popliteus--was notable for nearly as many EMG firing patterns as study participants. Variations included (a) activity throughout the entire gait cycle, (b) a paucity of EMG activity in both stance and swing at the other extreme, and (c) individuals with popliteus EMG (and presumably, force and power) in either stance or swing. In each case, the amplitude of the EMG signal was small, as would be force and power regardless of EMG timing. Thus, in the absence of a consistent pattern and little likely direct input to knee motion, further analysis of this muscle was not performed.  
Figure 4: Comparison of knee power produced by EFP (blue solid line, \(\pm\)one standard deviation) and KIN methods (red solid line, \(\pm\)one standard deviation). Percents gait cycle is shown on the abscissa. Horizontal bars (gray) indicate the EFP versus KIN analysis interval, where power is produced by muscles crossing the knee.  
Figure 5: Mean power produced during self-selected walking for the ten muscles crossing the knee. The percent gait cycle is shown on the abscissa; power produced by each muscle (in Watts) is shown on the ordinate. All figures are identically scaled. Stance phase duration is approximately 60%GC.