## 3 Data Acquisition and Processing
### EMG-to-Force Processing--Dynamic Electromyography (EMG)  
Electromyographic (EMG) was obtained from fourteen muscles crossing the knee. Included were four knee extensors (Vastus Lateralis (VL), Vastus Intermedius (VI), Vastus Medialis Oblique (VMO), Rectus Femoris (RF)), four knee flexors (Semitendinosis (STEND), Semimembranosis (SMEMB), Biceps Femoris, Long Head (BFLH), Biceps Femoris, Short Head BFSH)), plus other muscles with compound functions (Sartorius (SART), Gracilis (GRAC), Tensor Fascia Lata (TFL), Medial Gastrocnemius (MGAST), Lateral Gastrocnemius (LGAST), and Popliteus (POP)). Electromyographic activity was recorded with paired, insulated 50\(\upmu\) stainless steel wire electrodes. The insulation was removed 2 mm from the wire ends, the tips curved (to limit electrode migration),and inserted into each tested muscle near the presumed motor point of the muscle [57] with a hypodermic needle [58] (Figure 2). The needle was then withdrawn, and electrode placement was achieved via both voluntary muscle contraction and electrical stimulation via the indwelling electrode. A ground electrode was placed over a lower extremity bony landmark. The EMG system bandwidth was 10-1000 Hz, with an overall gain of 1000 [59] (Model MA-300, Motion Laboratory Systems, Baton Rouge, LA, USA). The limbs were positioned to ensure a maximum voluntary contraction (MVC) [60], and the signal was normalized to the MVC [61]. Maximum muscle tests were performed by a single Board-Certified rehabilitation physician (RB). The manual muscle test was five seconds in duration [61]. The one-second interval in that test with the highest mean voltage was established. A second maximum muscle test was performed at the end of each gait trial to assure continued electrode insertion integrity [62]. The higher value obtained by the two tests was used as the MMT reference value (one hundred percent maximum voluntary contraction reference value (100%MVC)). Footswitches were used to record the gait cycle interval and foot support patterns. EMG and footswitch data were collected at 2500 Hz. The digitized EMG data were then rectified, and baseline noise was corrected with a resting run. The %MVC value at each percent gait cycle interval was determined by normalization the stride data. EMG was input to the force processor model via two sequential processing steps. First, on a stride-by-stride basis, each muscle's EMG linear envelope was determined [45]. Here, the raw EMG signal was rectified, and the mean relative intensity was determined for the first fifty-sample interval. The analysis window was advanced one data point, and a new fifty-sample mean was established. This moving-window averaging process was repeated for the entire gait cycle [62]. The several thousand EMG data points per stride were reduced to one hundred EMG relative intensity values--one for each percent  
Figure 2: Graphical depiction of motion, external force, and EMG data acquisition environment. The figure shows the motion analysis laboratory setting (multiple video acquisition cameras and paired embedded force platforms in line with the walking direction (indicated by curved arrows). The middle figure shows the insertion site for intramuscular (“wire”) electrodes inserted at the motor point of the muscle (Vastus Lateralis, at blue arrow). Finally, several consecutive strides of rectified, non-normalized EMG from that same muscle are shown in the lower figure. gait cycle--via a linear interpolation method. The within-subject EMG profiles were then determined from individual stride EMG data [63]. A muscle's electromyographic activity predictably precedes that muscle's force production. This timing mismatch between the onset of a muscle's EMG and force production (electromechanical delay, EMD) typically is in the range of 20 ms [64]. EMG and MTU force production were also not temporally matched during the continued muscle contraction, with MTU force maintained after that muscle's EMG activity had returned to baseline.