Development of Stepping Figure 2. RLMs at E15. RLMs for 3 experiments are shown. A force transducer resting on the hip monitored general body displacements during recordings. Kinematic methods were not applied in these experiments. A: LG was rhythmically active for 3 cycles (3.261.0 Hz) and FT was coactive with LG. B: LG was rhythmically active for 3 cycles (6.460.5 Hz) and TA was reciprocally active with LG. C: TA was rhythmically active for 4 cycles (4.860.6 Hz), FT was coactive, and an LG burst was also detected by analysis methods (q). doi:10.1371/journal.pone.0006111.g002 only 7 of 17 experiments (R2.0.6). FT onset varied weakly in 8 experiments (R2,0.4), as exemplified by the E20 experiment in Figure 4B. In the latter experiment, FT bursts began early in cycles 2–6 (Figure 4B1), and during the experiment relative onsets fell mostly between 0.2–0.5 over a frequency range of 2–8 Hz (Figure 4B2, bottom plot). More than 60% of FT bursts began in the first half of the TA cycle in 10 of 17 experiments, and a bias for the latter half of the cycle was found in only 4 experiments. FT findings did not differ between E18 and E20. (See Tables S2 and S3 in supplementary materials for detailed summaries.) Burst durations were brief for all muscles at all TA burst frequencies. Average TA burst durations slightly exceeded 60 ms and averages for SA, LG, and FT ranged from 45 to 60 ms at E18 and E20. Regression analyses revealed that SA, LG, FT and TA Figure 3. SA and LG bursts exhibited reliable onset patterns by E18. Data for an E18 experiment are shown. A: This RLM was 6.5 s long. Dashes underscore 15 TA bursts (2.460.5 Hz). TA amplitude was reduced 50% in this record to clearly visualize FT and SA traces. B–C: Larger scatter plots include trend lines for regression analyses. The number of bursts (N) in each trend analysis is indicated. In the small plots offset to the right, SA and LG burst onsets are normalized to the concurrent TA cycle duration (0–1) and plotted against TA cycle frequency (Hz). B1: SA burst onsets clustered near 0 s (slope = 0.03, R2 = 0.02). B2: SA relative onsets clustered early in the cycle (0–0.2) at all frequencies. C1: LG onset latency strongly co- varied with cycle duration (slope = 0.78, R2 = 0.76). C2: LG bursts clustered in the latter half of TA cycles with 82.8% of bursts falling between relative onsets of 0.5 and 1.0. doi:10.1371/journal.pone.0006111.g003 PLoS ONE | www.plosone.org 4 July 2009 | Volume 4 | Issue 7 | e6111