ADAPTIVE-CONTROL FOR BACKWARD QUADRUPEDAL WALKING .5. MUTABLE ACTIVATION OF BIFUNCTIONAL THIGH MUSCLES

1. In this, the fifth article in a series to assess changes in posture , hindlimb dynamics, and muscle synergies associated with backward (BW D) quadrupedal walking, we compared the recruitment of three biarticul ar muscles of the cat's anterior thigh (anterior sartorius, SAa; media l sartorius, SAm; rectus femoris, RF) for forward (FWD) and BWD treadm ill walking. Electromyography (EMG) records from these muscles, along with those of two muscles (semitendinosus, ST; anterior biceps femoris , ABF) studied previously in this series, were synchronized with kinem atic data digitized from high-speed cine film for unperturbed steps an d steps in which a stumbling corrective reaction was elicited during s wing. 2. During swing, the relative timing of EMG activity for the uni functional SAm (hip and knee flexor) was similar for unperturbed steps of FWD and BWD walking. The SAm was active before paw lift off and re mained active during most of swing (75%) for both forms of walking, bu t there was a marked decrease in EMG amplitude after paw off during BW D and not FWD swing. In contrast, the relative timing of EMG activity for the SAa and RF, two bifunctional muscles (hip flexors, knee extens ors), was different for FWD and BWD swing. During FWD swing, the SAa a nd the RF (to a lesser extent) were coactive with the SAm; however, du ring BWD swing, the SAa and RF were active just before paw lift off an d then inactive for the rest of swing until just before paw contact (s ee 3). Thus the swing-phase activity of the SAa and RF was markedly sh orter for BWD than FWD swing. 3. Activity in SAa and RF was also diffe rent during FWD and BWD stance. The RF was consistently active from mi d-to-late stance of FWD walking, and the SAa was also active during th is period in some FWD steps. During the stance phase of BWD walking, h owever, the onset of activity in both muscles consistently shifted to early stance as both muscles became active just before paw contact (th e E1 phase). Activity in RF consistently persisted through most of BWD stance. The duration of SAa recruitment during BWD stance was more va riable across cats with offsets ranging from mid- to late stance. 4. T he activation patterns of the biarticular anterior thigh muscles durin g stumbling corrective reactions were, in general, similar to their di fferent activations during FWD and BWD swing. The initial response to a mechanical stimulus applied to the dorsum of the paw that obstructed FWD swing was an augmentation of knee flexion and increased activity in ST and SAm. A mechanical stimulus applied to the ventral surface of the paw to obstruct BWD swing resulted in an initial conversion of hi p extension to flexion and a slowing of knee flexion. There was a corr esponding recruitment of SAa and RF and an enhancement of background a ctivity in SAm. 5. The two forms of walking are differentiated by post ure and limb dynamics, yet muscles participating in the basic flexor a nd extensor synergies are unchanged. Although central pattern generati ng (CPG) circuits determine the basic timing of these synergies, chang es in the duration and waveform of muscle activity may depend on uniqu e interactions among the CPG, supraspinal inputs that set posture and the animal's goal (to walk BWD or FWD) and motion-related feedback fro m the hindlimb. Output mutability to each muscle may depend on the bal ance of this tripartite input; muscles with immutable patterns may rel y heavily on input from CPG circuits, whereas muscles with mutable pat terns may rely more on form-specific proprioceptive and supraspinal in puts.