Coupling of upper and lower limb pattern generators during human crawling at different arm/leg speed combinations

Area of Science:

• Neuroscience
• Human Motor Control
• Locomotion Biomechanics

Background:

• Understanding inter-limb coordination is crucial for deciphering motor control.
• Central pattern generators (CPGs) are neural circuits believed to generate rhythmic motor activity, like locomotion.
• The coordination between upper and lower limb CPGs in humans during crawling remains incompletely understood.

Purpose of the Study:

• To investigate inter-limb coupling patterns during human crawling.
• To explore the coordination between upper and lower limb central pattern generators (CPGs).
• To compare human crawling coordination with findings in other species, such as cats.

Main Methods:

• Healthy adults performed hands-and-feet crawling on separate, independently controlled treadmills for upper and lower limbs.
• Treadmill speeds were varied to examine their effect on inter-limb coordination and frequency relationships.
• Kinematic data and limb phase relationships were analyzed to assess coupling patterns.

Main Results:

• A 1:1 frequency relationship between upper and lower limbs was frequently observed, even with mismatched treadmill speeds.
• Relative stance durations in the upper limbs were primarily influenced by upper limb treadmill speed.
• Coupling ratio changes occurred more often when upper limbs moved slower than lower limbs, contrasting with feline locomotion.

Conclusions:

• Integer frequency ratios support the existence of coupled CPG control for human crawling.
• Human upper limb CPGs may be less influenced by lower limb speed changes, possibly due to weaker propriospinal connections or greater cortical influence.
• These findings highlight species-specific differences in limb CPG coordination during locomotion.