Dynamics of Neural Adjustments During Progressive Transitions to Unweighted and Reloaded Running.

Some neural adjustments remained incomplete at the end of the unweighting transition during the first run.

• Adjustments in hamstring preactivation, as well as triceps surae braking and push-off activity, remained incomplete at the end of the unweighting transition (UNWtr) in RUN1.
• The unweighting transition lasted 12 ± 2 seconds.
• The progressive transition moved runners from 100% to 60% body weight.
• A stabilizing muscle synergy was transiently recruited during UNWtr in RUN1.

Neural adjustments during unweighting transitions occurred more rapidly in the second run compared to the first run.

• In RUN2, the same adjustments that were incomplete in RUN1 (hamstring preactivation, triceps surae braking and push-off activity) occurred more rapidly.
• The authors interpret this as 'strong evidence of savings,' indicating motor learning between exposures.
• The study design involved two successive runs (RUN1, RUN2) on the LBPPT with identical protocol structure.
• Thirty-eight men participated in the study.

All neural adjustments were completed by the end of the reloading transition in both runs.

• In both RUN1 and RUN2, all neural adjustments were completed at the end of the reloading transition (RLDtr).
• Reloading transitioned runners back from 60% to 100% body weight.
• The authors attributed this to RLDtr reinstating a 'familiar sensorimotor context.'
• This contrasts with UNWtr, which represented a 'novel sensorimotor context' in RUN1.

A stabilizing muscle synergy was transiently recruited during the unweighting transition in the first run.

• Muscle synergy analysis was performed using surface EMG data from 11 lower limb muscles.
• This transient synergy recruitment was observed during UNWtr in RUN1 but not described as persisting in RUN2.
• Root mean square EMG and muscle synergies were analyzed for each running cycle separately.
• The recruitment of this synergy is interpreted as a response to the challenging novel sensorimotor context during unweighting.

The unweighting transition created a novel sensorimotor context where gradual decreases in mechanical constraints interacted with gradual increases in sensory and temporal constraints.

• The LBPPT enabled running at different percentages of body weight (100%, 60%, and 100% in successive stable phases of 3 minutes each).
• The transitions between weight conditions lasted 12 ± 2 seconds.
• The authors describe the context as one where 'the gradual decrease in mechanical constraints interacted with the gradual increase in sensory and temporal constraints.'
• This novel context challenged the nervous system in RUN1 but was 'integrated more rapidly in RUN2.'

The study protocol involved stable phases at three body weight levels separated by rapid progressive transitions recorded with surface EMG from 11 lower limb muscles.

• Thirty-eight men completed two runs on a lower body positive pressure treadmill (LBPPT).
• Each run comprised three stable phases: 100%, 60%, and 100% body weight (3 × 3 min each).
• Progressive unweighting and reloading transitions lasted 12 ± 2 seconds.
• Surface electromyographic activity was recorded from 11 lower limb muscles, and both root mean square EMG and muscle synergies were analyzed for each running cycle separately.

The authors propose that unweighting and reloading transitions are relevant tools for evaluating and rehabilitating locomotor adaptability.

• The results highlight the relevance of these transitions 'for evaluating and rehabilitating locomotor adaptability in clinical and athletic settings.'
• The evidence of savings (faster adaptation in RUN2) supports the use of repeated LBPPT exposures in rehabilitation.
• The contrast between incomplete adjustments during UNWtr and complete adjustments during RLDtr provides a framework for understanding neural adaptability.
• The study focused on healthy men, forming a baseline for future clinical applications.