Acute Effects of a Mini-Trampoline Training Session for Improving Normalized Symmetry Index in Participants with Higher Baseline Inter-Limb Asymmetry

Background: Inter-limb asymmetry has implications for both athletic performance and healthcare practice. High baseline inter-limb asymmetries have been associated with impaired mobility, increased fall risk, and musculoskeletal injuries across the lifespan. Exercise interventions able to stimulate the stretch-shortening cycle (e.g., plyometric training and jump training) have been shown to have a good impact on asymmetries. Among these, Mini-Trampoline Training (MTT) has recently emerged as potentially effective in reducing asymmetries. Objectives: The study aimed to evaluate the acute effects of a single MTT session on muscle power and inter-limb asymmetry in young adults. Methods: Twenty-eight recreationally active participants (25.6 ± 2.4 years) completed one MTT session. Before (PRE) and after (POST) the MTT session, single-leg 6 m Timed Hop (6MTH) and countermovement jump (CMJ) tests were administered. Additionally, 6MTH values of the dominant (DOM) and non-dominant (NODOM) limbs were used to stratify participants according to higher (HBIA) or lower (LBIA) baseline inter-limb asymmetry, based on a commonly adopted Normalized Symmetry Index (NSI) threshold (NSI ≥ 10%, n = 12; NSI < 10%, n = 16). Repeated-measures mixed models were used to evaluate the effects of the MTT session on 6MTH, NSI, and CMJ. Results: Regardless of group and limb, significant (p < 0.0001) improvements in 6MTH (PRE: 2.5 ± 0.06 s; POST: 2.3 ± 0.05 s) were found. Interestingly, the MTT session had a significant (p = 0.01) effect on both groups, with a significant (p = 0.003) interaction with NSI values, showing an improvement for HBIA (PRE = 15.4 ± 1.1%, POST = 11.3 ± 2.1%), whereas a decrement in LBIA was recorded (PRE = 5.1 ± 0.6%, POST = 9.6 ± 1.5%). CMJ did not show any changes in HBIA (PRE: 36.2 ± 0.9 cm; POST: 35.1 ± 0.7 cm), while a significant (p = 0.007) decrease was found in LBIA (PRE: 34.8 ± 1.2 cm; POST: 33.2 ± 1.3 cm). Conclusions: A single MTT session induced acute neuromuscular fatigue, reflected by reduced CMJ performance and improved (~8%) inter-limb control during hopping. The HBIA group preserved jump height (~36 cm) and demonstrated a significant reduction in asymmetry (NSI: -4.1%), suggesting more balanced lower-limb recruitment. Conversely, LBIA showed a significant decrease in CMJ and an increased NSI (+4.5%), possibly reflecting fatigue-related compensatory strategies. Overall, a single MTT elicited distinct responses according to baseline asymmetry, supporting its potential as an adaptable modality for enhancing neuromuscular function in HBIA.