Effects of static stretching and its combination with conditioning contractions on lower limb muscle synergy and squat jump performance at two initial knee joint angles

Static stretching (SS) may reduce maximal muscle force and power output, while short-duration, high-intensity conditioning contractions (CC) have the potential to increase force and power output. However, the precise effects of CC on athletic performance and lower limb muscle coordination after SS are not yet fully understood. This investigation sought to explore the effects of SS (four sets of 30 seconds each) and its combination with CC (10 repetitive drop jumps), denoted as SC, on the synergy patterns of key lower limb muscles and jump performance during squat jumps (SJ) executed at two distinct knee joint starting angles (90◦ and 120◦). Eleven participants were randomly assigned to three experimental conditions, with each condition encompassing three SJs at both angles. A three-dimensional motion capture system, force platform, and electromyography (EMG) system were employed to quantify jump height, extract ground contact time, and perform non-negative matrix factorization. Our findings revealed that at a knee joint starting angle of 120◦, both SS and SC altered the weighting of the five major muscles (cosine similarity: SS: r = 0.897; SC: r = 0.767) and augmented the activity strength of the primary synergy (SS: 59.6%; SC: 10.48%). Additionally, SC demonstrably advanced the phase shift (90◦: 14%; 120◦: 61%). Notably, neither SS nor SC exerted a statistically significant influence on jump height (p > 0.05). However, SS significantly increased ground contact time (p = 0.029). In conclusion, at a knee joint angle of 120◦, both SS and CC were observed to alter lower limb muscle synergy patterns and influence ground contact time. While SS led to an increase in ground contact time, CC effectively countered this rise. These findings suggest that athletes in disciplines demanding rapid movements might benefit from omitting SS in isolation during warm-ups or consider combining SS with CC to optimize performance.

Non-negative matrix factorization; Weight matrix; Activity intensity; Ground contact time; Jump height

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