Hip joint contact forces and muscle contributions between bounce and standard squats

Bounce squats, involving a rapid eccentric-concentric transition, are used in strength training to enhance force production via the stretch-shortening cycle. However, its effects on hip joint contact force (JCF) and muscle-specific contributions remain unclear. This study investigated the hip JCF and individual muscle contributions during bounce and standard squats. Nine healthy resistance-trained participants (7 males, 2 females; ≥3 years of resistance training experience) performed four trials each of bounce and standard squats across five loads (0%, 25%, 50%, 75%, and 100% of body weight). Three-dimensional motion capture, force plates, and electromyography data were used to estimate hip JCF and muscle contributions via musculoskeletal modeling and simulation. Two-way repeated-measures analysis of variance was used to compare hip JCF and muscle contributions between squat types and load conditions. Peak hip flexion angles (F(4, 32) = 3.58, p < 0.015, η2p = 0.31) and peak hip net joint moments (F (4, 32) = 3.44, p < 0.018, η2p = 0.30) were significantly greater in bounce than standard squats across loads. Peak hip JCF was also greater in bounce squats (F(1, 8) = 44.97, p < 0.001, η2p = 0.84). This increase was attributed to greater hamstrings (F(1, 8) = 23.98, p = 0.001, η2p = 0.75) and gluteus maximus (F(1, 8) = 13.40, p = 0.006, η2p = 0.63) contributions. Bounce squats increase hip JCF compared with standard squats, primarily due to greater hamstrings and gluteus maximus contributions. While the bounce strategy may offer performance benefits, the associated increase in mechanical hip joint loading may represent a potential risk factor that warrants consideration during resistance training.