Investigation of bench press muscle activity and kinematic parameters under stable and unstable load conditions

Unstable load training (ULT) has become increasingly popular for its potential to enhance muscle activation, strength, and neural development. Unlike unstable surface training (UST), ULT introduces instability through an unsteady load. Studies suggest ULT increases muscle activation in both stabilizing and primary muscles, particularly in multi-joint exercises. This study aims to assess whether ULT in the bench press improves stabilizer muscle activation, identify the most effective ULT methods, and examine their impact on kinematic parameters. Sixteen male participants (age: 25 ± 3.14 years, body mass: 85.05 ± 5.39 kg, height: 1.78 ± 0.06 m, lifting experience: 6.75 ± 1.56 years, and bench press 1RM: 127.31 ± 13.45 kg) performed the bench press exercise under four different load conditions: Standard bar with a stable load (75% of 1RM), Standard bar with a stable load (60% of 1RM), Standard bar with an unstable load (60% of 1RM), Flexible barbell (60% of 1RM). Displacement, velocity, movement duration, and time parameters were compared under four different conditions. Surface electromyography was used to detect the activity of the pectoralis major, triceps brachii, anterior deltoid, lateral deltoid, rectus abdominis, and external oblique muscles. The Stable Load 75% condition exhibited the highest muscle activation across most muscles. It showed greater activation than Unstable Load, Stable Load 60%, and flexible barbell in the Pectoralis Major, Triceps Brachii, Rectus Abdominus, External Oblique, and Lateral Deltoid (p < 0.002). In the Anterior Deltoid, the Standard Unstable Load (97.02%) showed higher activation than Stable Load 60% (76.84%) (p < 0.001), with no significant difference to flexible barbell (89.49%). The flexible barbell showed greater displacement (cm) (eccentric: 39.84 ± 4.5, concentric: 40.17 ± 4.62) and velocity (m/s) (eccentric: 0.62 ± 0.11, concentric: 0.54 ± 0.11) compared to the Standard Unstable Load and Stable Load 75% (p < 0.005). The Standard Unstable Load (eccentric: 1.14 ± 0.61, concentric: 1.07 ± 0.42) and Stable Load 75% (eccentric: 0.87 ± 0.28, concentric: 0.95 ± 0.21) had the longest movement duration (s), while flexible barbell (eccentric: 0.64 ± 0.15, concentric: 0.72 ± 0.13) exhibited the shortest durations (p < 0.000). This study demonstrates that Stable Load 75% resulted in the highest muscle activation across major muscle groups. Muscle activation depends on both the load and the phase of movement. Unstable Load Training (ULT) using a flexible barbell enhances movement dynamics by increasing displacement and velocity but does not show a significant difference from Stable Load 60% in most kinematic parameters. Therefore, load selection should align with training goals, whether the aim is to maximize muscle activation or improve movement efficiency.