Biomechanical analysis of single leg deadlift under the effect of instability

The single leg deadlift (SLD), one of resistance exercises, is widely prescribed to enhance muscular strength and balance due to its inherent unstable, unilateral nature. However, this instability may limit joint mobility and diminish strength training efficacy. This study aimed to investigate the biomechanical effect of surface-induced instability during SLD performance. Thirty healthy adult males performed SLDs under stable surface (flat ground) and unstable surface (foam-pad). Inertial measurement unit (IMU) and surface electromyography (EMG) were employed to assess joint kinematics, such as range of motion (ROM) and joint sway, and muscle activation patterns during stretch-shortening cycle. The unstable surface significantly increases ankle joint sway by 80.00% during the eccentric phase and 99.62% during the concentric phase (p < 0.05). While ankle joint ROM remains unaltered, instability reduces ROM at hip and knee joints by 6.04° and 3.81°, respectively. EMG analyses revealed that surface instability elevated Gluteus Maximus activation during the eccentric phase (13.27%) and Tibialis Anterior activation during both eccentric (6.56%) and concentric (15.21%) phases. Notably, Biceps Femoris activation increases across both phases under unstable conditions. Surface-induced instability during the SLD augments neuromuscular demand, particularly in the balance and stabilizing ankle musculature. However, this instability reduces hip and knee joint mobility, which may compromise the mechanical loading required for strength adaptation. These findings underscore the need for strategic integration of instability in resistance training programs depending on specific performance goals.