## 1 Introduction  
Spastic diplegic cerebral palsy (CP) is characterized by impaired motor control on both sides of the body with shortened or improperly developed muscles, and impaired joint movement in both lower extremities [1; 2; 3; 4; 5; 6]. These impairments impact directly on the body's mobility and stability throughout the gait cycle [7], not only for body-weight support during single-limb support (SLS), but also for body-weight transfer during double-limb support (DLS). In particular, during DLS, both limbs are on the ground, forming a closed kinetic chain. Thus, well-coordinated joint controls, both intra- and inter-limb, are needed for smooth and stable body-weight transfer and balance control during gait. Since the lower limbs exchange the roles of leading and trailing limbs between DLS phases,bilateral symmetry of the lower limb kinematics and kinetics is essential for walking. Tendon release surgery (TRS) lengthens the affected muscle-tendon unit to treat the motor dysfunctions and has been shown to help improve ranges of motion of lower limb joints during gait in children with spastic diplegic CP [8; 9; 10; 11]. Evaluation of the efficacy of the TRS on the inter-limb coordination and bilateral symmetry in body-weight transfer and balance control during CP gait is needed. Motion analysis technologies have been used to identify the deviations of pathological gait, as well as treatment effects on the affected limb during different phases of gait [8; 9; 10; 11]. Generally, deviations in CP gait involve complex interactions of the lower limb joints that are affected by factors such as muscle tightness, bony deformity and impaired motor control. Evaluating the disease progression and treatment efficacy in terms of deviations of individual joints can be very challenging [12], especially when attempting to identify the between-limb interactions. Thus, very few studies have examined how the lower limbs coordinate with each other for the observed gait patterns, especially during weight transfer between limbs in DLS. Previous studies have shown that leg stiffness, incorporating information of both the kinematics and kinetics of the lower limb joints, is a useful measure and should be included in future clinical gait analysis for a more complete assessment of gait in children with CP [13; 14; 15]. For assessing the effects of pathology and the efficacy of TRS on the inter-limb coordination and bilateral symmetry in body-weight transfer and balance control during CP gait, quantitative analysis of leg stiffness during the phases of gait for both limbs can be a useful approach. The control of the lower limbs as a whole in supporting the body during gait can be modeled as a mass-spring system with a point body-mass supported by the leg simulated as a non-linear spring. To support the body against collapse and to meet the demands of movement during the stance phase of gait, the stiffness of the spring (called leg stiffness) can be controlled by changing the joint torques provided by the musculotendinous units (i.e., net internal moments) and thus the skeletal alignment of the lower limb. The changes in the leg stiffness describe the modulation of the motions and joint loads of the lower limbs, and thus are useful in assessing pathology and treatment efficacy [15; 16; 17]. For example, leg stiffness has been used to study the effects of disease on the control of the lower limbs as a whole during walking [15; 16; 17], and the efficacy of treatment in children with CP, showing unaltered leg stiffness but significantly decreased joint stiffness at the hip and knee after TRS [13]. The improvement in the joint kinematics and kinetics of the lower limb may either change the coordination between limbs (change of biomechanical condition) or remain unchanged (maintain walking habits), which are not easily revealed through traditional gait analysis variables.