Research in Sport Medicine and Technology

Balance and strength are important abilities that athletes should have them. This study was designed to investigate the relationship between maximum leg press (non-weight bearing closed kinetic chain) and squat (weight bearing closed kinetic chain) strength with statistic and dynamic balance. Thirty (15 men and 15 women) physically active participants (age, 22.13 ± 1.54 years; height, 171.26 ± 6.92 cm; weight, 66.02 ± 8.27 kg) completed the study. The Biodex Balance System was used to evaluate balance performance. Subjects completed two, 40 seconds trials attempting to maintain their statistic and dynamic balance and Overall Stability Index, Anterior/Posterior Index and Medial/Lateral Index were obtained. Maximum strength also was measured with a 1RM squat and leg press. Significant correlations were found between the measurements of 1RM squat with static anterior-posterior indices (r, 0.019; P=-0.541) and dynamic medial-lateral indices (r, 0.021, P=-0.532) in men and with static anterior-posterior indices (r, 0.016, P=-0.552) and dynamic anterior-posterior indices (r, 0.032; P=-0.491) in women. No significant correlations were found between the measures of 1RM leg press and balance indices in both groups. The results of this study indicate that maximum squat strength has more relation with some indices of static and dynamic balance than maximum leg press.
 

Hamstrings (Hams) to Quadriceps (Quadr) strength ratio is known as a risk factor for Anterior Cruciate Ligament (ACL) injury and affects by knee and hip flexion angles. Gluteus Maximus (Gmax) muscle acts as a synergist for Hams in hip extension in a closed kinetic chain. The aim of this study was to investigate the effect of Gmax strengthening exercises on the kinetic and kinematic risk factors of ACL injury when single-leg landing from a jump. 25 volunteer women aged 18-30 years were assigned into control (n =13) and experimental (n =12) groups. Maximum isometric strength of Gmax, hip and knee joint angles and ground reaction force (GRF) was measured at landing, by dynamometer, 2D imaging and foot scan respectively,. After 8 weeks of 3 sessions of Gmax resistance training, Following the significant within-subject difference for GRF (F=5.245, P=0.032) by using Two-way mixed model ANOVA, Pre and post-test Differences were significant with 16.63% decrease (P= 0.038, t=2.354). Differences between pre and post-test peak force, time to peak, mean joint angles, impact and load rate were not significant (α = 0.05). According to a 4.55% and 3.47% increase in Gmax strength and time to peak force, and considering the fact that at the beginning of landing, the mechanical advantage of Hams is lower than that of Gmax, the risk of ACL injury could be reduced by reducing GRF following an increase in the Gmax strength.