Isokinetic Strength and Strength Asymmetries of Lower Extremities in Professional Soccer Players

Abstract

Muscle strength (MS) is an important component of physical fitness in soccer players (SP). Side-to-side differences may be developed due to the unilateral nature of certain soccer skills and movement patterns. Constant loading on one side of the body over time may lead to strength asymmetry and imbalances in tissue adaptation. PURPOSE: To investigate isokinetic strength asymmetry (SA) and magnitude in professional male SP. METHODS: Professional male SP from the first division of Czech Republic (n = 148, age 25.1±4.8 years) volunteered to participate in the study. SP performed isokinetic strength testing (Humac Norm, CYBEX, USA) concentrically at angular velocities of 60, 180 and 300°·s-1.The peak muscle torque of knee extensors (PTE) and flexors (PTF) in both legs, hamstrings and quadriceps strength ratio of muscle torque for both preferred and non-preferred extremities (H:Qp and H:Qn, respectively), bilateral ratio between the exerted strength of knee extensors (Q:Q) and flexors (H:H) were calculated. Mixed-design RM ANOVA, Bonferroni’s post hoc tests and partial eta square (ηp2) were used for statistical assessment. RESULTS: Knee flexors showed significantly higher SA (H:H=9.77±0.40%) compared to the extensors (Q:Q=7.24±0.40 %) (F1,294=20.49, p=0.00, η2=0.07). Greater hamstrings and quadriceps strength ratio was found in preferred leg (H:Qp = 59.89%) compare to non-preferred (H:Qn = 58.10±0.63 %) (F2,588=4.01, p=0.04, η2=0.01). Contraction velocity speed did not have a significant difference on SA (p>0.05). MS of knee extensors reached the highest value at the lowest velocity for both legs (PTE60=2.94±0.36 N·m·kg-1). Exerted MS significantly decreased with increasing velocity in both: extensors (F2,588=4218.92, p=0.00, η2=0.94) and flexors (F2,588=2932.69, p=0.00, η2=0.91). At the highest velocity (300°·s-1), MS achieved ~ 55% (PTE) respectively ~53% (PTF) of MS exerted at the lowest velocity (300°·s-1). CONCLUSIONS: Greater SA was found in knee flexion (H:H) rather than knee extension (Q:Q) in a group of professional male SP. Also, greater hamstrings and quadriceps strength ratio was found in preferred leg (H:Qp) compared to non-preferred leg (H:Qn). The findings of this study will be useful in the design of injury prevention and performance enhancement programs.