Isokinetic Performance of Knee Flexor and Extensor Muscles in American Football Players from Brazil

Abstract The isokinetic performance of thigh muscles has been related to athletic performance and risk for non-contact injuries, such as anterior cruciate ligament ruptures and hamstring strains. Although isokinetic profile of American football players from United States (USA) is widely described, there is a lack of studies comprising players acting outside the USA. The primary objective of this study was to describe the isokinetic performance of thigh muscles in elite American football players in Brazil. Secondarily, we aimed to compare the playing positions and compare the Brazilian players with high-level athletes from USA. Knee extensor (KE) and flexor (KF) muscles of 72 Brazilian players were assessed through isokinetic tests at 60°∙s-1. KE concentric peak torque was 276±56 N∙m, while KF had concentric and eccentric peak torques of 151±37 N∙m and 220±40 N∙m, respectively. Offensive linemen players presented greater peak torque values than defensive lineman, halfbacks, and wide receivers (all comparisons are provided in the article). Brazilian players had lower scores than USA athletes for KE and KF peak torque values. In addition, a conventional torque ratio (concentric/concentric) lower than 0.6 was found in 76-83% of athletes, and a functional ratio (eccentric/eccentric) below to 1.0 in 94%. Bilateral asymmetry greater than 10% was verified in 26% and 43% of athletes for KE and KF muscles, respectively. Elite players in Brazil present high incidence of strength imbalance in thigh muscles, and they are below USA players in relation to torque production capacity of KE and KF muscles.

摘要
大腿肌肉的等速（isokinetic）肌力表现与运动表现及非接触性损伤风险密切相关，例如前交叉韧带断裂与腘绳肌拉伤。尽管针对美国（USA）美式橄榄球运动员的等速肌力特征已有广泛研究，但针对美国境外参赛运动员的相关研究仍较为匮乏。
本研究的首要目标为描述巴西精英美式橄榄球运动员的大腿肌肉等速肌力表现；次要目标为对比不同场上位置运动员的肌力差异，并将巴西运动员与美国高水平运动员进行对比。
本研究采用60°∙s⁻¹的等速测试方案，对72名巴西运动员的膝伸肌（knee extensor, KE）与膝屈肌（knee flexor, KF）进行了评估。结果显示，膝伸肌的向心峰值扭矩为276±56 N∙m，膝屈肌的向心与离心峰值扭矩分别为151±37 N∙m与220±40 N∙m。进攻锋线球员的峰值扭矩显著高于防守锋线、跑卫与外接手（所有位置间的对比细节详见本文）。巴西运动员的膝伸肌与膝屈肌峰值扭矩均低于美国运动员。此外，76%~83%的运动员存在常规扭矩比（向心/向心）低于0.6的情况，94%的运动员的功能性扭矩比（离心/离心）低于1.0。膝伸肌与膝屈肌分别有26%与43%的运动员存在超过10%的双侧肌力不对称。
巴西精英美式橄榄球运动员的大腿肌肉肌力失衡发生率较高，且其膝伸肌与膝屈肌的肌力产出能力均低于美国运动员。