Anterior cruciate ligament (ACL) is a primary structure and a commonly injured ligament of the knee joint. Some patients with ACL deficiency (ACLD) experience joint instability and require a reconstructive surgery to return to daily routines, some can adapt by limiting their activities while others, called copers, can return to high-level activities with no instability. We investigated the effects of alterations in the knee flexion angle (KFA) and muscle force activations on the stability and biomechanics of ACLD joints at 25, 50, and 75% periods of gait stance. ACLD joint stability is controlled by variations in both KFA and knee muscle forces. For the latter, a parameter called activity index is defined as the ratio of forces in ACL antagonists (quadriceps and gastrocnemii) to those in ACL agonists (hamstrings). Under a greater KFA (2-6^o beyond the mean of reported values in healthy subjects), an ACLD joint regains its pre-injury stability levels. The ACLD joint stability also markedly improves at smaller quadriceps and larger hamstrings forces (activity indices of 2.0-3.6 at 25%) at the first half of stance and smaller gastrocnemii and larger hamstrings forces (activity indices of 0.1-1.1 at 50% and 0.1-1.2 at 75 %) at the second half of stance. Activity index and KFA are both crucial when assessing the dynamic stability of an ACLD joint. These results are helpful in our understanding of the biomechanics and stability of ACLD joints towards improved prevention and treatment strategies.

前十字韧带（ACL）是膝关节的主要结构和常见的韧带。一些患有ACL缺乏症（ACLD）的患者会经历关节不稳，需要进行整形手术才能恢复到日常工作，一些患者可以通过限制自己的活动来适应，而另一些被称为copers的患者可以恢复到高水平的活动而没有不稳定。我们研究了25、50和75％步态站立时膝关节屈曲角度（KFA）和肌肉力量激活对ACLD关节稳定性和生物力学的影响。ACLD关节的稳定性受KFA和膝盖肌肉力量的变化控制。对于后者，称为活动指数的参数定义为ACL拮抗剂（股四头肌和腓肠肌）的力量与ACL激动剂（）绳肌）的力量之比。在更大的KFA下（2-6^o超出健康受试者报告值的平均值），ACLD关节恢复到损伤前的稳定水平。在站立的前半部分，较小的股四头肌和较大的肌力量（25％时的活动指数为2.0-3.6），腓肠肌较小和较大的肌力量（50％和0.1时的活动指数为0.1-1.1）时，ACLD关节稳定性也明显改善。在下半场则为-1.2（75％）。在评估ACLD关节的动态稳定性时，活动指数和KFA都是至关重要的。这些结果有助于我们了解ACLD关节的生物力学和稳定性，从而改善预防和治疗策略。