Botulinum toxin type A (BTX-A) effects on the mechanics of non-injected antagonistic muscles are unknown. The aim was to test the following hypotheses in a rat model: BTX-A injected into gastrocnemius medialis (GM) and lateralis (GL) (1) decreases forces of the antagonistic tibialis anterior (TA) and extensor digitorum longus (EDL), (2) reduces length range of force exertion and (3) increases passive forces of the TA, and (4) changes inter-antagonistic and inter-synergistic epimuscular myofascial force transmission (EMFT).

Two groups of Wistar rats were tested: BTX (0.1 units of BTX-A were injected to the GM and GL, each) and Control (saline injected). Five-days post, TA, EDL, GM-GL, and soleus distal and EDL proximal isometric forces were measured after TA lengthening.

BTX-A exposure caused forces of all muscles to decrease significantly. TA and EDL active force drops (maximally by 37.3%) show inter-compartmental spread. Length range of force exertion of the TA did not change, but its passive force increased significantly (by 25%). The percentages of intramuscular connective tissue content of the TA and EDL was higher (BTX: 20.0 ± 4.9% and 19.3 ± 4.1% vs. control: 13.1 ± 5.4% and 14.5 ± 4.0%, respectively). Calf muscles’ forces were not affected by TA length changes for both groups indicating lacking inter-antagonistic EMFT. However, BTX-A altered EDL proximo-distal force differences hence, inter-synergistic EMFT.

A major novel finding is that BTX-A affects mechanics of non-injected antagonistic muscles in test conditions involving only limited EMFT. The effects indicating a stiffer muscle with no length range increase contradict some treatment aims, which require clinical testing.

A型肉毒毒素（BTX-A）对未注射的拮抗性肌肉力学的影响尚不清楚。目的是在大鼠模型中检验以下假设：将BTX-A注射到腓肠肌（GM）和外侧肌（GL）（1）中可降低拮抗性胫骨前（TA）和趾长伸肌（EDL）的力量，（ 2）减小作用力的长度范围，（3）增加TA的被动力，并且（4）改变对立和协同作用之间的肌腱肌筋膜肌力传递（EMFT）。

测试了两组Wistar大鼠：BTX（分别向GM和GL注射了0.1单位的BTX-A）和对照组（注射了盐水）。术后5天，TA延长后测量TA，EDL，GM-GL，比目鱼肌远侧和EDL近侧等距力。

BTX-A暴露导致所有肌肉的力量显着降低。TA和EDL主动力下降（最大37.3％）显示室间扩散。TA施加的力的长度范围没有变化，但其被动力显着增加（增加了25％）。TA和EDL的肌内结缔组织含量百分比更高（BTX：相对于对照：分别为13.1±5.4％和14.5±4.0％，分别为20.0±4.9％和19.3±4.1％）。两组的小腿肌肉力量不受TA长度变化的影响，表明缺乏对抗性的EMFT。但是，BTX-A改变了EDL的近-远力差异，因此，发生了协同增效EMFT。

一个主要的新发现是，在仅涉及有限的EMFT的测试条件下，BTX-A会影响未注射的拮抗肌肉的力学。表明没有长度范围增加的肌肉僵硬的效果与某些治疗目的相矛盾，这需要临床测试。