背景:老年人在废用萎缩后的肌肉质量恢复比年轻人慢。在较小的尺度上,肌肉纤维横截面积(即,平行的肉瘤)表现出相同的模式。鲜为人知,然而,关于肌肉纤维长度恢复的年龄相关差异,由连续肌节数(SSN)的增加驱动,在废弃之后。这项研究的目的是调查与年龄相关的SSN适应和肌肉机械功能在肌肉固定期间和之后的差异。我们假设老年成年大鼠在固定过程中会经历类似程度的SSN损失,然而,去除石膏后,恢复SSN需要比年轻更长的时间,这将限制肌肉机械功能的恢复。
方法:我们将幼年(8个月)和老年(32个月)雄性大鼠的足底屈肌缩短放置2周,并评估4周自愿步行期间的恢复情况。牺牲之后,将腿固定在福尔马林中以测量比目鱼SSN和生理横截面积(PCSA),未浇铸的比目鱼作为对照。每周进行一次悬垂角(PA)和肌肉厚度(MT)的超声检查。体内主动和被动扭矩-角度关系是预制的,播后,经过4周的恢复。
结果:从前到后,年轻和年长的成年大鼠经历了类似的SSN下降(-20%,P<0.001),肌肉湿重(-25%,P<0.001),MT(-30%),PA(-15%,P<0.001),和最大等距扭矩(-40%,P<0.001),但是老年人的被动扭矩增加更大(+180%,P<0.001)与年轻成年大鼠相比(68%,P=0.006)。拆除铸件后,年轻人表现出比老年人更快的SSN和MT恢复,但在年轻人和老年人中,SSN比PA和MT恢复得更快。幼年大鼠的PCSA几乎恢复,主动扭矩完全恢复,而在老年成年大鼠中,这些仍未恢复到75%。
结论:这项研究表明,与去除铸型后的平行肌肉形态相比,老年成年大鼠保持了更好的纵向恢复能力,使SSN成为康复早期改善老年人群肌肉功能的高度适应性目标。
BACKGROUND: Older adults exhibit a slower recovery of muscle mass following disuse atrophy than young adults. At a smaller scale, muscle fibre cross-sectional area (i.e., sarcomeres in parallel) exhibits this same pattern. Less is known, however, about age-related differences in the recovery of muscle fibre length, driven by increases in serial sarcomere number (SSN), following disuse. The purpose of this study was to investigate age-related differences in SSN adaptations and muscle mechanical function during and following muscle immobilization. We hypothesized that older adult rats would experience a similar magnitude of SSN loss during immobilization, however, take longer to recover SSN than young following cast removal, which would limit the recovery of muscle mechanical function.
METHODS: We casted the plantar flexors of young (8 months) and old (32 months) male rats in a shortened position for 2 weeks, and assessed recovery during 4 weeks of voluntary ambulation. Following sacrifice, legs were fixed in formalin for measurement of soleus SSN and physiological cross-sectional area (PCSA) with the un-casted soleus acting as a control. Ultrasonographic measurements of pennation angle (PA) and muscle thickness (MT) were conducted weekly. In-vivo active and passive torque-angle relationships were constructed pre-cast, post-cast, and following 4 weeks of recovery.
RESULTS: From pre- to post-cast, young and older adult rats experienced similar decreases in SSN (-20%, P < 0.001), muscle wet weight (-25%, P < 0.001), MT (-30%), PA (-15%, P < 0.001), and maximum isometric torque (-40%, P < 0.001), but there was a greater increase in passive torque in older (+ 180%, P < 0.001) compared to young adult rats (+ 68%, P = 0.006). Following cast removal, young exhibited quicker recovery of SSN and MT than old, but SSN recovered sooner than PA and MT in both young and old. PCSA nearly recovered and active torque fully recovered in young adult rats, whereas in older adult rats these remained unrecovered at ∼ 75%.
CONCLUSIONS: This study showed that older adult rats retain a better ability to recover longitudinal compared to parallel muscle morphology following cast removal, making SSN a highly adaptable target for improving muscle function in elderly populations early on during rehabilitation.