Sarcomeres are the most basic contractile element of skeletal muscle, with lengths of around 3 µm in humans. The lengths and contractile dynamics of sarcomeres can greatly influence muscle function and be affected by disease, but the small scale of these structures deep within the body has made in vivo measurements difficult. Recently, second-harmonic generation (SHG) microendoscopy has been introduced to image sarcomeres in a minimally invasive manner. This thesis presents the first in vivo measurements of sarcomere lengths in important muscles for gait, the vastus lateralis and the soleus, and of contractile dynamics in a mouse model of ALS.

We imaged sarcomeres in healthy humans in the vastus lateralis and the soleus muscles at multiple joint angles to better understand their force generating capacity across a range of joint angles. In the vastus lateralis, we acquired in vivo sarcomere images of several muscle fibers of the resting vastus lateralis in six healthy individuals. Mean sarcomere lengths increased from 2.84±0.16 µm at 50° of knee flexion to 3.17±0.13 µm (mean ± standard deviation) at 110° of knee flexion. The standard deviation of sarcomere lengths among different fibers within a muscle was 0.21±0.09 µm. Our results suggest that the sarcomeres of the vastus lateralis at 50° of knee flexion are near optimal length, enabling the muscle to generate approximately its peak isometric force. At a knee flexion angle of 110° the sarcomeres of vastus lateralis are longer than optimal length, reducing the muscle’s active force-generating capacity to approximately 74% of its peak isometric force.

In the soleus, we used microendoscopy to measure resting sarcomere lengths at 10° plantarflexion and 20° dorsiflexion in 7 healthy individuals. Mean sarcomere lengths at 10° plantarflexion are 2.84±0.24 µm (mean ± standard deviation), suggesting that the muscle generates near maximum force in this posture. Sarcomere lengths are 3.43±0.25 µm at 20° dorsiflexion, indicating that they are longer than optimal length when the ankle is in dorsiflexion and the muscle is inactive. Our results in both the vastus lateralis and the soleus indicate a smaller change in sarcomere length with joint flexion compared to estimates from musculoskeletal models and suggest why these models may underestimate the force-generating capacity of the soleus.

We also measured muscle twitch contractile dynamics in a mouse model of ALS, a fatal disease in which motor neurons connected to muscles progressively die off. Through longitudinal measurements of motor unit twitch contractions in a B6.SOD1G93A mouse model of ALS from a presymptomatic stage to end stage, we observed a presymptomatic elongation in rise times and half relaxation times that increased in the later stage of the disease. We constructed a composite twitch time taken from measurements in each animal that effectively diagnoses individuals, and changes with progression of the disease. This quantitative, minimally invasive approach to assess motor unit contractile timing with ALS progression supports episodes of specific types of motor neuron loss, providing new useful information for preclinical and clinical studies. Together, these studies demonstrate the utility of muscle microendoscopy in research and the clinic.