Locomotion, particularly running, generates significant impact forces upon the body. These forces must be effectively dampened to prevent injury and muscle fatigue. While joint stiffness plays a role, skeletal muscle vibration is believed to be a crucial mechanism for dissipating impact energy. Previous research has used indirect methods like skin-mounted accelerometers, which suffer from motion artifacts and difficulty in isolating muscle-specific oscillations. This study aimed to directly measure in vivo muscle oscillations using dynamic 2D ultrasound imaging, focusing on the soleus muscle, a major ankle plantar flexor, to determine if oscillation characteristics vary with changes in locomotion speed and thus impact force.

Existing research suggests that soft-tissue oscillations contribute significantly to impact force dissipation during locomotion. Studies using computer models and skin-mounted accelerometers have demonstrated the potential of soft-tissue wobble in attenuating impact energy. Further evidence from combined accelerometer and muscle activity measurements shows muscles can modulate their activity and oscillations in response to impact force. However, direct in vivo quantification of skeletal muscle oscillations during locomotion has been lacking due to limitations of previous measurement techniques, particularly the difficulty in separating skin movement artifacts from underlying tissue motion. This study addresses this gap by employing ultrasound imaging to directly visualize and quantify muscle oscillations.

Ten participants (9 male, 1 female) walked and ran on a treadmill at various speeds. Data included marker trajectories (8-camera motion capture system at 250 Hz), ground reaction force (instrumented treadmill at 2000 Hz), dynamic ultrasound images (B-mode ultrasound system at 80 Hz), and soleus muscle EMG (telemetered system at 2000 Hz). Ultrasound images were processed using an active shape model to segment and track landmarks on the superficial and deep aponeuroses of the soleus. Transverse displacement of these landmarks was analyzed to quantify oscillations. Input signal frequency was determined from the vertical ground reaction force. Power spectral analysis was used to determine the power and frequency content of the muscle oscillations. Statistical analysis involved one-way and two-way repeated measures ANOVAs.

Input signal frequency, derived from ground reaction forces, increased significantly with increasing walking and running speeds (p<0.001). The soleus muscle showed activity during stance phase across all locomotion conditions, with peak EMG intensity increasing with speed. Transverse displacement of aponeuroses showed an initial downward displacement followed by a return to initial position and a subsequent downward displacement during push-off. Faster walking resulted in greater initial displacement, while faster running showed reduced peak-to-peak displacement. Power spectral analysis revealed significant main effects for both power and frequency with locomotion condition and image region. Peak power increased significantly (p<0.001 for both superficial and deep aponeuroses) with increased locomotion speed, showing a relationship with input frequency. Cumulative frequency (frequency accounting for 50% of total signal power) also increased significantly (p<0.001) with locomotion speed and varied significantly between image regions (distal, mid, proximal). The relationship between peak power and cumulative frequency with input frequency was evident across the superficial and deep aponeuroses.

This study provides the first in vivo evidence that ultrasound imaging can measure the influence of external impact stimuli on muscle dissipative behaviors. The observed input frequencies (10-20 Hz) align with previous studies. The positive association between input frequency and oscillation power and frequency supports the role of muscle oscillations in impact force dissipation. However, the lower cumulative frequencies reported here compared to some previous studies highlight differences in measurement methods and the importance of muscle-specific versus composite tissue measurements. The effects of probe compression on the superficial muscles (lateral gastrocnemius) highlight a limitation, suggesting the approach may be more suitable for deeper muscles. Differences in oscillations between aponeurosis regions suggest that the measurements reflect in vivo muscle behavior rather than artifact. Potential factors influencing the relationship between input frequency and oscillation power include differences in EMG amplitude and timing, as well as foot-strike patterns. Further research is needed to investigate these influences.

This study successfully employed ultrasound imaging to quantify in vivo muscle oscillations in the soleus during locomotion. The findings demonstrate a clear relationship between increasing input frequency and increased muscle oscillations. This technique offers potential for future research examining muscle-specific responses to impact forces under varying external conditions (e.g., shoe material, ground surface).

The study's limitations include the potential influence of probe compression on superficial muscle measurements, and the limited sampling rate of ultrasound which may not fully capture higher frequency oscillations. Foot-strike patterns were not strictly controlled. Analysis focused on transverse displacement, neglecting longitudinal oscillations which would provide a more complete picture of muscle response to impact forces.