Poster E2 - The Effects of Calf Muscle Length on Local Muscle Fatigability

Saturday, October 26, 2024

10:00 AM - 11:00 AM EST

Location: Exhibit Hall E, F & G

Presenting Author(s)

Anh D. Nguyen

Undergraduate Research Assistant
Joint Department of Biomedical Engineering at UNC Chapel Hill & NC State University
Chapel Hill, North Carolina, United States

Co-Author(s)

Aubrey Gray (she/her/hers)

Graduate Research Assistant
Joint Department of Biomedical Engineering at UNC Chapel Hill and NC State University, United States
GS

Gregory Sawicki

Associate Professor
George W. Woodruff School of Mechanical Engineering, United States

Primary Investigator(s)

JF

Jason Franz

Associate Professor
University of North Carolina - Chapel Hill/North Carolina State University, United States

Introduction: Preventing walking disability among our rapidly aging population presents a significant public health challenge. A key factor is that older adults consume metabolic energy more rapidly than younger adults during walking, making them more prone to fatigue and exhaustion. We and others have shown that, compared to younger adults, older adults operate their calf muscles at shorter lengths which associates with a diminished capacity to enhance push-off intensity during walking. According to principles of muscle physiology, shorter muscle lengths require higher activation levels to produce a requisite force. Conceptually, those higher activations should precipitate an earlier onset of local muscle fatigue. However, the specific role of muscle length in governing the onset of local muscle fatigue has yet to be empirically established, even in healthy young adults. The goal of this study was to empirically examine the effect of muscle length on the onset of local calf muscle fatigue in a cohort of young adult participants. We focused on the calf muscles due to their relevance to walking performance and age-related mobility decline. We hypothesized that, compared to those when operating at longer fascicle lengths, calf muscle activity would increase and time to onset of fatigue would decrease when performing fixed-end calf muscle contractions to failure at shorter calf muscle lengths.

Materials and

Methods: We used a repeated measures design to investigate the effects in a cohort of 16 young adults (age: 22.7±4.5 yrs). All testing was completed on the right leg in a computerized robotic dynamometer (System 4 Pro, Biodex) with knee flexed to 110° to isolate force production to uniarticular ankle muscles (Fig. 1A) . We collected resting in vivo B-mode ultrasound images of the medial gastrocnemius at 15° of dorsiflexion (DF15) and 15° of plantarflexion (PF15) to assess fascicle length, the relative change in which we assumed would be indistinguishable from that of the soleus. Throughout testing, we recorded posterolateral soleus surface EMG. Prior to fatiguing trials, participants performed 3 maximal voluntary isometric contractions (MVIC) at a neutral ankle position to determine their maximum ankle moment. Thereafter, participants performed repeated fixed-end calf muscle contractions at DF15 and PF15, in randomized order, while a real-time biofeedback system implemented in Matlab monitored instantaneous net ankle moment (Fig. 1B). We instructed participants to push against the dynamometer pedal to replicate a continuous stream of target square waves displayed on a screen (0.5 Hz, 35% duty cycle, 75% MVIC ankle moment amplitude). We defined the onset of fatigue as an inability to reach the target over 5 consecutive pushes (Fig. 1C). Participants were provided a 25-minute rest between the two postures to allow recovery. As a neuromuscular surrogate of fatigue, we quantified median frequency and rms amplitude of soleus EMG, which we averaged over the first 5 and last 5 pushes.

Results, Conclusions, and Discussions: Changes in ankle joint posture elicited anticipated changes in muscle fascicle lengths, which were 21% (8.9±4.4 mm; p< 0.001) shorter for PF15 than for DF15 (Fig. 2A). Consistent with principles of muscle physiology, these shorter lengths were accompanied by roughly 2-fold higher EMG rms amplitudes (p < 0.01) (Figs. 2C & 2D). This supports our theoretical motivation to interrogate the effects of shorter muscle lengths, and the higher requisite activation they require, on local muscle fatigability. Indeed, as one of the primary contributions of this study and consistent with our hypothesis, we found that the shorter fascicle lengths at PF15 precipitated, on average, a 60% reduction in time to fatigue onset compared to the longer lengths at DF15 (Fig. 2B). As another validation of our study protocol, soleus fatigue was also associated with hallmark changes in EMG independent of ankle joint posture and thus fascicle length; compared to earlier contractions, later contractions exhibited lower EMG median frequency (DF15, p=0.046; PF15, p< 0.01) – a neuromuscular marker of fatigue (Fig. 3). An unanticipated modest but significant negative correlation between decreases in fascicle length (i.e., from DF15 to PF15) and change in time to onset of fatigue (r=-0.540, p=0.046) requires further investigation.
Our results point to an ecological impact of the increased physiological demand placed on calf muscles when operating at shorter lengths – namely accelerated fatigue. We contend that these findings have clinical relevance for mechanisms associated with mobility impairment among older adults, who often operate their calf muscles at shorter lengths. These insights have practical implications for personalized rehabilitation and training protocols and may help refine ways – such as assistive devices – to mitigate fatigue in older adults.

Acknowledgements (Optional): We thank Andrew Shelton for assistance with EMG processing. Supported in part by NIH (R01AG058615).