Fibre operating lengths of human lower limb muscles during walking

Abstract

Muscles actuate movement by generating forces. The forces generated by muscles are highly dependent on their fibre lengths, yet it is difficult to measure the lengths over which muscle fibres operate during movement. We combined experimental measurements of joint angles and muscle activation patterns during walking with a musculoskeletal model that captures the relationships between muscle fibre lengths, joint angles and muscle activations for muscles of the lower limb. We used this musculoskeletal model to produce a simulation of muscle-tendon dynamics during walking and calculated fibre operating lengths (i.e. the length of muscle fibres relative to their optimal fibre length) for 17 lower limb muscles. Our results indicate that when musculotendon compliance is low, the muscle fibre operating length is determined predominantly by the joint angles and muscle moment arms. If musculotendon compliance is high, muscle fibre operating length is more dependent on activation level and force-length -velocity effects. We found that muscles operate on multiple limbs of the force -length curve (i.e. ascending, plateau and descending limbs) during the gait cycle, but are active within a smaller portion of their total operating range.

On the ascent: the soleus operating length is conserved to the ascending limb of the force-length curve across gait mechanics in humans

“…It is also worth highlighting that despite operating on the ascending limb, the normalized muscle length of 0.88 L 0 when peak EMG is recorded indicates that the SOL is not greatly restricted in its length-dependent force-generating capacity (~95%) when the greatest muscle activation and force is probably required (electromechanical delay is not expected to greatly affect this conclusion given the small changes in muscle length spanning the peak in EMG). Although previous experimental data for normalized SOL lengths during walking are (to the best of our knowledge) lacking for human gait, the measured changes in SOL normalized lengths corroborates a recent optimization-based computer simulation (Arnold and Delp, 2011), which predicted a similar pattern across both the stance and swing phases of walking. When these authors used a compliant tendon in their model they predict similar, albeit somewhat longer, normalized muscle lengths that function in a stretch-shorten cycle primarily over the ascending limb and plateau region, and exhibit an isometric phase in mid-stance.…”

“…This is due, in part, to a single participant whose SOL operated to a large extent on the plateau and descending limb of its F-L curve. Although this length is considerably longer than the other participants, it resides in a physiologically plausible range (Arnold and Delp, 2011) and thus we have retained it in our analysis. As a result, the variability in the reported normalized muscle lengths during gait is greater than if the absolute muscle length is expressed as a percentage of its length at heel strike (mean coefficient of variation of 18.7 vs 9.7%, respectively).…”

“…Muscle lengths were digitized in ImageJ, filtered using a fourth-order zerolag 5Hz low-pass Butterworth filter (MATLAB), and divided by L 0 in order to describe the portion of the F-L curve occupied by the muscle. Regions of the F-L curve occupied by the muscle were defined based on a whole muscle F-L curve: L/L 0 <0.75, steep ascending limb; 0.75≤L/L 0 <0.95, shallow ascending limb; 0.95≤L/L 0 ≤1.05, plateau region; 1.05<L/L 0 , descending limb (Arnold and Delp, 2011). These regions differ slightly from the theoretical sarcomere F-L curve and reflect sarcomere length heterogeneity in whole muscle.…”

“…This limitation is important, especially considering that L 0 may vary between individuals, since muscle architecture is known to be different between cadavers and agematched live individuals (Martin et al, 2001) and is affected by age and disuse (Narici and Maganaris, 2007). Furthermore, while computer-modelling approaches have several important merits, there exist uncertainties regarding how variables such as tendon stiffness and activation levels affect predictions of fibre lengths (Arnold and Delp, 2011). Given its fundamental significance to the mechanics of human movement, it is surprising that more direct experimental measurements of normalized length operating ranges of muscle during gait are lacking.…”

“…Although the region of the F-L curve occupied by muscles during locomotion has been primarily measured in non-human animals (Burkholder and Lieber, 2001), a handful of estimates have been made for leg muscles in human gait using both anatomical and computational approaches (Cutts, 1989;Fukunaga et al, 2002;Arnold and Delp, 2011). These studies have begun to shape our understanding of human muscle operating lengths during various locomotor tasks.…”