Rightward shift of optimal fascicle length with decreasing voluntary activity level in the soleus and lateral gastrocnemius muscles

Speed-specific optimal contractile conditions of the human soleus muscle from slow to maximum running speed

The soleus is the main muscle for propulsion during human running but its operating behavior across the spectrum of physiological running speeds is currently unknown. This study experimentally investigated the soleus muscle activation patterns and contractile conditions for force generation, power production and efficient work production (i.e. force-length potential, force-velocity potential, power-velocity potential and enthalpy efficiency) at seven running speeds (3.0 m s-1 to individual maximum). During submaximal running (3.0-6.0 m s-1), the soleus fascicles shortened close to optimal length and at a velocity close to the efficiency maximum, two contractile conditions for economical work production. At higher running speeds (7.0 m s-1 to maximum), the soleus muscle fascicles still operated near optimum length, yet the fascicle shortening velocity increased and shifted towards the optimum for mechanical power production with a simultaneous increase in muscle activation, providing evidence for three cumulative mechanisms to enhance mechanical power production. Using the experimentally determined force-length-velocity potentials and muscle activation as inputs in a Hill-type muscle model, a reduction in maximum soleus muscle force at speeds ≥7.0 m s-1 and a continuous increase in maximum mechanical power with speed were predicted. The reduction in soleus maximum force was associated with a reduced force-velocity potential. The increase in maximum power was explained by an enhancement of muscle activation and contractile conditions until 7.0 m s-1, but mainly by increased muscle activation at high to maximal running speed.

The force-length relation of the young adult human tibialis anterior

Background

Knowledge of the muscle's lengths at which maximum active isometric force is attained is important for predicting forces during movement. However, there is limited information about the in vivo force-length properties of a human muscle that plays crucial roles during locomotion; the tibialis anterior (TA). We therefore aimed to estimate TA's force-length relation from dorsiflexor torque-angle curves constructed from eight women and eight men.

Methods

Participants performed maximal voluntary fixed-end contractions with their right ankle dorsiflexors from 0° to 30° plantar flexion. Muscle fascicle lengths were estimated from B-mode ultrasound images, and net ankle joint torques were measured using dynamometry. Fascicle forces were estimated by dividing maximal active torques by literature-derived, angle-specific tendon moment arm lengths while assuming a fixed 50% force contribution of TA to the total dorsiflexor force and accounting for fascicle angles.

Results

Maximal active torques were higher at 15° than 20° and 30° plantar flexion (2.4-6.4 Nm, p ≤ 0.012), whereas maximal active TA fascicle forces were higher at 15° than 0°, 20° and 30° plantar flexion (25-61 N, p ≤ 0.042), but not different between 15° and 10° plantar flexion (15 N, p = 0.277). TA fascicle shortening magnitudes during fixed-end contractions were larger at 15° than 30° plantar flexion (3.9 mm, p = 0.012), but less at 15° than 0° plantar flexion (-2.4 mm, p = 0.001), with no significant differences (≤0.7 mm, p = 0.871) between TA's superficial and deep muscle compartments. Series elastic element stiffness was lowest and highest at lengths 5% shorter and 5% longer than optimum fascicle length, respectively (-30 and 15 N/mm, p ≤ 0.003).

Discussion

TA produced its maximum active force at 10-15° plantar flexion, and its normalized force-length relation had ascending and descending limbs that agreed with a simple scaled sarcomere model when active fascicle lengths from within TA's superficial or deep muscle compartment were considered. These findings can be used to inform the properties of the contractile and series elastic elements of Hill-type muscle models.

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Triceps surae muscle force potential and force demand shift with altering stride frequency in running

While it is well recognized that the preferred stride frequency (PSF) in running closely corresponds to the metabolically optimal frequency, the underlying mechanisms are still unclear. Changes in joint kinematics when altering stride frequency will affect the muscle-tendon unit lengths and potentially the efficiency of muscles crossing these joints. Here, we investigated how fascicle kinematics and forces of the triceps surae muscle, a highly energy consuming muscle, are affected when running at different stride frequencies. Twelve runners ran on a force measuring treadmill, adopting five different frequencies (PSF; PSF±8%; PSF±15%), while we measured joint kinematics, whole-body energy expenditure, triceps surae muscle activity, and soleus (SOL; N = 10) and gastrocnemius medialis (GM; N = 12) fascicle kinematics. In addition, we used dynamic optimization to estimate SOL and GM muscle forces. We found that SOL and GM mean muscle fascicle length during stance followed an inverted U-relationship with the longest fascicle lengths occurring at PSF. Fascicle lengths were shortest at frequencies lower than PSF. In addition, average SOL force was greater at PSF-15% compared to PSF. Overall, our results suggest that reduced SOL and GM muscle fascicle lengths, associated with reduced muscle force potential, together with greater SOL force demand, contribute to the increased whole-body energy expenditure when running at lower than PSF. At higher stride frequencies, triceps surae muscle kinematics and force production were less affected suggesting that increased energy expenditure is rather related to higher cost of leg swing and greater cost of force production.

Effects of Knee Joint Angle and Contraction Intensity on the Triceps Surae Stiffness

Purpose: Monitoring the contractility of muscles assists the clinician in understanding how muscle functions as part of the kinetic system. This study investigated the effect of knee joint angles under different resistance on the stiffness of the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL) muscles using the shear wave elastography (SWE) technique.

Methods: A total of 22 females were recruited. During isometric plantar flexion, at knee 0-degree (fully extended) and knee 90-degree (flexed 90°), the shear modulus on the MG, LG, and SOL was measured by shear wave elastography at no contraction and two intensities (40% and 80%) of maximal voluntary contraction (MVC). Shear modulus is a mechanical parameter to describe stiffness, and stiffness is a proxy for muscle contractility.

Results: There were moderate-to high-positive correlations between the active stiffness of triceps surae muscles and isometric contraction intensity (r: 0.57–0.91, p＜0.001). The active stiffness in MG and LG with extended knees was higher than that with flexed knees (p＜0.001). The active stiffness in SOL with flexed knee was higher than that with extended knee (p＜0.001).

Conclusion: Active stiffness can be considered a quantitative indicator generated by the force output of the triceps surae. Different knee joint angles cause three triceps surae muscles to exhibit non-uniform mechanical properties, which may explain part of the mechanism of soft tissue injury during physical exercise.

Achilles tendon mechanical properties during walking and running are underestimated when its curvature is not accounted for

Achilles tendon (AT) mechanical properties can be estimated using an inverse dynamic approach, taking into account the tendon internal moment arm (IMA) and its kinematic behavior. Although AT presents a curvilinear line of action, a straight-line function to estimate IMA and AT length is often utilized in the literature. In this study, we combined kinetic, kinematic and ultrasound data to understand the impact of two different approaches (straight-line vs. curvilinear) in determining AT mechanical properties in vivo (during walking and running at the self-selected speed). AT force and power were calculated based on data of AT IMA and AT length derived by both respective methods. All investigated parameters were significantly affected by the method utilized (paired t-test; p < 0.05): when using the curvilinear method IMA was about 5% lower and AT length about 1.2% higher, whereas peak and mean values of AT force and power were 5% higher when compared to the straight-line method (both in walking and running). Statistic-parametric mapping (SMP) analysis revealed significant differences in IMA during the early and the late stance phase of walking and during the late stance phase of running (p < 0.01); SPM revealed significant differences also in AT length during the entire stance phase in both locomotion modes (p < 0.01). These results confirm and extend previous findings to human locomotion: neglecting the AT curvature might be a source of error, resulting in underestimates not only of internal moment arm and tendon length, but also of tendon force and power.