Gauging force by tapping tendons? Inaccurately in the human patellar tendon.

Accurately estimating in vivo tendon load non-invasively remains a major challenge in biomechanics, which might be overcome by shear-wave tensiometry. Shear-wave tensiometry measures the speed of mechanically induced tendon shear waves by skin-mounted accelerometers. To gauge the feasibility and accuracy of this novel technique, we obtained patellar tendon shear wave speeds via shear-wave tensiometry during sustained or ramp voluntary contractions of the knee extensors in two experiments (n = 8 in both). In experiment one, participants produced a constant knee extension torque of âˆ¼ 50 Nm at five different knee joint angles (i.e. variable tendon load), which resulted in estimated patellar tendon forces between 1005 ± 6N and 1182 ± 16 N. However, wave speed squared did not correlate with estimated tendon force within participants (rrm(31) = -0.19, p = 0.278). In experiment two, averaged correlation coefficients between normalized wave speed squared and torque of maximal and submaximal voluntary contractions across participants ranged between r = 0.43 and r = 0.94, while the time-varying correlation between these normalized signals ranged from r = -0.99 to r = 1.00. Further, the mean absolute errors (MAEs) between normalized wave speed squared and normalized torque across participants ranged between 0.03 and 0.54, which were larger than the MAEs between normalized torque and normalized summed EMG amplitude from the superficial quadriceps muscles (0.03-0.54 versus 0.06-0.26, respectively). In conclusion, there was no simple relation between shear wave speed squared and patellar tendon load, which severely limits the feasibility of shear-wave tensiometry for accurately estimating in vivo tendon load at the knee joint.

Residual force depression is not related to positive muscle fascicle work during submaximal voluntary dorsiflexion contractions in humans.

Residual force depression (rFD) following active muscle shortening is assumed to correlate most strongly with muscle work, but this has not been tested during voluntary contractions in humans. Using dynamometry, we compared steady-state ankle joint torques (N = 16) following tibialis anterior (TA) muscle-tendon unit (MTU) lengthening and shortening to the time-matched torque during submaximal voluntary fixed-end dorsiflexion reference contractions (REF) at a matched MTU length and EMG amplitude. Ultrasound revealed significantly reduced (P < 0.001) TA fascicle shortening amplitudes during MTU lengthening without a preload over small and medium amplitudes, respectively, relative to REF. MTU lengthening with a preload over a large amplitude significantly (P < 0.001) increased fascicle shortening relative to REF, as well as stretch amplitudes relative to MTU lengthening without a preload (P = 0.001). Significant (P = 0.028) steady-state fascicle force enhancement relative to REF was observed following MTU lengthening, and was similar among MTU lengthening-hold conditions (3-5%). MTU shortening with and without a preload over small and large amplitudes significantly (P < 0.001) increased positive fascicle and MTU work relative to REF, but significant (P = 0.006) rFD was observed following MTU shortening with a preload (7-10%) only. rFD was linearly related to positive MTU work [rrm (47) = 0.48, P < 0.001], but not positive fascicle work [rrm (47) = 0.16, P = 0.277]. Our findings indicate that MTU lengthening without substantial fascicle stretch enhances steady-state force output, which might arise from less shortening-induced rFD. Our findings also indicate similar rFD following different amounts of positive fascicle/MTU work, which cautions against using work to predict rFD during submaximal voluntary contractions. KEY POINTS: Accurately predicting muscle force is challenging because active muscle shortening depresses force output. The residual force depression (rFD) that exists following active muscle shortening is commonly assumed to correlate strongly and positively with muscle work. We found that tibialis anterior muscle fascicle work and muscle-tendon unit work did not accurately predict rFD during submaximal voluntary dorsiflexion contractions. Fascicle shortening during fixed-end reference contractions also potentially induced rFD of 3-5%, which was similar to the rFD following muscle-tendon unit shortening without a preload. A higher number of active muscle fibres during shortening probably increased rFD, which suggests that motor unit recruitment during shortening might predict rFD.

Aponeurosis behaviour during muscular contraction: A narrative review.

There is an abundance of evidence that suggests elastic tendons can enhance both animal and human muscle performance. However, in many terrestrial animals, including humans, a large proportion of the elastic tissue within the muscle-tendon unit is located within the muscle. This continuous elastic sheet, which provides muscle fibre attachment, is known as the aponeurosis. The aponeurosis has a much more complicated shape than the free tendon and it undergoes a more complicated loading regime during contraction, due to its relationship with the bulging muscle fibres, which remain isovolumetric during force production. Muscle contraction may dynamically modulate the stiffness of the aponeurosis at the same active versus passive force, by increasing the intramuscular pressure and transverse forces within the muscle, which may stretch the aponeurosis in width and subsequently reduce its longitudinal strain. Some evidence also suggests that the aponeurosis mechanical properties may be affected by muscle length, which appears to reduce the fascicle strains for a given muscle force at longer muscle lengths. This narrative review outlines the animal and human studies that have investigated aponeurosis behaviour during contraction and discusses how an elastic sheet with a variable stiffness under activation might be beneficial for muscle performance. While it is clear that our understanding of the role of aponeurosis is lacking, it is hoped that further work will attempt to determine how this tissue contributes to power amplification and elastic energy savings during locomotion and potentially uncover how aponeurosis behaviour contributes to injury risk.

Muscle-tendon length and force affect human tibialis anterior central aponeurosis stiffness in vivo.

The factors that drive variable aponeurosis behaviors in active versus passive muscle may alter the longitudinal stiffness of the aponeurosis during contraction, which may change the fascicle strains for a given muscle force. However, it remains unknown whether these factors can drive variable aponeurosis behaviors across different muscle-tendon unit (MTU) lengths and influence the subsequent fascicle strains during contraction. Here, we used ultrasound and elastography techniques to examine in vivo muscle fascicle behavior and central aponeurosis deformations of human tibialis anterior (TA) during force-matched voluntary isometric dorsiflexion contractions at three MTU lengths. We found that increases in TA MTU length increased both the length and apparent longitudinal stiffness of the central aponeurosis at low and moderate muscle forces (P < 0.01). We also found that increased aponeurosis stiffness was directly related to reduced magnitudes of TA muscle fascicle shortening for the same change in force (P < 0.01). The increase in slope and shift to longer overall lengths of the active aponeurosis force-length relationship as MTU length increased was likely due to a combination of parallel lengthening of aponeurosis and greater transverse aponeurosis strains. This study provides in vivo evidence that human aponeurosis stiffness is increased from low to moderate forces and that the fascicle strains for a given muscle force are MTU length dependent. Further testing is warranted to determine whether MTU length-dependent stiffness is a fundamental property of the aponeurosis in pennate muscles and evaluate whether this property can enhance muscle performance.

Elastic ankle muscle-tendon interactions are adjusted to produce acceleration during walking in humans.

Humans and other cursorial mammals have distal leg muscles with high in-series compliance that aid locomotor economy. This muscle-tendon design is considered sub-optimal for injecting net positive mechanical work. However, humans change speed frequently when walking and any acceleration requires net positive ankle work. The present study unveiled how the muscle-tendon interaction of human ankle plantar flexors are adjusted and integrated with body mechanics to provide net positive work during accelerative walking. We found that for accelerative walking, a greater amount of active plantar flexor fascicle shortening early in the stance phase occurred and was transitioned through series elastic tissue stretch and recoil. Reorientation of the leg during early stance for acceleration allowed the ankle and whole soleus muscle-tendon complex to remain isometric while its fascicles actively shortened, stretching in-series elastic tissues for subsequent recoil and net positive joint work. This muscle-tendon behaviour is fundamentally different from constant-speed walking, where the ankle and soleus muscle-tendon complex undergo a period of negative work to store energy in series elastic tissues before subsequent recoil, minimizing net joint work. Muscles with high in-series compliance can therefore contribute to net positive work for accelerative walking and here we show a mechanism for how in human ankle muscles.