Daniel Lambertz

Changes in stiffness induced by hindlimb suspension in rat Achilles tendon

Abstract The aim of this study was to measure the effects of hindlimb suspension on mechanical properties of the rat Achilles tendon. Adult male Wistar rats were randomly assigned to groups to be either suspended, or a control. After 21 days, Achilles tendons were removed for mechanical analysis. Classical tests of tensile performance were made, and mechanical parameters were derived from a stress-strain relationship. The tendons of animals that had been suspended presented values for maximal stress and tangent modulus which were 37.5% (P < 0.01) and 41% (P < 0.01), respectively, lower than the tendons of the control rats. In a similar way, the energy absorption capacity had largely decreased in animals that had been suspended. However, the maximal strain was similar in the two groups. These results showed that hindlimb suspension in rats has an important detrimental effect on mechanical properties of the Achilles tendon. Differences in tendon stiffness obtained here, along with those found by other investigators, encourage the hypothesis that homeostatic responses of soft tissues are due to changes in limb loadings. This study may be useful in providing a better understanding of the adaptation of human skeletal muscle when exposed to microgravity.

Changes in mechanical properties of human plantar flexor muscles in ageing.

Changes in contractile and elastic properties of human plantar flexor muscles in ageing, were investigated in 12 young (19-24 years, YG) and 11 old (61-74 year, OG) men. Maximal isometric and concentric voluntary torques, at several angular velocities, were measured to construct torque-angular velocity relationship. This led to the calculation of an index of maximal shorting velocity (VImax) at low torque. Two methods were then used to calculate musculotendinous (MT, quick-release movements) and musculoarticular (MA, sinusoidal perturbations) stiffness. In both cases, stiffness was linearly related to torque, leading to the calculation of a stiffness index (SI) as the slope of the stiffness-torque relationship: SI(MT) and SI(MA), respectively. MA stiffness under passive conditions (Kp) was also determined. Surface electromyograms were useful to control agonist and antagonist myoelectrical activities. As expected, maximal isometric (P<0.005) and concentric torques (P<0.05) as well as VImax(p<0.05) were lower in OG compared to YG. SI(MT) values were higher for OG compared to YG (P<0.05) leading to a mean difference of 55%, whereas SI(MA) and Kp were not significantly different between the two groups. Thus, older men were weaker and exhibited higher SI(MT) values. These impairments seem to be principally due to muscular atrophy and modifications in both muscle fibre-type distribution and fibre composition, in ageing. Invariance of SI(MA) and Kp would suggest an adaptive mechanism in articular structures to avoid the continuous integration of the ankle joint stiffness by the central nervous system, what may simplify most daily motor tasks.

Age-Related Changes in Twitch Properties of Plantar Flexor Muscles in Prepubertal Children

The twitch of the triceps surae muscle (TS), which characterizes the contractile properties independently of volition, differs in amplitude, but not in time course, when evoked in pre or postpubertal children. The aim of the present study was to compare the TS twitch contractile properties in prepubertal children (7 to 11 y). M-wave and twitch were recorded at rest by supramaximal electrical stimulations of the posterior tibial nerve. Twitches were characterized by peak torque (Pt), contraction time (CT), half relaxation time (HRT), and rate of torque development (dPt/dt). Electromechanical delay (EMD) was quantified with regard to the TS M-wave onset. Pt values increased significantly with the age of the prepubertal children but remained lower than that for adult subjects. CT and HRT values did not change with age. Thus, dPt/dt increased significantly between the 7-year-old and the 11-year-old children but remained significantly lower than that for adults. Despite EMD values decreased with age, they remained significantly higher than those of adult subjects. These results confirmed the link between growth processes and the increase in twitch torque for prepubertal children within a limited range of age. However, the time-course characteristics were not affected by age. The increase in dPt/dt and the decrease in the EMD could be used as indirect indicators of changes in contractile kinetics and in musculo-tendinous stiffness with the age of the prepubertal children. The age-related relationships established by this study will serve as reference values for clinical testing of the TS performances in relation to muscle disease or disuse.

Intrinsic ankle and hopping leg-spring stiffness in distance runners and aerobic gymnasts.

The objective of this study was to examine the contribution of intrinsic ankle stiffness to leg-spring stiffness in high level athletes using various musculotendinous solicitations. 8 aerobic gymnasts (G), 10 long-distance runners (R) and 7 controls (C) were evaluated using quick-release and sinusoidal perturbation tests in order to quantify their respective plantarflexor musculotendinous ( SI(MT)), ankle musculoarticular active ( SI(MA)) and passive ( K(P)) stiffness. Leg-spring stiffness ( K(leg)) was measured during vertical hopping. Runners and gymnasts presented significantly higher SI(MT) values ( P < 0.01) than controls: 60.4 (± 14.1) rad (-1).kg (2/3) for G, 72.7 (± 23.8) rad (-1).kg (2/3) for R and 38.8 (± 6.5) for C. In addition, normalized K(leg) was not significantly different between G, R and C. It appeared that intrinsic ankle stiffness had no influence on leg-spring stiffness. The adaptation of SI (MT) seems to concern specifically the active part of the series elastic component in runners. The results suggested that the number of stretch-shortening cycles during daily practice sessions, rather than their intensity, act as the determinant for this component.

Musculotendinous Stiffness of Triceps Surae, Maximal Rate of Force Development, and Vertical Jump Performance

The relationships between ankle plantar flexor musculotendinous stiffness (MTS) and performance in a countermovement vertical jump (CMJ) and maximal rate of torque development (MRTD) were studied in 27 active men. MTS was studied by means of quick releases at 20 (S0.2), 40 (S0.4), 60 (S0.6), and 80% (S0.8) of maximal voluntary torque (T(MVC)). CMJ was not correlated with strength indices but was positively correlated with MRTD/BM, S 0.4/BM. The slope α 2 and intercept β 2 of the torque-stiffness relationships from 40 to 80% T(MVC) were correlated negatively (α 2) and positively (β 2) with CMJ. The different stiffness indices were not correlated with MRTD. The prediction of CMJ was improved by the introduction of MRTD in multiple regressions between CMJ and stiffness. CMJ was also negatively correlated with indices of curvature of the torque-stiffness relationship. The subjects were subdivided in 3 groups in function of CMJ (groups H, M, and L for high, medium, and low performers, resp.). There was a downward curvature of the torque-stiffness relationship at high torques in group H or M and the torque-stiffness regression was linear in group L only. These results suggested that torque-stiffness relationships with a plateau at high torques are more frequent in the best jumpers.

Reflex and stiffness of the triceps surae for prepubertal children of different ages

Some neuromechanical properties of the triceps surae have presented adaptations with the age of prepubertal children. This has been demonstrated for the musculotendinous stiffness (Lambertz et al. 2003) and for the characteristics of a twitch in response to a maximal electrical stimulation of the motor nerve (Grosset et al. 2005). We have also briefly reported an increase in the amplitude of the reflexes mechanically induced with the age of the children. The objective of the present presentation was to seek correlated changes in reflex amplitude and stiffness parameters

Does musculo-skeletal stiffness in frequency-imposed hopping conditions relate to ankle intrinsic stiffness?

During hopping in place or running, human legs normally behave like compression springs, so that the overall musculo-skeletal (MS) system can be described as a simple spring-mass model (McMahon and Cheng 1990; Farley et al. 1991). The MS stiffness (Kleg) is very important to hopping mechanics because it influences peak ground reaction force, ground contact time, center of mass (COM) displacement and hopping frequency (Farley et al. 1991). Farley and Morgenroth (1999) have demonstrated that in vertical hopping conditions, the modulation of the ankle stiffness is the main mechanism implied in the adjustment of the overall MS stiffness, which is relatively insensitive to changes in knee or hip stiffness. However, recent studies failed to observe a significant influence of ankle intrinsic elastic properties (measured under controlled and initial constant conditions) on joint or MS stiffness measured during hopping in place conditions (McLachlan et al. 2006; Kubo et al. 2007; Rabita et al. 2008). These observations concerned anatomical structures at different hierarchical levels, i.e. tendons (Kubo et al. 2007), musculo-tendinous (MT; Rabita et al. 2008) or musculo-articular (MA; McLachlan et al. 2006; Rabita et al. 2008) systems. However, among these studies, only one has analysed the relationships between intrinsic and global MS stiffness in frequency-imposed hopping tests (McLachlan et al. 2006). McLachlan et al. (2006) did not report influence of the intrinsic ankle MA stiffness on the MS stiffness during vertical hopping. In comparison to MA stiffness, representing the effects of the whole ankle joint, the MT stiffness is a more specific parameter. It represents the series elastic component (SEC). To our best knowledge, no study has analysed the influence of MT stiffness on MS stiffness during frequency-imposed hopping. It was the objective of the present study. 2. Methods

Comparison in ankle stiffness between French international tennis table players and sedentary subjects

It is well known that the performance in stretch-shortening cycle exercises is greatly influenced by the elastic properties of muscle-tendon complexes in the lower limbs. Recent studies have shown that musculotendinous stiffness in well-trained athletes differed from sedentary subjects. For example, we have shown that long and triple jumpers presented an increased musculotendinous stiffness in plantarflexors (not published). However, in long distance runners, while stiffness of the vastus lateralis tendon was shown to be increased (Kubo et al. 2000a), no changes were observed in stress-strain characteristics of the Achilles tendon (Rosager et al. 2002). Moreover, it is well known that an increased musculotendinous (MT) stiffness improves the muscle force transmission to the skeletal structures and thus lead to a reduced stretchshortening cycle time. In the present study, it is hypothesized that high level sportsmen in physical activities which implies displacements with reduced ground contact time (and so reduced stretch-shortening cycle time) present such an increased MT stiffness. The aim of the present study was to compare the MT and the musculoarticular (MA) stiffness in plantarflexors between international tennis-table players and sedentary subjects.