Structure and function of the ankle dorsiflexor muscles in young and moderately active men and women.

The aim was to investigate determinants of ankle dorsiflexor muscle (DF) strength and size in moderately active young men and women (n = 30; age 20-31 yr). Concentric (Con) and eccentric (Ecc) strength were measured isokinetically. Magnetic resonance imaging was used to determine the muscle cross-sectional area (CSA). Multiple biopsies were obtained from the tibialis anterior muscle to determine total numbers, areas (Area I and II) and proportions (Prop I and II) of type I and II fibers, respectively, and relative contents of myosin heavy chain (MHC) isoforms MHC1, MHC2a, and MHC2x. Women had lower Con and Ecc strength (24 and 27%; P < 0.01), smaller CSA (19%; P < 0.001), lower Ecc DF specific strength (strength/CSA) (10%; P < 0.01), and smaller Area I and Area II (21 and 31%; P < 0.01) than men. Prop I, MHC1, estimated total number of fibers, and Con DF specific strength were similar for both sexes. Con DF strength was up to 72% determined by CSA and Prop I, and Ecc DF strength was up to 81% determined by CSA, Prop I, and sex; variables other than CSA explained at most 9%. Body weight and fiber areas explained >50% of the variation in CSA. In conclusion, CSA was the predominant determinant of DF strength, CSA was to a great extent determined by the body weight and the sizes of muscle fibers, and sex differences in Ecc specific strength require further study.     

Relation between the ankle joint angle and the maximum isometric force of the toe flexor muscles

The purpose of this study was to investigate the relationships between the ankle joint angle and maximum isometric force of the toe flexor muscles. Toe flexor strength and electromyography activity of the foot muscles were measured in 12 healthy men at 6 different ankle joint angles with the knee joint at 90 deg in the sitting position. To measure the maximum isometric force of the toe flexor muscles, subjects exerted maximum force on a toe grip dynamometer while the activity levels of the intrinsic and extrinsic plantar muscles were measured. The relation between ankle joint angle and maximum isometric force of the toe flexor muscles was determined, and the isometric force exhibited a peak when the ankle joint was at 70–90 deg on average. From this optimal neutral position, the isometric force gradually decreased and reached its nadir in the plantar flexion position (i.e., 120 deg). The EMG activity of the abductor hallucis (intrinsic plantar muscle) and peroneus longus (extrinsic plantar muscle) did not differ at any ankle joint angles. The results of this study suggest that the force generation of toe flexor muscles is regulated at the ankle joint and that changes in the length-tension relations of the extrinsic plantar muscle could be a reason for the force-generating capacity at the metatarsophalangeal joint when the ankle joint angle is changed.     

Effect of temperature on post-tetanic potentiation in human dorsiflexor muscles

The effect of temperature on post-tetanic potentiation (PTP) has been examined in the muscles of small mammals but not in human skeletal muscle. We examined PTP in the ankle dorsiflexor muscles of 10 young men by evoking twitches before and after a 7-second tetanus at 100 Hz in a control (room air approximately 21 degrees C) condition and after immersion of the lower leg in warm (45 degrees C) and cold (10 degrees C) water baths for 30 min. Exposure to cold decreased tetanus and pre-tetanus twitch peak torque, but increased rise time, half-relaxation time, and muscle action potential (M-wave) amplitude; exposure to warm water had little effect. PTP was smallest in cold exposure 5 s post-tetanus, but persisted throughout the 12 min test period, whereas PTP had subsided by 6 min post-tetanus in control and warm exposures. M-wave amplitude initially decreased after exposure to warm water, recovered, then decreased again by 11 min post-tetanus. In contrast, exposure to cold had no initial effect but did increase the M-wave amplitude during the last half of the 12 min test period, similar to that seen in the control. The greatest immediate decrease in rise time and half-relaxation time was observed in the control; however, by 12 min post-tetanus warm exposure showed the greatest increase in rise time and half-relaxation time above pre-tetanus values. The decrease in the unpotentiated twitch torque with cooling in human dorsiflexors is typical for muscles with a predominance of type I (slow) fibres. The effect of cold on PTP is similar to that seen previously in mammalian muscles with a predominance of type II (fast) fibres, although the underlying mechanism of the cooling effect appears to differ.     

A parametric model of muscle moment arm as a function of joint angle: Application to the dorsiflexor muscle group in mice

A parametric model was developed to describe the relationship between muscle moment arm and joint angle. The model was applied to the dorsiflexor muscle group in mice, for which the moment arm was determined as a function of ankle angle. The moment arm was calculated from the torque measured about the ankle upon application of a known force along the line of action of the dorsiflexor muscle group. The dependence of the dorsiflexor moment arm on ankle angle was modeled as r=R sin(a+Δ), where r is the moment arm calculated from the measured torque and a is the joint angle. A least-squares curve fit yielded values for R, the maximum moment arm, and Δ, the angle at which the maximum moment arm occurs as offset from 90°. Parametric models were developed for two strains of mice, and no differences were found between the moment arms determined for each strain. Values for the maximum moment arm, R, for the two different strains were 0.99 and 1.14 mm, in agreement with the limited data available from the literature. While in some cases moment arm data may be better fitted by a polynomial, use of the parametric model provides a moment arm relationship with meaningful anatomical constants, allowing for the direct comparison of moment arm characteristics between different strains and species.     

Contractile changes in opposing muscles of the human ankle joint with aging

The effects of aging on maximal voluntary strength and on the isometric twitch were determined in the ankle dorsiflexor and plantarflexor muscles of 111 healthy men and women aged 20-100 yr. Men were found to be stronger than women at all ages. In both sexes, the average values for maximum voluntary strength of the dorsiflexors and plantarflexors began to decline in the 6th decade. Although the absolute loss of strength was greater for the plantarflexor muscles, the relative losses were similar in the two muscle groups. During maximum voluntary effort, stimulation of motor nerves produced no additional torque in the majority of elderly men and women, indicating that these subjects remained able to utilize their descending motor pathways for optimal muscle activation. Comparisons of muscle compound action potentials, twitch torques, and muscle cross-sectional areas suggested that a decrease in excitable muscle mass was entirely responsible for the lower strength of the elderly. An additional effect of aging was the gradual prolongation of twitch contraction and half-relaxation times throughout the adult life-span.     

Effects of joint angle and age on ankle dorsi- and plantar-flexor strength.

This study aimed at examining the effects of joint angle and age on the maximal voluntary contraction (MVC) torque, for the agonist and antagonist muscle groups around the ankle, i.e., the dorsi- and plantar-flexors. To this aim, neural and muscular factors were investigated in two groups of healthy men: 11 young (mean age, 24 years) and 18 older (mean age, 78 years). Plantar-flexion (PF) and dorsiflexion (DF) isometric MVC torques were measured in three different ankle joint angles and surface electromyographic activities of the triceps surae and of the tibialis anterior muscles were recorded. The main findings were that the DF-to-PF MVC torque ratio varied with joint angle and age, indicating that aging affected at different rates the two muscle groups: this ratio was always higher in older adults because of the PF strength decline with aging. Furthermore, the DF MVC torque-angle relationship appeared to be especially explained by neural factors, whereas the relationship in PF seemed to be mainly due to muscular parameters. These relationships would not be a discriminating factor between the two age groups. As a consequence, measurements at one ankle joint angle, whatever the angle, are thus enough to examine the differences within age groups and to perform a rapid assessment of the imbalance at the ankle joint.     

Passive and active components of the internal moment developed about the ankle joint during human ambulation

The internal moment developed about a joint during a functional activity is the result of contraction of muscles and the visco-elastic properties of the joint and its surrounding soft tissues. In this study, the contribution of each one of these mechanisms to the total internal moment developed about the ankle joint during human level walking was assessed. The results indicate that during normal level walking the internal moment about the ankle is mainly due to contraction of muscles surrounding the joint. The contribution of the passive component was found to be negligible. These results, however, were found to be different for the pathological case tested. The results indicated that in a subject with a mild equinus ankle deformity, a substantial portion (21%) of the total internal moment was contributed by the passive resistance of the joint and its surrounding structures.     

Unloaded shortening velocity of voluntarily and electrically activated human dorsiflexor muscles in vivo

We have previously shown that unloaded shortening velocity (V ₀) of human plantar flexors can be determined in vivo, by applying the “slack test” to submaximal voluntary contractions (J Physiol 567:1047–1056, 2005). In the present study, to investigate the effect of motor unit recruitment pattern on V ₀ of human muscle, we modified the slack test and applied this method to both voluntary and electrically elicited contractions of dorsiflexors. A series of quick releases (i.e., rapid ankle joint rotation driven by an electrical dynamometer) was applied to voluntarily activated dorsiflexor muscles at three different contraction intensities (15, 50, and 85% of maximal voluntary contraction; MVC). The quick-release trials were also performed on electrically activated dorsiflexor muscles, in which three stimulus conditions were used: submaximal (equal to 15%MVC) 50-Hz stimulation, supramaximal 50-Hz stimulation, and supramaximal 20-Hz stimulation. Modification of the slack test in vivo resulted in good reproducibility of V ₀, with an intraclass correlation coefficient of 0.87 (95% confidence interval: 0.68–0.95). Regression analysis showed that V ₀ of voluntarily activated dorsiflexor muscles significantly increased with increasing contraction intensity (R ² = 0.52, P<0.001). By contrast, V ₀ of electrically activated dorsiflexor muscles remained unchanged (R ²<0.001, P = 0.98) among three different stimulus conditions showing a large variation of tetanic torque. These results suggest that the recruitment pattern of motor units, which is quite different between voluntary and electrically elicited contractions, plays an important role in determining shortening velocity of human skeletal muscle in vivo.     

Estimation of passive ankle joint moment during standing and walking

This study estimated the passive ankle joint moment during standing and walking initiation and its contribution to total ankle joint moment during that time. The decrement of passive joint moment due to muscle fascicle shortening upon contraction was taken into account. Muscle fascicle length in the medial gastrocnemius, which was assumed to represent muscle fascicle length in plantarflexors, was measured using ultrasonography during standing, walking initiation, and cyclical slow passive ankle joint motion. Total ankle joint moment during standing and walking initiation was calculated from ground reaction forces and joint kinematics. Passive ankle joint moment during the cyclical ankle joint motion was measured via a dynamometer. Passive ankle joint moment during standing and at the time (Tp) when the MG muscle-tendon complex length was longest in the stance phase during walking initiation were 2.3 and 5.4 Nm, respectively. The muscle fascicle shortened by 2.9 mm during standing compared with the length at rest, which decreased the contribution of passive joint moment from 19.9% to 17.4%. The muscle fascicle shortened by 4.3 mm at Tp compared with the length at rest, which decreased the contribution of passive joint moment from 8.0% to 5.8%. These findings suggest that (a) passive ankle joint moment plays an important role during standing and walking initiation even in view of the decrement of passive joint moment due to muscle fascicle shortening upon muscle contraction, and (b) muscle fascicle shortening upon muscle contraction must be taken into account when estimating passive joint moment during movements.     

A geometric model of the human ankle joint.

A two-dimensional four-bar linkage model of the ankle joint is formulated to describe dorsi/plantarflexion in unloaded conditions as observed in passive tests on ankle complex specimens. The experiments demonstrated that the human ankle joint complex behaves as a single-degree-of-freedom system during passive motion, with a moving axis of rotation. The bulk of the movement occurred at the level of the ankle. Fibres within the calcaneofibular and tibiocalcaneal ligaments remained approximately isometric. The experiments showed that passive kinematics of the ankle complex is governed only by the articular surfaces and the ligaments. It was deduced that the ankle is a single-degree-of-freedom mechanism where mobility is allowed by the sliding of the articular surfaces upon each other and the isometric rotation of two ligaments about their origins and insertions, without tissue deformation. The linkage model is formed by the tibia/fibula and talus/calcaneus bone segments and by the calcaneofibular and tibiocalcaneal ligament segments. The model predicts the path of calcaneus motion, ligament orientations, instantaneous axis of rotation, and conjugate talus surface profile as observed in the experiments. Many features of ankle kinematics such as rolling and multiaxial rotation are elucidated. The geometrical model is a necessary preliminary step to the study of ankle joint stability in response to applied loads and can be used to predict the effects of changes to the original geometry of the intact joint. Careful reconstruction of the original geometry of the ligaments is necessary after injury or during total ankle replacement.     

A non-invasive protocol to determine the personalized moment arms of knee and ankle muscles

One difficulty that comes with predicting muscular forces is the accuracy of experimental data, particularly the assessment of muscle moment arms with respect to each joint rotation axis. This paper presents a non-invasive experimental protocol to obtain the personalized muscle moment arms with respect to the ankle and knee joints. A specific pointer is used by a specialist of lower limb anatomy in order to define the local portion of the line of action of the different muscles closed to the joint on the standing subject's lower limb. With this pointer, the three-dimensional coordinates of several points representing the line of action of 12 ankle and knee muscles are collected by a Motion Analysis system. The collection is done five times by the same operator and one time by two different operators. From this data, the intra and inter operator repeatabilities are tested. Relative (ICC) and absolute (SEM) reliabilities are determined in order to evaluate the intra operator repeatability of this non-invasive protocol. The ICC values obtained are higher than 0.91 for 10 among 12 muscles. The intra operator repeatability is thus confirmed. From the records realized by the two operators, the differences are negligible. Thus, the inter operator repeatability is also confirmed. The moments arms obtained using this non-invasive experimental protocol are compared with those calculated from origin and insertion points reported in the literature, according to the work of Whites, Pierrynowskis and Kepples, respectively. The estimations obtained using the non-invasive experimental protocol are found, for some muscles, more realistic than those calculated using the literature data and are always coherent with the role of the muscles described in anatomical books.     

Effect of ankle joint position and electrode placement on the estimation of the antagonistic moment during maximal plantarflexion.

During maximal efforts, antagonistic activity can significantly influence the joint moment. During maximal voluntary "isometric" contractions, certain joint rotation can not be avoided. This can influence the estimation of the antagonistic moment from the EMG activity. Our study aimed to quantify the influence on the calculated agonistic moment produced during maximal voluntary isometric plantarflexions (a) when estimating antagonistic moments at different ankle angles and (b) when placing the EMG electrodes at different portions over the m. tibialis anterior. Ten subjects performed maximal voluntary isometric plantarflexions at 90 degrees ankle angle. In order to estimate the antagonistic moment, submaximal isometric dorsiflexions were performed at various ankle angles. Moment and EMG signals from mm. triceps surae and tibialis anterior were measured. The RMS differences between plantarflexors moment calculated considering the antagonistic cocontraction estimated at the same ankle angle at which the maximal plantarflexion moment was achieved and at different ankle angles ranged from 0.10 to 2.94 Nm. The location of the electrodes led to greater RMS differences (2.35-5.18 Nm). In conclusion, an angle 10 degrees greater than the initial plantarflexion angle is enough to minimize the effect of the change in length of the m. tibialis anterior during the plantarflexion on the estimation of the plantarflexors moment. The localisation of the electrodes over the m. tibialis anterior can influence the estimation of its cocontraction during maximal plantarflexion efforts.     

Mathematical model of the human ankle joint

It has been suspected that the mechanical environment in which a particular joint functions has an effect on the initiation or progression of degenerative joint disease. The objective of this study is to define the mechanical environment of the ankle joint, specifically, the contact areas and pressure distributions, through the development and analysis of a simplified mathematical model. Since the state of pressure across articular surfaces during function is influenced by joint incongruity, cartilage thickness profile and the geometry of the opposing surfaces, these factors have been incorporated into the model formulation. Mathematical analysis of the model has resulted in pressure distributions in both the anterior-posterior and medial-lateral directions and contact area growth plots which correlate well with observed ankle contact patterns obtained from in vitro investigations. The significance of joint incongruity to these pressure distributions and to the relative immunity of the ankle joint to primary osteoarthritis is discussed.     

Influence of the moment exerted by the athlete on the pole in pole-vaulting performance

Current studies on pole-vaulting focus mostly on energy transfer data [Ekevad, M., Lundberg, B., 1995. Simulation of "smart" pole vaulting. Journal of Biomechanics 28, 1079-1090; Ekevad, M., Lundberg, B., 1997. Influence of pole length and stiffness on the energy conversion in pole-vaulting. Journal of Biomechanics, 30, 259-264; Linthorne, N.P., 2000. Energy loss in the pole vault take-off and the advantage of the flexible pole. Sports Engineering 3, 205-218; Schade, F., Arampatzis, A., Bruggemann, G.P., 2006. Reproducibility of energy parameters in the pole vault. Journal of Biomechanics 39, 146-147.] and often fail to take into account the actions exerted on the pole [Arampatzis, Schade, Bruggemann, 2004. Effect of the pole-human body interaction on pole-vaulting performance. Journal of Biomechanics 37, 1353-1360]. The present study integrates the 3D kinematics data of the athlete but also the actions measured at the end of the pole in the planting box and on the track during the last stride before take-off. It proposes a mechanical model allowing determination of the pole-vaulter's actions on the pole. The model is based on a global mechanical approach. The pole-vaulter's action on his upper and lower hand is concentrated on one middle point to solve the dynamics problem. The model was applied to seven experienced pole-vaulters. The force and the moment exerted on the pole by the pole-vaulter during the last stride before take off and during jump stage, were calculated. This analysis of the compressive force and bending moment for seven pole-vaulters helps to highlight the impact of the moment in the performance. The conclusion is confirmed by an additional comparative study carried out on two pole-vaulters, with comparable morphologies and performing with the same pole.     

The passive elastic moment at the knee and its influence on human gait

The elastic component of the passive moment at the knee was measured in situ. The force needed to manually range the knee from approximately 90 degrees of flexion to full extension was measured. Hip and ankle angle were held fixed. The passive knee moment, computed from the force and knee angle data, was compared to the total knee moment required for normal gait. This comparison suggested that the passive moment can contribute a significant portion of the total joint moment during some phases of the gait cycle.     

Estimates of forces exerted by the jaw muscles of some reptiles

Masses and muscle fibre lengths have been determined for the jaw muscles of Chrysemys (Chelonia). Varanus (Lacertilia) and Caiman (Crocodilia). Hence the force that each muscle can exert has been estimated. The position and direction of the line of action of each force have also been determined. The results are used to calculate the magnitude and direction of the bite forces that each reptile could exert, at particular positions in its mouth, and the magnitude and direction of the reactions at the jaw articulation.     

Foot and ankle forces during an automobile collision: the influence of muscles

Muscles have a potentially important effect on lower extremity injuries during an automobile collision. Computational modeling can be a powerful tool to predict these effects and develop protective interventions. Our purpose was to determine how muscles influence peak foot and ankle forces during an automobile collision. A 2-D bilateral musculoskeletal model was constructed with seven segments. Six muscle groups were included in the right lower extremity, each represented by a Hill muscle model. Vehicle deceleration data were applied as input and the resulting movements were simulated. Three models were evaluated: no muscles (NM), minimal muscle activation at a brake pedal force of 400 N (MN), and maximal muscle activation to simulate panic braking (MX). Muscle activation always resulted in large increases in peak joint force. Peak ankle joint force was greatest for MX (10120 N), yet this model also had the lowest peak rearfoot force (629 N). Peak force on the Achilles tendon was 4.5 times greater, during MX (6446 N) compared to MN (1430 N). We conclude that (1). external and internal forces are dependent on muscles, (2). muscle activation level could exacerbate axial loading injuries, (3). external and internal forces can be inversely related once muscle properties are included.