Muscle force depends on the amount of transversal muscle loading

Skeletal muscles are embedded in an environment of other muscles, connective tissue, and bones, which may transfer transversal forces to the muscle tissue, thereby compressing it. In a recent study we demonstrated that transversal loading of a muscle with 1.3 N cm⁻² reduces maximum isometric force (F_im) and rate of force development by approximately 5% and 25%, respectively. The aim of the present study was to examine the influence of increasing transversal muscle loading on contraction dynamics.     

A hill-type muscle model expansion accounting for effects of varying transverse muscle load

Recent studies demonstrated that uniaxial transverse loading (F_G) of a rat gastrocnemius medialis muscle resulted in a considerable reduction of maximum isometric muscle force (ΔF_im). A hill-type muscle model assuming an identical gearing G between both ΔF_im and F_G as well as lifting height of the load (Δh) and longitudinal muscle shortening (Δl_CC) reproduced experimental data for a single load.Here we tested if this model is able to reproduce experimental changes in ΔF_im and Δh for increasing transverse loads (0.64 N, 1.13 N, 1.62 N, 2.11 N, 2.60 N). Three different gearing ratios were tested: (I) constant G_c representing the idea of a muscle specific gearing parameter (e.g. predefined by the muscle geometry), (II) G_exp determined in experiments with varying transverse load, and (III) G_f that reproduced experimental ΔF_im for each transverse load.Simulations using G_c overestimated ΔF_im (up to 59%) and Δh (up to 136%) for increasing load. Although the model assumption (equal G for forces and length changes) held for the three lower loads using G_exp and G_f, simulations resulted in underestimation of ΔF_im by 38% and overestimation of Δh by 58% for the largest load, respectively. To simultaneously reproduce experimental ΔF_im and Δh for the two larger loads, it was necessary to reduce F_im by 1.9% and 4.6%, respectively. The model seems applicable to account for effects of muscle deformation within a range of transverse loading when using a linear load-dependent function for G.     

A novel approach to evaluate abdominal coactivities for optimal spinal stability and compression force in lifting

A novel optimisation algorithm is developed to predict coactivity of abdominal muscles while accounting for both trunk stability via the lowest buckling load (P _cr) and tissue loading via the axial compression (F _c). A nonlinear multi-joint kinematics-driven model of the spine along with the response surface methodology are used to establish empirical expressions for P _cr and F _c as functions of abdominal muscle coactivities and external load magnitude during lifting in upright standing posture. A two-component objective function involving F _c and P _cr is defined. Due to opposite demands, abdominal coactivities that simultaneously maximise P _cr and minimise F _c cannot exist. Optimal solutions are thus identified while striking a compromise between requirements on trunk stability and risk of injury. The oblique muscles are found most efficient as compared with the rectus abdominus. Results indicate that higher abdominal coactivities should be avoided during heavier lifting tasks as they reduce stability margin while increasing spinal loads.     

Architectural Properties of Sloth Forelimb Muscles (Pilosa: Bradypodidae)

Tree sloths have reduced skeletal muscle mass, and yet they are able to perform suspensory behaviors that require both strength and fatigue resistance to suspend their body mass for extended periods of time. The muscle architecture of sloths is hypothesized to be modified in ways that will enhance force production to compensate for this reduction in limb muscle mass. Our objective is to test this hypothesis by quantifying architecture properties in the forelimb musculature of the brown-throated three-toed sloth (Bradypus variegatus: N = 4). We evaluated architecture from 52 forelimb muscles by measuring muscle moment arm (r _m), muscle mass (MM), belly length (ML), fascicle length (L_F), pennation angle (θ), and physiological cross-sectional area (PCSA), and these metrics were used to estimate isometric force, joint torque, and power. Overall, the musculature becomes progressively more pennate from the extrinsic to intrinsic regions of the forelimb, and the flexors are more well developed than the extensors as predicted. However, most muscles are indicative of a mechanical design for fast joint rotational velocity instead of large joint torque (i.e., strength), although certain large, parallel-fibered shoulder (e.g., m. latissimus dorsi) and elbow (e.g., m. brachioradialis) flexors are capable of producing appreciable torques by having elongated moment arms. This type of functional tradeoff between joint rotational velocity and mechanical advantage is further exemplified by muscle gearing in Bradypus that pairs synergistic muscles with opposing L_F/r _m ratios in each functional group. These properties are suggested to facilitate the slow, controlled movements in sloths. In addition, the carpal/digital flexors have variable architectural properties, but their collective PCSA and joint torque indicates the capability for maintaining grip force and carpal stability while distributing load from the manus to the shoulder. The observed specializations provide a basis for understanding sustained suspension in sloths.     

Redistribution of cell membrane probes following contraction-induced injury of mouse soleus muscle

Our aim was to study how mouse skeletal muscle membranes are altered by eccentric and isometric contractions. A fluorescent dialkyl carbocyanine dye (DiOC₁₈(3)) was used to label muscle membranes, and the membranes accessible to the dye were observed by confocal laser scanning microscopy. Experiments were done on normal mouse soleus muscles and soleus muscles injured by 20 eccentric or 20 isometric contractions. Longitudinal optical sections of control muscle fibers revealed DiOC₁₈(3) staining of the plasmalemma and regularly spaced transverse bands corresponding in location to the T-tubular system. Transverse optical sections showed an extensive reticular network with the DiOC₁₈(3) staining. Injured muscle fibers showed distinctively different staining patterns in both longitudinal and transverse optical sections. Longitudinal optical sections of the injured fibers revealed staining in a longitudinally-oriented pattern. No correlations were found between the abnormal DiOC₁₈(3) staining and the reductions in maximal isometric tetanic force or release of lactate dehydrogenase (P0.32). Additionally, no difference in the extent of abnormal staining was found between muscles performing eccentric contractions and those performing the less damaging isometric contractions. However, many fibers in muscles injured by eccentric contractions showed swollen regions with marked loss of membrane integrity and an elevated free cytosolic calcium concentration as observed in Fluo-3 images. In conclusion, a loss of cell membrane integrity results from contractile activity, enabling DiOC₁₈(3) staining of internal membranes. The resulting staining pattern is striking and fibers with damaged cell membranes are easily distinguished from uninjured ones.     

Injectability Evaluation: An Open Issue

The current work aimed to propose a system of scoring to rationalize and support the selection of the optimal diameter and length of needles. Four formulations at different viscosity and needles ranging from 21 to 26 G and length ranging from 16 to 40 mm were used. Plunger-stopper breakloose force, maximum force (F _max), and dynamic glide force were measured by a texture analyzer at the crosshead speed of 1 mm/s. Testing was carried out into air or human subcutaneous tissue. The manual injectability of the highest viscosity product was assessed by ten evaluators. The comparison of the panel test score and the quantitative measurements of the forces permitted to score a given needle-syringe-formulation system keeping also in consideration the pressure created in the subcutaneous space and muscles at the injection site. In particular, the following relationship was drawn: at the F _max up to 250 mPa, the injection was practically impossible; at F _max ranging from 160 to 250 mPa, the injection was very difficult; at F _max in the 125–160 mPa range, the injection was feasible, though with some difficulty; when the values of F _max were lower 125 mPa, the injection went smoothly. On the basis of these preliminary data, a system of scoring the needle-syringe-formulation system is proposed to rationalize and support the selection of the optimal diameter and length of needles, keeping also in consideration the pressure created in the subcutaneous space and muscles at the injection site.     

Mechanical similarity across ontogeny of digging muscles in an Australian marsupial (Isoodon fusciventer)

Many mammals dig, either during foraging to access subsurface food resources, or in creating burrows for shelter. Digging requires large forces produced by muscles and transmitted to the soil via the skeletal system; thus fossorial mammals tend to have characteristic modifications of the musculoskeletal system that reflect their digging ability. Bandicoots (Marsupialia: Peramelidae) scratch-dig mainly to source food, searching for subterranean food items including invertebrates, seeds, and fungi. They have musculoskeletal features for digging, including shortened, robust forelimb bones, large muscles, and enlarged muscle attachment areas. Here, we compared changes in the ontogenetic development of muscles associated with digging in the Quenda (Isoodon fusciventer). We measured muscle mass (m _m), pennation angle, and fiber length (FL) to calculate physiological cross-sectional area (PCSA; a proxy of maximum isometric force) as well as estimate the maximum isometric force (Fmax) for 34 individuals ranging in body size from 124 to 2,390 g. Males grow larger than females in this bandicoot species, however, we found negligible sex differences in mass-specific m _m, PCSA or FL for our sample. Majority of the forelimb muscles PCSA showed a positive allometric relationship with total body mass, while m _m and FL in the majority of forelimb muscles showed isometry. Mechanical similarity was tested, and two thirds of forelimb muscles maximum isometric forces (Fmax) scaled with isometry; therefore the forelimb is primarily mechanical similar throughout ontogeny. PCSA showed a significant difference between scaling slopes between main movers in the power stroke, and main movers of the recovery stroke of scratch-digging. This suggests that some forelimb muscles grow with positive allometry, specially these associated with the power stroke of digging. Intraspecific variation in PCSA is rarely considered in the literature, and thus this is an important study quantifying changes in muscle architectural properties with growth in a mammalian model of scratch-digging.     

Cupiennius salei: biomechanical properties of the tibia–metatarsus joint and its flexing muscles

Hunting spiders are well adapted to fast locomotion. Space saving hydraulic leg extension enables leg segments, which consist almost soley of flexor muscles. As a result, the muscle cross sectional area is high despite slender legs. Considering these morphological features in context with the spider’s segmented C-shaped legs, these specifics might influence the spider’s muscle properties. Moreover, these properties have to be known for modeling of spider locomotion. Cupiennius salei (n = 5) were fixed in a metal frame allowing exclusive flexion of the tibia–metatarsus joint of the second leg (counted from anterior). Its flexing muscles were stimulated supramaximally using needle electrodes. Accounting for the joint geometry, the force–length and the force–velocity relationships were determined. The spider muscles produce 0.07 N cm maximum isometric moment (corresponding to 25 N/cm² maximum stress) at 160° tibia–metatarsus joint angle. When overextended to the dorsal limit at approximately 200°, the maximum isometric moments decrease to 72%, and, when flexed to the ventral hinge stop at 85°, they drop to 11%. The force–velocity relation shows the typical hyperbolic shape. The mean maximum shortening velocity is 5.7 optimum muscle lengths per second and the mean curvature (a/F _iso) of the Hill-function is 0.34. The spider muscle’s properties which were determined are similar to those of other species acting as motors during locomotion (working range, curvature of Hill hyperbola, peak power at the preferred speeds), but they are relatively slow. In conjunction with the low mechanical advantage (muscle lever/load arm), the arrangement of three considerably actuated joints in series may nonetheless enable high locomotion velocities.     

Characterisation of a phenomenological model for commercial pneumatic muscle actuators

This study focuses on the parameter characterisation of a three-element phenomenological model for commercially available pneumatic muscle actuators (PMAs). This model consists of a spring, damping and contractile element arranged in parallel. Data collected from static loading, contraction and relaxation experiments were fitted to theoretical solutions of the governing equation for the three-element model resulting in prediction profiles for the spring, damping and contractile force coefficient. For the spring coefficient, K N/mm, the following relationships were found: K = 32.7 ? 0.0321P for 150 ≤ P ≤ 314 kPa and K = 17 + 0.0179P for 314 ≤ P ≤ 550 kPa. For the damping coefficient, B Ns/mm, the following relationship was found during contraction: B = 2.90 for 150 ≤ P ≤ 550 kPa. During relaxation, B = 1.57 for 150 ≤ P ≤ 372 kPa and B = 0.311 + 0.00338P for 372 ≤ P ≤ 550. The following relationship for the contractile force coefficient, F _ce N, was also determined: F _ce = 2.91P+44.6 for 150 ≤ P ≤ 550 kPa. The model was then validated by reasonably predicting the response of the PMA to a triangular wave input in pressure under a constant load on a dynamic test station.     

Relationships between range of motion,Lo, and passive force in five strap-like muscles of the feline hind limb

The relationships between range of motion, optimal length for force production (l_o), and passive force provide useful insights into the structure and function of muscles but are unknown for most individual muscles. We measured these values and examined their relationships in five strap-like muscles of the cat hind limb: caudofemoralis, semitendinosus, sartorius anterior, tenuissimus, and biceps femoris anterior. The range of motion relative to l_o was found to vary significantly between different muscles and even between different specimens of the same muscle. The passive force-length (FL) curve was found to be correlated with both l_o and l_max (maximal in situ muscle length) but was correlated more strongly with l_max. The mean passive force produced by these muscles at l_max was less than 7% of estimated maximal isometric force, suggesting that passive force may not be important in these muscles during normal activation patterns. The variance in passive FL curves between specimens of the same muscle was found to be significantly lower when length was scaled by l_max as opposed to l_o. These results suggest that l_max may provide a more useful scaling factor for generic models of muscle. However, the passive length-tension properties of mammalian muscle appear to reflect a complex mix of structures at both the myofilament and connective tissue levels that may differ depending on muscle-fiber architecture and perhaps on the history of trophic influences on a particular specimen. © 1996 Wiley-Liss, Inc.     

Effects of oxygen fraction in inspired air on force production and electromyogram activity during ergometer rowing

Six male rowers rowed maximally for 2500 m in ergometer tests during normoxia (fractional concentration of oxygen in inspired air, F _IO₂ 0.209), in hyperoxia (F _IO₂ 0.622) and in hypoxia (F _IO₂ 0.158) in a randomized single-blind fashion. Oxygen consumption (V˙O₂), force production of strokes as well as integrated electromyographs (iEMG) and mean power frequency (MPF) from seven muscles were measured in 500-m intervals. The iEMG signals from individual muscles were summed to represent overall electrical activity of these muscles (sum-iEMG). Maximal force of a stroke (F _max) decreased from the 100% pre-exercise maximal value to 67 (SD 12)%, 63 (SD 15)% and 76 (SD 13)% (P<0.05 to normoxia, ANOVA) and impulse to 78 (SD 4)%, 75 (SD 14)% and 84 (SD 7)% (P<0.05) in normoxia, hypoxia and hyperoxia, respectively. A strong correlation between F _max and V˙O₂ was found in normoxia but not in hypoxia and hyperoxia. The mean sum-iEMG tended to be lower (P<0.05) in hypoxia than in normoxia but hyperoxia had no significant effect on it. In general, F _IO₂ did not affect MPF of individual muscles. In conclusion, it was found that force output during ergometer rowing was impaired during hypoxia and improved during hyperoxia when compared with normoxia. Moreover, the changes in force output were only partly accompanied by changes in muscle electrical activity as sum-iEMG was affected by hypoxic but not by hyperoxic gas. The lack of a significant correlation between F _max and V˙O₂ during hypoxia and hyperoxia may suggest a partial uncoupling of these processes and the existence of other limiting factors in addition to V˙O₂. Accepted: 2 June 1997     

Investigation of the Peculiarities of Two-joint Muscles Using a 3 DOF Model of the Human Upper Limb in the Sagittal Plane: an Optimization Approach

Abstract

The purpose of this paper is an investigation of the peculiarities of biarticular muscles by means of modelling and analytical solution of the indeterminate problem. The basic model includes 10 muscle elements performing flexio/extensio in the shoulder, elbow and wrist. Four of them are biarticular muscles. Two modifications of the model with only monoarticular muscles are developed. The indeterminate problem is solved analytically using the objective criterion σc_iF_i ² where F( is the module of the i-th muscle force and Cj is a weight coefficient. The predicted muscle forces, joint reactions and moments are compared in-between the basic model and its two modifications for different joint angles, external loading and weight coefficients. The main conclusions are: it is impossible to formulate strict advantages of the biarticular muscles under quasistatical conditions, their peculiarities depend on limb position, external loading and neural control; in general, monoarticular muscles are more powerful than biarticular ones; the biarticular muscles fine tune muscle coordination, their control is more precise and graceful; the presence of biarticular muscles leads to an increase of the joint reactions and moments, thus stabilizing the limb.     

Comparison of surface electromyographic (sEMG) activity of submental muscles between the head lift and tongue press exercises as a therapeutic exercise for pharyngeal dysphagia

Objective: The present study compared surface electromyographic (sEMG) activity obtained from the submental muscle group for a tongue press and a head lift exercise as potential therapeutic exercises for dysphagic elderly. Materials and methods: Fifty‐three healthy volunteers with a mean age of 35.3 participated in this study. Subjects were required to perform an isometric task, pressing their tongue against the hard palate, and an isotonic task requiring sustained lingual force against the hard palate. Pressure sensors were used to measure the amount of lingual pressure against the hard palate. Submental sEMG data from these tasks were compared with those obtained from the isometric and isotonic aspects of a head lift exercise. Results: No sEMG differences were identified between the isometric tongue press task and head lift exercise. Isotonic tongue press exercises resulted in significantly higher maximum and mean sEMG values compared with the isotonic head lift exercise (p < 0.05). The submental sEMG activity from the tongue press exercise was equal (isometric) to, or greater (isotonic) than comparable muscle activation obtained during the head lift exercise. Conclusions: The tongue press exercise may be less strenuous than the head lift exercise while achieving the same therapeutic effect.     

Influence of muscle activity on the forces in the femur: An in vivo study

Experiments were performed on two patients with custom-made instrumented massive proximal femoral prostheses implanted after tumour resection. In vivo axial forces transmitted along the prostheses were telemetered during level walking, single- and double-leg stance, and isometric exercises of the hip muscles. These activities varied the lever arms available to the external loads: minimum for double-leg stance and maximum for hip isometric exercises. Kinematic, force plate, EMG and telemetered force data were recorded simultaneously. The force magnification ration (FMR; the ratio of the telemetered axial force to the external force) was calculated. The FMRs ranged from 1.3 (during double-leg stance) to 29.8 (during abductors test), indicating that a major part of the axial force in the long bones is a response to muscle activity, the strength of which depends on the lever arms available to the external loads. From these results, it was shown that the bulk of the bending moment along limbs is transmitted by a combination of tensile forces in muscles and compressive forces in bones, so moments transmitted by the bones are smaller than the limb moments. It was concluded that appropriate simulation of muscle forces is important in experimental or theoretical studies of load transmission along bones.     

Mechanical properties of aponeurosis and tendon of the cat soleus muscle during whole-muscle isometric contractions

Recent studies have suggested that the mechanical properties of aponeurosis are not similar to the properties of external tendon. In the present study, the lengths of aponeurosis, tendon, and muscle fascicles were recorded individually, using piezoelectric crystals attached to the surface of each structure during isometric contractions in the cat soleus muscle. We used a surgical microscope to observe the surface of the aponeurosis, which revealed a confounding effect on measures of aponeurosis length due to sliding of a thin layer of epimysium over the proximal aponeurosis. After correcting for this artifact, the stiffness computed for aponeurosis was similar to tendon, with both increasing from around 8 F₀/L_c (F₀ is maximum isometric force and L_c is tissue length) at 0.1 F₀ to 30 F₀/L_c at forces greater than 0.4 F₀. At low force levels only (0.1 F₀), aponeurotic stiffness increased somewhat as fascicle length increased. There was a gradient in the thickness of the aponeurosis along its length: its thickness was minimal at the proximal end and maximal at the distal end, where it converged to form the external tendon. This gradient in thickness appeared to match the gradient in tension transmitted along this structure. We conclude that the specific mechanical properties of aponeurosis are similar to those of tendon. © 1995 Wiley-Liss, Inc.     

Using individual-muscle specific instead of across-muscle mean data halves muscle simulation error

Hill-type parameter values measured in experiments on single muscles show large across-muscle variation. Using individual-muscle specific values instead of the more standard approach of across-muscle means might therefore improve muscle model performance. We show here that using mean values increased simulation normalized RMS error in all tested motor nerve stimulation paradigms in both isotonic and isometric conditions, doubling mean simulation error from 9 to 18 (different at p?<?0.0001). These data suggest muscle-specific measurement of Hill-type model parameters is necessary in work requiring highly accurate muscle model construction. Maximum muscle force (F _max) showed large (fourfold) across-muscle variation. To test the role of F _max in model performance we compared the errors of models using mean F _max and muscle-specific values for the other model parameters, and models using muscle-specific F _max values and mean values for the other model parameters. Using muscle-specific F _max values did not improve model performance compared to using mean values for all parameters, but using muscle-specific values for all parameters but F _max did (to an error of 14, different from muscle-specific, mean all parameters, and mean only F _max errors at p?≤ 0.014). Significantly improving model performance thus required muscle-specific values for at least a subset of parameters other than F _max, and best performance required muscle-specific values for this subset and F _max. Detailed consideration of model performance suggested that remaining model error likely stemmed from activation of both fast and slow motor neurons in our experiments and inadequate specification of model activation dynamics.     

A comparison of models explaining muscle activation patterns for isometric contractions

One of the main problems in motor-control research is the muscle load sharing problem, which originates from the fact that the number of muscles spanning a joint exceeds the number of degrees of freedom of the joint. As a consequence, many different possibilities exist for the activation of muscles in order to produce a desired joint torque. Several models describing muscle activation have been hypothesized over the last few decades to solve this problem. This study presents theoretical analyses of the various models and compares the predictions of these models with new data on muscle activation patterns for isometric contractions in various directions. None of the existing models fitted the experimental data in all aspects. The best fit was obtained by models based on minimization of the squared sum of muscle forces (∑ _m φ² _m , which is almost equivalent to the Moore-Penrose pseudo-inverse solution), muscle stress σ (∑ _m σ _m ²) or muscle activation α (∑ _m α _m ²). Since muscle activation patterns are different for isometric contractions and for movements, it could well be that other models or optimization criteria are better suited to describe muscle activation patterns for movements. The results of our simulations demonstrate that the predicted muscle activation patterns do not depend critically on the parameters in the model. This may explain why muscle activation patterns are highly stereotyped for all subjects irrespective of differences between subjects in many neuro-anatomical aspects, such as, for example, in the physiological cross-sectional area of muscle. Received: 24 September 1998 / Accepted in revised form: 1 March 1999     

31P nuclear magnetic resonance study on changes in phosphocreatine and the intracellular pH in rat skeletal muscle during exercise at various inspired oxygen contents

We measured ATP, phosphocreatine (PCr), inorganic phosphate (P_i), and the intracellular pH in rat hindlimb muscles during submaximal isometric exercise with various O₂ deliveries using³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) to evaluate changes in energy metabolism in relation to O₂ availability. Delivery of O₂ to muscles was altered by controlling the fractional concentration of inspired oxygen (F _IO₂) at 0.50, 0.28, 0.21, 0.11 and 0.08 with monitoring partial pressure of oxygen and carbon dioxide, and bicarbonate at the femoral artery. The steady-state ratio of PCr : (PCr + P_i) during exercise decreased as a function ofF _IO₂ even at 0.21. Significant acidification of the intracellular pH during exercise occurred at 0.08F _IO₂. Change in the PCr : (PCr + P_i) ratio demonstrated that the oxidative capacity, i.e. the maximal rate of the oxidative phosphorylation reaction, in muscle was not limited by O₂ delivery at 0.50F _IO₂, but was significantly limited at 0.21F _IO₂ or below. Change in the intracellular pH at 0.08F _IO₂ could be interpreted as an increase in lactate, suggesting activation of glycolysis. Correlation between the PCr : (PCr + P_i) ratio and the intracellular pH revealed the existence of a critical PCr : (PCr + P_i) ratio and pH for glycolysis activation at around 0.4 and 6.7, respectively.     

Subject-specific thumb muscle activity during functional tasks of daily life

BackgroundThe trapeziometacarpal joint is subjected to high compressive forces during powerful pinch and grasp tasks due to muscle loading. In addition, muscle contraction is important for stability of the joint. The aim of the present study is to explore if different muscle activation patterns can be found between three functional tasks.MethodsIsometric forces and fine-wire electromyographic (fEMG) activity produced by three intrinsic and four extrinsic thumb muscles were measured in 10 healthy female volunteers. The participants performed isometric contractions in a lateral key pinch, a power grasp and a jar twist task. The tasks were executed with and without EMG recording to verify if electrode placement influenced force production.ResultsA subject-specific muscle recruitment was found which remained largely unchanged across tasks. Extrinsic thumb muscles were significantly more active than intrinsic muscles in all tasks. Insertion of the fEMG electrodes decreased force production significantly in all tasks.ConclusionThe thumb muscles display a high variability in muscle activity during functional tasks of daily life. The results of this study suggest that to produce a substantial amount of force, a well-integrated, but subject-specific, co-contraction between the intrinsic and extrinsic thumb muscles is necessary.     

Force–velocity,force–power relationships of bilateral and unilateral leg multi-joint movements in young and elderly women

The present study investigated force–velocity and force–power relationships of bilateral and unilateral knee-hip extension movement in young and elderly women. Twelve healthy young (age, 19–31 yr) and 12 healthy elderly (age, 60–82 yr) women performed bilateral and unilateral knee-hip extension movements on the dynamometer against loads controlled by the servo system. Under the isotonic force condition, force–velocity relationships were measured. The maximum isometric force (F_max), unloaded velocity (V_max) and power output (P_max) of the movements were calculated from extrapolating force–velocity and force–power relationships. F_max and P_max of bilateral and unilateral knee-hip extension movements were 20–30% lower in elderly than in young women. On the other hand, there were no significant differences in V_max between young and elderly women and between bilateral and unilateral movements. Bilateral deficit was larger as the generation of force was larger in both young and elderly women. Also, bilateral deficit of F_max and P_max were not different between young and elderly women. The results were that lower maximum power output of bilateral and unilateral leg multi-joint movements in elderly women did not depend on the intrinsic shortening velocity of muscle action, but largely on reduction in force generating capacity. This suggests the importance of preventing a loss of force generating capacity of muscles during leg multi-joint movements in elderly women.