The effects of crank power and cadence on muscle fascicle shortening velocity, muscle activation and joint-specific power during cycling.

Whilst people typically choose to locomote in the most economical fashion, during bicycling they will, unusually, chose cadences that are higher than metabolically optimal. Empirical measurements of the intrinsic contractile properties of the vastus lateralis (VL) muscle during submaximal cycling suggest that the cadences that people self-selected might allow for optimal muscle fascicle shortening velocity for the production of knee extensor muscle power. It remains unclear, however, whether this is consistent across different power outputs where the self-selected cadence (SSC) varies. We examined the effect of cadence and external power requirements on muscle neuromechanics and joint power during cycling. VL fascicle shortening velocity, muscle activation and joint-specific power were measured during cycling between 60 and 120 rpm (including SSC), while participants produced 10%, 30% and 50% of peak maximal power. VL shortening velocity increased as cadence increased but was similar across the different power outputs. Although no differences were found in the distribution of joint power across cadence conditions, the absolute knee joint power increased with increasing crank power output. Muscle fascicle shortening velocity increased in VL at the SSC as pedal power demands increased from submaximal towards maximal cycling. A secondary analysis of muscle activation patterns showed minimized activation of VL and other muscles near the SSC at the 10% and 30% power conditions. Minimization of activation with progressively increasing fascicle shortening velocities at the SSC may be consistent with the theory that the optimum shortening velocity for maximizing power increases with the intensity of exercise and recruitment of fast twitch fibers.

Linking muscle mechanics to the metabolic cost of human hopping.

Many models have been developed to predict metabolic energy expenditure based on biomechanical proxies of muscle function. However, current models may only perform well for select forms of locomotion, not only because the models are rarely rigorously tested across subtle and broad changes in locomotor task but also because previous research has not adequately characterised different forms of locomotion to account for the potential variability in muscle function and thus metabolic energy expenditure. To help to address the latter point, the present study imposed frequency and height constraints to hopping and quantified gross metabolic power as well as the activation requirements of medial gastrocnemius (MG), lateral gastrocnemius (GL), soleus (SOL), tibialis anterior (TA), vastus lateralis (VL), rectus femoris (RF) and biceps femoris (BF), and the work requirements of GL, SOL and VL. Gross metabolic power increased with a decrease in hop frequency and increase in hop height. There was no hop frequency or hop height effect on the mean electromyography (EMG) data of ankle musculature; however, the mean EMG of VL and RF increased with a decrease in hop frequency and that of BF increased with an increase in hop height. With a reduction in hop frequency, GL, SOL and VL fascicle shortening, fascicle shortening velocity and fascicle to MTU shortening ratio increased, whereas with an increase in hop height, only SOL fascicle shortening velocity increased. Therefore, within the constraints that we imposed, decreases in hop frequency and increases in hop height resulted in increases in metabolic power that could be explained by increases in the activation requirements of knee musculature and/or increases in the work requirements of both knee and ankle musculature.

Increased muscle force does not induce greater stretch-induced damage to calf muscles during work-matched heel drop exercise.

PURPOSE: To investigate the effect of muscle force during active stretch on quantitative and qualitative indicators of exercise-induced muscle damage (EIMD) in the medial gastrocnemius (MG) muscle. METHODS: Twelve recreationally active volunteers performed two trials of an eccentric heel drop exercise. Participants performed a single bout of low-load (body weight) and high-load (body weight + 30% body weight) exercises on separate legs. The total mechanical work output for each condition was matched between legs. Before, two hours and 48 h after each bout of eccentric exercise, electrically stimulated triceps surae twitch torque, muscle soreness, MG active fascicle length at maximum twitch torque and muscle passive stiffness were collected. Triceps surae electromyographic (EMG) activity, MG fascicle stretch and MG muscle-tendon unit (MTU) length were measured during the eccentric tasks. RESULTS: The high-load condition increased triceps surae muscle activity by 6-9%, but reduced MG fascicle stretch (p < 0.001). MTU stretch was similar between conditions. The greater muscle force during stretch did not give rise to additional torque loss (5 vs 6%) or intensify muscle soreness. CONCLUSIONS: Adding 30% body weight during eccentric contractions has a modest impact on exercise-induced muscle damage in the medial gastrocnemius muscle. These results suggest that muscle load may not be an important determinant of stretch-induced muscle damage in the human MG muscle. The muscle investigated does exhibit large pennation angles and high series elastic compliance; architectural features that likely buffer muscle fibres against stretch and damage.

The acute effects of higher versus lower load duration and intensity on morphological and mechanical properties of the healthy Achilles tendon: a randomized crossover trial.

The Achilles tendon (AT) exhibits volume changes related to fluid flow under acute load which may be linked to changes in stiffness. Fluid flow provides a mechanical signal for cellular activity and may be one mechanism that facilitates tendon adaptation. This study aimed to investigate whether isometric intervention involving a high level of load duration and intensity could maximize the immediate reduction in AT volume and stiffness compared with interventions involving a lower level of load duration and intensity. Sixteen healthy participants (12 males, 4 females; age 24.4±9.4 years, body mass 70.9±16.1 kg, height 1.7±0.1 m) performed three isometric interventions of varying levels of load duration (2 s and 8 s) and intensity (35% and 75% maximal voluntary isometric contraction) over a 3 week period. Freehand 3D ultrasound was used to measure free AT volume (at rest) and length (at 35%, 55% and 75% of maximum plantarflexion force) pre- and post-interventions. The slope of the force-elongation curve over these force levels represented individual stiffness (N mm-1). Large reductions in free AT volume and stiffness resulted in response to long-duration high-intensity loading whilst less reduction was produced with a lower load intensity. In contrast, no change in free AT volume and a small increase in AT stiffness occurred with lower load duration. These findings suggest that the applied load on the AT must be heavy and sustained for a long duration to maximize immediate volume reduction, which might be an acute response that enables optimal long-term tendon adaptation via mechanotransduction pathways.

Flexor digitorum brevis utilizes elastic strain energy to contribute to both work generation and energy absorption at the foot.

The central nervous system utilizes tendon compliance of the intrinsic foot muscles to aid the foot's arch spring, storing and returning energy in its tendinous tissues. Recently, the intrinsic foot muscles have been shown to adapt their energetic contributions during a variety of locomotor tasks to fulfil centre of mass work demands. However, the mechanism by which the small intrinsic foot muscles are able to make versatile energetic contributions remains unknown. Therefore, we examined the muscle-tendon dynamics of the flexor digitorum brevis during stepping, jumping and landing tasks to see whether the central nervous system regulates muscle activation magnitude and timing to enable energy storage and return to enhance energetic contributions. In step-ups and jumps, energy was stored in the tendinous tissue during arch compression; during arch recoil, the fascicles shortened at a slower rate than the tendinous tissues while the foot generated energy. In step-downs and landings, the tendinous tissues elongated more and at greater rates than the fascicles during arch compression while the foot absorbed energy. These results indicate that the central nervous system utilizes arch compression to store elastic energy in the tendinous tissues of the intrinsic foot muscles to add or remove mechanical energy when the body accelerates or decelerates. This study provides evidence for an adaptive mechanism to enable the foot's energetic versatility and further indicates the value of tendon compliance in distal lower limb muscle-tendon units in locomotion.

The functional importance of human foot muscles for bipedal locomotion.

Human feet have evolved to facilitate bipedal locomotion, losing an opposable digit that grasped branches in favor of a longitudinal arch (LA) that stiffens the foot and aids bipedal gait. Passive elastic structures are credited with supporting the LA, but recent evidence suggests that plantar intrinsic muscles (PIMs) within the foot actively contribute to foot stiffness. To test the functional significance of the PIMs, we compared foot and lower limb mechanics with and without a tibial nerve block that prevented contraction of these muscles. Comparisons were made during controlled limb loading, walking, and running in healthy humans. An inability to activate the PIMs caused slightly greater compression of the LA when controlled loads were applied to the lower limb by a linear actuator. However, when greater loads were experienced during ground contact in walking and running, the stiffness of the LA was not altered by the block, indicating that the PIMs' contribution to LA stiffness is minimal, probably because of their small size. With the PIMs blocked, the distal joints of the foot could not be stiffened sufficiently to provide normal push-off against the ground during late stance. This led to an increase in stride rate and compensatory power generated by the hip musculature, but no increase in the metabolic cost of transport. The results reveal that the PIMs have a minimal effect on the stiffness of the LA when absorbing high loads, but help stiffen the distal foot to aid push-off against the ground when walking or running bipedally.

2021 ISB World Athletics Award for Biomechanics: The Subtalar Joint Maintains "Spring-Like" Function While Running in Footwear That Perturbs Foot Pronation.

Humans have the remarkable ability to run over variable terrains. During locomotion, however, humans are unstable in the mediolateral direction and this instability must be controlled actively-a goal that could be achieved in more ways than one. Walking research indicates that the subtalar joint absorbs energy in early stance and returns it in late stance, an attribute that is credited to the tibialis posterior muscle-tendon unit. The purpose of this study was to determine how humans (n = 11) adapt to mediolateral perturbations induced by custom-made 3D-printed "footwear" that either enhanced or reduced pronation of the subtalar joint (modeled as motion in 3 planes) while running (3 m/s). In all conditions, the subtalar joint absorbed energy (ie, negative mechanical work) in early stance followed by an immediate return of energy (ie, positive mechanical work) in late stance, demonstrating a "spring-like" behavior. These effects increased and decreased in footwear conditions that enhanced or reduced pronation (P ≤ .05), respectively. Of the recorded muscles, the tibialis posterior (P ≤ .05) appeared to actively change its activation in concert with the changes in joint energetics. We suggest that the "spring-like" behavior of the subtalar joint may be an inherent function that enables the lower limb to respond to mediolateral instabilities during running.

The extensibility of the plantar fascia influences the windlass mechanism during human running.

The arch of the human foot is unique among hominins as it is compliant at ground contact but sufficiently stiff to enable push-off. These behaviours are partly facilitated by the ligamentous plantar fascia whose role is central to two mechanisms. The ideal windlass mechanism assumes that the plantar fascia has a nearly constant length to directly couple toe dorsiflexion with a change in arch shape. However, the plantar fascia also stretches and then shortens throughout gait as the arch-spring stores and releases elastic energy. We aimed to understand how the extensible plantar fascia could behave as an ideal windlass when it has been shown to strain throughout gait, potentially compromising the one-to-one coupling between toe arc length and arch length. We measured foot bone motion and plantar fascia elongation using high-speed X-ray during running. We discovered that toe plantarflexion delays plantar fascia stretching at foot strike, which probably modifies the distribution of the load through other arch tissues. Through a pure windlass effect in propulsion, a quasi-isometric plantar fascia's shortening is delayed to later in stance. The plantar fascia then shortens concurrently to the windlass mechanism, likely enhancing arch recoil at push-off.

The energetic function of the human foot and its muscles during accelerations and decelerations.

The human foot is known to aid propulsion by storing and returning elastic energy during steady-state locomotion. While its function during other tasks is less clear, recent evidence suggests the foot and its intrinsic muscles can also generate or dissipate energy based on the energetic requirements of the center of mass during non-steady-state locomotion. In order to examine contributions of the foot and its muscles to non-steady-state locomotion, we compared the energetics of the foot and ankle joint while jumping and landing before and after the application of a tibial nerve block. Under normal conditions, energetic contributions of the foot rose as work demands increased, while the relative contributions of the foot to center of mass work remained constant with increasing work demands. Under the nerve block, foot contributions to both jumping and landing decreased. Additionally, ankle contributions were also decreased under the influence of the block for both tasks. Our results reinforce findings that foot and ankle function mirror the energetic requirements of the center of mass and provide novel evidence that foot contributions remain relatively constant under increasing energetic demands. Also, while the intrinsic muscles can modulate the energetic capacity of the foot, their removal accounted for only a 3% decrement in total center of mass work. Therefore, the small size of intrinsic muscles appears to limit their capacity to contribute to center of mass work. However, their role in contributing to ankle work capacity is likely important for the energetics of movement.

Regional changes in muscle activity do not underlie the repeated bout effect in the human gastrocnemius muscle.

The repeated bout effect (RBE) confers protection following exercise-induced muscle damage. Typical signs of this protective effect are significantly less muscle soreness and faster recovery of strength after the second bout. The aim of this study was to compare regional changes in medial gastrocnemius (MG) muscle activity and mechanical hyperalgesia after repeated bouts of eccentric exercise. Twelve healthy male participants performed two bouts of eccentric heel drop exercise (separated by 7 days) while wearing a vest equivalent to 20% of their body weight. High-density MG electromyographic amplitude maps and topographical pressure pain sensitivity maps were created before, two hours (2H), and two days (2D) after both exercise bouts. Statistical parametric mapping was used to identify RBE effects on muscle activity and mechanical hyperalgesia, using pixel-level statistics when comparing maps. The results revealed a RBE, as a lower strength loss (17% less; P < .01) and less soreness (50% less; P < .01) were found after the second bout. However, different muscle regions were activated 2H and 2D after the initial bout but not following the repeated bout. Further, no overall changes in EMG distribution or mechanical hyperalgesia were found between bouts. These results indicate that muscle activation is unevenly distributed during the initial bout, possibly to maintain muscle function during localized mechanical fatigue. However, this does not reflect a strategy to confer protection during the repeated bout by activating undamaged/non-fatigued muscle areas.

Plantar flexor voluntary activation capacity, strength and function in cerebral palsy.

PURPOSE: Distal lower limb motor impairment impacts gait mechanics in individuals with cerebral palsy (CP), however, the contribution of impairments of muscle activation to reduced gross motor function (GMF) is not clear. This study aimed to investigate deficits in plantar flexion voluntary activation capacity in CP compared to typically developed (TD) peers, and evaluate relationships between voluntary activation capacity, strength and GMF. METHODS: Fifteen ambulant individuals with spastic CP (23 ± 6 years, GMFCS I-III) and 14 TD (22 ± 2 years) people participated. Plantar- and dorsiflexion strength were assessed with a dynamometer. Voluntary activation capacity was assessed using the interpolated twitch technique via single twitch supramaximal tibial nerve stimulation. GMF was assessed using the timed upstairs test, 10 m walk test, muscle power sprint test and six-minute walk test. RESULTS: Plantar- and dorsiflexion strength were 55.6% and 60.7% lower in CP than TD (p < 0.001). Although voluntary activation capacity was 17.9% lower on average for CP than TD (p = 0.039), 46.7% of individuals with CP achieved a sufficiently high activation to fall within one standard deviation of the TD mean. Plantar flexion voluntary activation capacity did not correlate with strength (R2 = 0.092, p = 0.314) or GMF measures in the high functioning CP group (GMFCS I-II). CONCLUSION: In contrast to previous research, plantar flexion activation capacity did not strongly predict weakness or reduced GMF. We propose that muscle size contributes more to weakness than voluntary activation capacity in high functioning individuals with CP and that relationships between muscle activation and functional capacity are complicated by effects at multiple joints.

Cyclic eccentric stretching induces more damage and improved subsequent protection than stretched isometric contractions in the lower limb.

PURPOSE: Controversy remains about whether exercise-induced muscle damage (EIMD) and the subsequent repeated bout effect (RBE) are caused by the stretching of an activated muscle, or the production of high force at long, but constant, muscle lengths. The aim of this study was to determine the influence of muscle fascicle stretch elicited during different muscle contraction types on the magnitude of EIMD and the RBE. METHODS: Fourteen participants performed an initial bout of lower limb exercise of the triceps surae. One leg performed sustained static contractions at a constant long muscle length (ISO), whereas the contralateral leg performed a bout of eccentric heel drop exercise (ECC). Time under tension was matched between the ECC and ISO conditions. Seven days later, both legs performed ECC. Plantar flexor twitch torque, medial gastrocnemius (MG) fascicle length and muscle soreness were assessed before, 2 h and 2 days after each exercise bout. MG fascicle length and triceps surae surface electromyography were examined across the bouts of exercise. RESULTS: We found that both ECC and ISO conditions elicited EIMD and a RBE. ISO caused less damage 2 h after the initial bout (14% less drop in twitch torque, P = 0.03) and less protection from soreness 2 days after the repeated bout (56% higher soreness, P = 0.01). No differences were found when comparing neuromechanical properties across exercise bouts. CONCLUSION: For MG, the action of stretching an active muscle seems to be more important for causing damage than a sustained contraction at a long length.

Comparisons of laboratory-based methods to calculate jump height and improvements to the field-based flight-time method.

Laboratory methods that are required to calculate highly precise jump heights during experimental research have never been sufficiently compared and examined. Our first aim was to compare jumping outcome measures of the same jump, using four different methods (double integration from force plate data, rigid-body modeling from motion capture data, marker-based video tracking, and a hybrid method), separately for countermovement and squat jumps. Additionally, laboratory methods are often unsuitable for field use due to equipment or time restrictions. Therefore, our second aim was to improve an additional field-based method (flight-time method), by combining this method with an anthropometrically scaled constant. Motion capture and ground reaction forces were used to calculate jump height of twenty-four participants who performed five maximal countermovement jumps and five maximal squat jumps. Within-participant mean and standard deviation of jump height, flight distance, heel-lift, and take-off velocity were compared for each of the four methods. All four methods calculated countermovement jump height with low variability and are suitable for research applications. The double integration method had significant errors in squat jump height due to integration drift, and all other methods had low variability and are therefore suitable for research applications. Rigid-body modeling was unable to determine the position of the center of mass at take-off in both jumping movements and should not be used to calculate heel-lift or flight distance. The flight-time method was greatly improved with the addition of an anthropometrically scaled heel-lift constant, enabling this method to estimate jump height and subsequently estimate power output in the field.

Tibialis anterior tendinous tissue plays a key role in energy absorption during human walking.

The elastic tendinous tissues of distal lower limb muscles can improve the economy of walking and running, amplify the power generated by a muscle and absorb energy. This paper explores the behaviour of the tibialis anterior (TA) muscle and its tendinous tissue during gait, as it absorbs energy during contact and controls foot position during swing. Simultaneous measurements of ultrasound, surface electromyography and 3D motion capture with musculoskeletal modelling from 12 healthy participants were recorded as they walked at preferred and fast walking speeds. We quantified the length changes and velocities of the TA muscle-tendon unit (MTU) and its fascicles across the stride at each speed. Fascicle length changes and velocities were relatively consistent across speeds, although the magnitude of fascicle length change differed between the deep and superficial regions. At contact, when the TA is actively generating force, the fascicles remained relatively isometric as the MTU actively lengthened, presumably stretching the TA tendinous tissue and absorbing energy. This potentially protects the muscle fibres from damage during weight acceptance and allows energy to be returned to the system later in the stride. During early swing, the fascicles and MTU both actively shortened to dorsiflex the foot, clearing the toes from the ground; however, at the fast walking velocity, the majority of shortening occurred through tendinous tissue recoil, highlighting its role in accelerating ankle dorsiflexion to power rapid foot clearance in swing.

The influence of added mass on muscle activation and contractile mechanics during submaximal and maximal countermovement jumping in humans.

Muscle contractile mechanics induced by the changing demands of human movement have the potential to influence our movement strategies. This study examined fascicle length changes of the triceps surae during jumping with added mass or increasing jump height to determine whether the chosen movement strategies were associated with relevant changes in muscle contractile properties. Sixteen participants jumped at sub-maximal and maximal intensities while total net work was matched via two distinct paradigms: (1) adding mass to the participant or (2) increasing jump height. Electromyography (EMG) and ultrasound analyses were performed to examine muscle activation, fascicle length and fascicle velocity changes of the triceps surae during jumping. Integrated EMG was significantly higher in the added mass paradigm with no difference in mean or maximal EMG, indicating that the muscle was activated for a significantly longer period of time but not activated to a greater intensity. Fascicle shortening velocity was slower with added mass compared than with increasing jump height; therefore, intrinsic force-velocity properties probably enabled increased force production. Improved fascicle contractile mechanics paired with a longer activation period probably produced a consistently larger fascicle force, enabling a greater impulse about the ankle joint. This may explain why previous research found that participants used an ankle-centred strategy for work production in the added mass paradigm and not in the jump height paradigm. The varied architecture of muscles within the lower limb may influence which muscles we choose to employ for work production under different task constraints.

Functional Capacity in Adults With Cerebral Palsy: Lower Limb Muscle Strength Matters.

OBJECTIVE: To investigate the relation between lower limb muscle strength, passive muscle properties, and functional capacity outcomes in adults with cerebral palsy (CP). DESIGN: Cross-sectional study. SETTING: Tertiary institution biomechanics laboratory. PARTICIPANTS: Adults with spastic-type CP (N=33; mean age, 25y; range, 15-51y; mean body mass, 70.15±21.35kg) who were either Gross Motor Function Classification System (GMFCS) level I (n=20) or level II (n=13). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Six-minute walk test (6MWT) distance (m), lateral step-up (LSU) test performance (total repetitions), timed up-stairs (TUS) performance (s), maximum voluntary isometric strength of plantar flexors (PF) and dorsiflexors (DF) (Nm.kg-1), and passive ankle joint and muscle stiffness. RESULTS: Maximum isometric PF strength independently explained 61% of variance in 6MWT performance, 57% of variance in LSU test performance, and 50% of variance in TUS test performance. GMFCS level was significantly and independently related to all 3 functional capacity outcomes, and age was retained as a significant independent predictor of LSU and TUS test performance. Passive medial gastrocnemius muscle fascicle stiffness and ankle joint stiffness were not significantly related to functional capacity measures in any of the multiple regression models. CONCLUSIONS: Low isometric PF strength was the most important independent variable related to distance walked on the 6MWT, fewer repetitions on the LSU test, and slower TUS test performance. These findings suggest lower isometric muscle strength contributes to the decline in functional capacity in adults with CP.

Effects of series elastic compliance on muscle force summation and the rate of force rise.

Compliant tendons permit mechanically unfavourable fascicle dynamics during fixed-end contractions. The purpose of this study was to reduce the effective compliance of tendon and investigate how small reductions in active shortening affect twitch kinetics and contractile performance in response to a second stimulus. The series elastic element (SEE) of the human triceps surae (N=15) was effectively stiffened by applying a 55 ms rotation to the ankle, through a range of 5 deg, at the onset of twitch and doublet [interstimulus interval (ISI) of 80 ms] stimulation. Ultrasonography was employed to quantify lateral gastrocnemius and soleus fascicle lengths. Rotation increased twitch torque (40-75%), rate of torque development (RTD; 124-154%) and torque-time integral (TTI; 70-110%) relative to constant-length contractions at the initial and final joint positions, yet caused only modest reductions in shortening amplitude and velocity. The torque contribution of the second pulse increased when stimulation was preceded by rotation, a finding unable to be explained on the basis of fascicle length or SEE stiffness during contraction post-rotation. A further increase in torque contribution was not demonstrated, nor was an increase in doublet TTI, when the second pulse was delivered during rotation and shortly after the initial pulse (ISI of 10 ms). The depressant effect of active shortening on subsequent torque generation suggests that compliant tendons, by affording large length changes, may limit torque summation. Our findings indicate that changes in tendon compliance shown to occur in response to resistance training or unloading are likely sufficient to considerably alter contractile performance, particularly maximal RTD.

The role of human ankle plantar flexor muscle-tendon interaction and architecture in maximal vertical jumping examined in vivo.

Humans utilise elastic tendons of lower limb muscles to store and return energy during walking, running and jumping. Anuran and insect species use skeletal structures and/or dynamics in conjunction with similarly compliant structures to amplify muscle power output during jumping. We sought to examine whether human jumpers use similar mechanisms to aid elastic energy usage in the plantar flexor muscles during maximal vertical jumping. Ten male athletes performed maximal vertical squat jumps. Three-dimensional motion capture and a musculoskeletal model were used to determine lower limb kinematics that were combined with ground reaction force data in an inverse dynamics analysis. B-mode ultrasound imaging of the lateral gastrocnemius (GAS) and soleus (SOL) muscles was used to measure muscle fascicle lengths and pennation angles during jumping. Our results highlighted that both GAS and SOL utilised stretch and recoil of their series elastic elements (SEEs) in a catapult-like fashion, which likely serves to maximise ankle joint power. The resistance of supporting of body weight allowed initial stretch of both GAS and SOL SEEs. A proximal-to-distal sequence of joint moments and decreasing effective mechanical advantage early in the extension phase of the jumping movement were observed. This facilitated a further stretch of the SEE of the biarticular GAS and delayed recoil of the SOL SEE. However, effective mechanical advantage did not increase late in the jump to aid recoil of elastic tissues.

Additional in-series compliance reduces muscle force summation and alters the time course of force relaxation during fixed-end contractions.

There are high mechanical demands placed on skeletal muscles in movements requiring rapid acceleration of the body or its limbs. Tendons are responsible for transmitting muscle forces, but, because of their elasticity, can manipulate the mechanics of the internal contractile apparatus. Shortening of the contractile apparatus against the stretch of tendon affects force generation according to known mechanical properties; however, the extent to which differences in tendon compliance alter force development in response to a burst of electrical impulses is unclear. To establish the influence of series compliance on force summation, we studied electrically evoked doublet contractions in the cane toad peroneus muscle in the presence and absence of a compliant artificial tendon. Additional series compliance reduced tetanic force by two-thirds, a finding predicted based on the force-length property of skeletal muscle. Doublet force and force-time integral expressed relative to the twitch were also reduced by additional series compliance. Active shortening over a larger range of the ascending limb of the force-length curve and at a higher velocity, leading to a progressive reduction in force-generating potential, could be responsible. Muscle-tendon interaction may also explain the accelerated time course of force relaxation in the presence of additional compliance. Our findings suggest that a compliant tendon limits force summation under constant-length conditions. However, high series compliance can be mechanically advantageous when a muscle-tendon unit is actively stretched, permitting muscle fibres to generate force almost isometrically, as shown during stretch-shorten cycles in locomotor activities. Restricting active shortening would likely favour rapid force development.