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. 2018 Apr 25;596(11):2121–2129. doi: 10.1113/JP275527

History‐dependence of muscle slack length following contraction and stretch in the human vastus lateralis

Peter W Stubbs 1,3, Lee D Walsh 4, Arkiev D'Souza 1,2, Martin E Héroux 1,2, Bart Bolsterlee 1,2, Simon C Gandevia 1,2, Robert D Herbert 1,2,✉
PMCID: PMC5983182  PMID: 29604053

Abstract

Key points

  • In reduced muscle preparations, the slack length and passive stiffness of muscle fibres have been shown to be influenced by previous muscle contraction or stretch. In human muscles, such behaviours have been inferred from measures of muscle force, joint stiffness and reflex magnitudes and latencies.

  • Using ultrasound imaging, we directly observed that isometric contraction of the vastus lateralis muscle at short lengths reduces the slack lengths of the muscle–tendon unit and muscle fascicles. The effect is apparent 60 s after the contraction.

  • These observations imply that muscle contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles.

Abstract

In reduced muscle preparations, stretch and muscle contraction change the properties of relaxed muscle fibres. In humans, effects of stretch and contraction on properties of relaxed muscles have been inferred from measurements of time taken to develop force, joint stiffness and reflex latencies. The current study used ultrasound imaging to directly observe the effects of stretch and contraction on muscle–tendon slack length and fascicle slack length of the human vastus lateralis muscle in vivo. The muscle was conditioned by (a) strong isometric contractions at long muscle–tendon lengths, (b) strong isometric contractions at short muscle–tendon lengths, (c) weak isometric contractions at long muscle–tendon lengths and (d) slow stretches. One minute after conditioning, ultrasound images were acquired from the relaxed muscle as it was slowly lengthened through its physiological range. The ultrasound image sequences were used to identify muscle–tendon slack angles and fascicle slack lengths. Contraction at short muscle–tendon lengths caused a mean 13.5 degree (95% CI 11.8–15.0 degree) shift in the muscle–tendon slack angle towards shorter muscle–tendon lengths, and a mean 5 mm (95% CI 2–8 mm) reduction in fascicle slack length, compared to the other conditions. A supplementary experiment showed the effect could be demonstrated if the muscle was conditioned by contraction at short lengths but not if the relaxed muscle was held at short lengths, confirming the role of muscle contraction. These observations imply that muscle contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles.

Keywords: thixotropy, ultrasound, muscle fascicles

Key points

  • In reduced muscle preparations, the slack length and passive stiffness of muscle fibres have been shown to be influenced by previous muscle contraction or stretch. In human muscles, such behaviours have been inferred from measures of muscle force, joint stiffness and reflex magnitudes and latencies.

  • Using ultrasound imaging, we directly observed that isometric contraction of the vastus lateralis muscle at short lengths reduces the slack lengths of the muscle–tendon unit and muscle fascicles. The effect is apparent 60 s after the contraction.

  • These observations imply that muscle contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles.

Introduction

Relaxed skeletal muscles generate passive tension when stretched (Hill, 1950). However, at least some human muscle–tendon units fall slack at short muscle lengths (Hoang et al. 2007; Herbert et al. 2011). This can be demonstrated by holding the knee in flexion and the ankle in plantarflexion, so that the gastrocnemius muscle is short. With ultrasonography it is observed that, when the ankle is passively dorsiflexed, the muscle fascicles in the gastrocnemius do not initially change length (Herbert et al. 2011, 2015). This occurs because the muscle is slack at very short lengths. With further dorsiflexion of the ankle, the slack is taken up and muscle fascicles begin to lengthen.

Studies on reduced muscle preparations suggest that the muscle–tendon length at which muscle fascicles start to lengthen (the muscle–tendon ‘slack length’) is influenced by previous muscle stretch (Lakie & Robson, 1988a, 1990) and contraction (Jewell & Wilkie, 1958, 1960; McCarter et al. 1971; Lakie & Robson, 1988b; Proske et al. 1993). For example, in isolated frog muscle fibres held at short lengths, the delay between the delivery of an electrical stimulus and the onset of active tension is reduced when the electrical stimulus is preceded by a brief tetanus (Proske et al. 1993). It is thought that conditioning the muscle in this way removes slack that is otherwise present in muscle fibres at short lengths, allowing active tension to be produced more rapidly.

Studies conducted on human muscles in vivo also suggest that muscle stretch (Lakie et al. 1980, 1984; Jahnke et al. 1989; Hagbarth et al. 1995; Sakanaka et al. 2016) and contraction (Hagbarth et al. 1985, 1995; Jahnke et al. 1989) can change muscle slack lengths. For example, Hagbarth et al. (1985) compared the passive stiffness of human finger flexor muscles after they were conditioned with voluntary isometric, concentric or eccentric contractions. Stiffness was highest after the concentric flexion contractions to short muscle lengths. In that study, the after‐effects of muscle contraction on the muscle–tendon and fascicle slack length were inferred from measurements of passive joint stiffness. Other studies of history‐dependent changes in muscle have investigated effects on time taken to develop force or the magnitude and onset latencies of reflexes (e.g. Hagbarth et al. 1995). Although these outcomes can be used as surrogate measures of muscle–tendon and fascicle slack length, history‐dependence of muscle slack lengths has not been directly observed.

In the present study, ultrasound imaging was used to directly observe muscle fascicles in the human vastus lateralis muscle in vivo. Pilot experiments were conducted on the medial gastrocnemius muscle, but the present study was conducted on the vastus lateralis muscle because better quality images could be obtained from the vastus lateralis than the gastrocnemius. The present study tested the hypothesis that conditioning the muscle with contractions at short lengths would reduce muscle–tendon and fascicle slack lengths observed when the muscle was passively lengthened, compared to conditioning the muscle with contractions at long muscle–tendon lengths or with stretches.

Methods

Ethical approval

The study was approved by the University of New South Wales Human Research Ethics Committee (approval number: 15/006) and performed in accordance with the Declaration of Helsinki (2013) except for registration in a database. Subjects gave informed consent in writing.

Subjects

Eleven healthy adults were recruited for the experiment. One subject did not successfully complete the experiment due to a persistent inability to relax after the conditioning muscle contractions, so data are reported from 10 subjects (1 female, 9 males; mean age: 28 ± 5 (SD) years; mean height: 1.78 ± 0.09 m; mean distance from the greater trochanter to lateral femoral condyle: 0.43 ± 0.03 m). Based on the results of the main experiment, a supplementary experiment was performed on five subjects (5 males; mean age: 35 ± 11 years; mean height: 1.81 ± 0.11 m; mean distance from the greater trochanter to lateral femoral condyle: 0.42 ± 0.05 m). No subject had a history of recent fracture, muscle or tendon strain, or orthopaedic surgery to the tested leg.

Experimental set‐up

Subjects were seated with the shank of the left leg firmly strapped into a dynamometer (Cybex Norm with Humac, CSMi, Stoughton, MA, USA) that cyclically rotated the knee from maximum comfortable extension (main experiment: 3.7 ± 1.8°; supplementary experiment: 5.2 ± 2.4°) to ∼90° of flexion. The dynamometer generated knee angle and torque signals that were sampled at 50 Hz. Two surface EMG electrodes (30 mm diameter (VS30); Verity Medical Ltd, Romsey, UK) were placed on the skin over the rectus femoris (spacing ∼100 mm) and a ground electrode was placed over the greater trochanter. The EMG signal was amplified, bandpass filtered (20–500 Hz) and sampled at 2000 Hz.

Ultrasound images of the vastus lateralis muscle fascicles were obtained using two 46 mm linear array ultrasound transducers (MyLab25 with LA522E transducers, 7.5–12 MHz operating at 12 MHz; Esaote, Firenze, Italy) held together in a custom‐built mould. The use of two ultrasound transducers increased the field of view. The transducers were placed over the mid‐belly of the vastus lateralis and the orientation of the transducers was adjusted to generate the clearest possible image of the muscle fascicles. Prior to testing, tape was placed on the skin to mark transducer placement and facilitate imaging of the same part of the muscle with repeated measurements. Images from the two ultrasound transducers were sampled at 10 Hz and synchronised with the knee angle and EMG signals using Spike2 software (CED, Cambridge, UK, with S2video plug‐in).

Testing protocol

The protocol is shown in Fig. 1 A–D. The vastus lateralis muscle was conditioned in four ways, presented in random order: contract‐short, contract‐long‐weak, contract‐long, and stretch. All conditions were preceded by a submaximal (weak) pre‐conditioning contraction at ∼90° of knee flexion.

Figure 1. Experimental protocol for the main experiment and supplementary experiment.

Figure 1

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For the main experiment there were four conditions: contract‐short (A), contract‐long‐weak (B), contract‐long (C), and stretch (D). For the supplementary experiment there were three conditions: contract‐short (A), contract‐long (C), and hold‐short (E). In each panel, time proceeds from left to right. The vertical axis indicates knee angle. Thick coloured lines show the knee angle as a function of time. ‘Weak’ and ‘strong’ refer to weak contractions and strong contractions, respectively. The arrow indicates the transducer placement, which took ∼5 s. ‘Test image’ indicates the period over which the test image sequence was recorded. Subjects were asked to remain relaxed except when performing the weak or strong contractions. Note that all conditions began with a weak contraction with the knee flexed (i.e. with the vastus lateralis muscle in the lengthened position).

The rationale for including both a contract‐long and a contract‐long‐weak condition was that the knee extensor muscles can generate more active force when the knee is flexed (i.e. at long lengths) than when the knee is extended. The contract‐long‐weak condition was included to explore whether any differences between contract‐short and contract‐long conditions should be attributed to the angle at which the isometric contraction was performed or the contraction force. As it was possible to measure knee torque but not force generated by the vastus lateralis muscle, we matched knee extensor torque rather than vastus lateralis force at the two joint angles. The target active torque in the contract‐long‐weak condition was the active torque produced in a maximal contraction at short lengths. The maximal active torque produced at short lengths was measured at the beginning of the experiment.

For contract‐short conditioning, the subject performed a 3‐s maximal isometric knee extension contraction with the knee extended. For contract‐long‐weak conditioning, the subject performed a 3‐s weak knee extension contraction with the knee at ∼90° of knee flexion. For contract‐long conditioning, the subject performed a 3‐s maximal isometric knee extension contraction with the knee at ∼90° of knee flexion. For stretch conditioning, the knee was flexed and extended ∼90° at 10° s−1. After conditioning, ultrasound images were recorded while the knee was passively flexed from the extended position at 5° s−1. Subjects were asked to remain relaxed except when performing the isometric contractions.

After analysis of data from the main experiment, a supplementary experiment was performed (Fig. 1 A, C and E). For this experiment, the vastus lateralis was conditioned in three ways, presented in a random order: hold‐short, contract‐short and contract‐long. The hold‐short condition was the same as the contract‐short condition without the contraction. The contract‐short and contract‐long conditions were the same as in the main experiment.

Occasionally, subjects were unable to relax their muscles following the contractions, as shown by EMG above baseline levels. When this was observed, the trial was repeated. Five successful trials were conducted for each conditioning protocol.

Data analysis

The two ultrasound image sequences were stitched together to provide a composite image with a 110 mm field of view. The image had an 18 mm gap between the two transducers. Muscle fascicles were tracked on the composite image sequence using a semi‐automated procedure (Herbert et al. 2011, 2015; Diong et al. 2012; Kwah et al. 2012). One muscle fascicle from each image sequence was tracked. Tracking involved identifying the location of the proximal and distal ends of the fascicle in each frame of the image sequence. The investigator who supervised the semi‐automated analysis was only provided with video data from the final (testing) part of each trial and was blinded to the subject and contraction condition. Muscle fascicle length was defined as the distance between the proximal and distal ends of the muscle fascicle. Fascicle length was plotted as a function of knee angle.

The muscle–tendon slack angle is the knee angle at which the muscle fascicle begins to lengthen when the knee is passively flexed from an extended position. Presumably this occurs when the slack has been taken up in the muscle–tendon unit and tension transmitted to the fascicle causes an abrupt lengthening of the fascicle. The muscle–tendon slack angle is a surrogate measure for muscle–tendon slack length. Fascicle slack length is the fascicle length at the muscle–tendon slack angle.

A semi‐automated procedure was used to determine the joint angle corresponding to the muscle–tendon slack length. For each trial, two linear regressions were fitted to the relationship between fascicle length and knee angle: one to data from the first 14 frames of the image sequence (i.e. frames 1–14) and another to the next 14 frames (frames 15–28). The ratio of the slopes of the two regressions was determined. Then the procedure was repeated, each time at longer muscle lengths (i.e. on frames 2–15 and 16–29, then frames 3–16 and 17–30, and so on). The knee angle corresponding to muscle–tendon slack length was the knee angle corresponding to the midpoint between two sets of frames for which the absolute ratio of the slopes was greatest. Sometimes it appeared that the muscle–tendon slack angle may have been reached before the 14th frame and regressions were conducted using seven frames. If the muscle–tendon slack angle was reached before the seventh frame, the reported muscle–tendon slack angle and fascicle slack length was the angle and length at the seventh frame. This algorithm worked well in most trials and generated estimates of the muscle–tendon slack angle that agreed with visual inspection. However, in a few trials the algorithm generated more than one distinct peak. When this occurred, the peak that agreed with visual inspection was used to define the muscle–tendon slack angle. All estimates of muscle–tendon slack angle were visually checked by two or three investigators who were blind to the condition of the trial. If there was uncertainty about the muscle–tendon slack angle, the trial was excluded from subsequent analyses.

Statistical analysis

Linear mixed models were used to determine the effect on muscle–tendon slack angle and fascicle slack length of contract‐short, contract‐long‐weak, contract‐long and stretch‐only conditioning for the main experiment and contract‐short, contract‐long and hold‐short conditioning for the supplementary experiment. In these models, there were random intercepts for subjects. Condition (contract‐short, contract‐long‐weak, contract‐long and stretch conditioning for the main experiment and contract‐short, contract‐long and hold‐short for the supplementary experiment) was a fixed factor. Data analysis was conducted using SPSS Statistics v. 21 (IBM Corp., Armonk, NY, USA) and replicated with Stata v. 15 (StataCorp, College Station, TX, USA). Statistical significance was set at P < 0.05.

Results

In total 228/275 image sequences were retained in the final analyses of the main experiment and the supplementary experiment. Fourteen image sequences were excluded from the analysis because muscle twitches could be seen on the images and nine image sequences were excluded due to poor image quality. A further 24 image sequences were excluded because it was not possible to clearly identify the muscle–tendon slack angle. In total, for the main experiment, five contract‐short, 13 contract‐long‐weak, 10 contract‐long and nine stretch image sequences were excluded by the blinded investigators, leaving 45/50 contract‐short, 37/50 contract‐long‐weak, 40/50 contract‐long and 41/50 stretch image sequences. In total, for the supplementary experiment, three contract‐short, five contract‐long and two hold‐short image sequences were excluded by the blinded investigators, leaving 22/25 contract‐short, 20/25 contract‐long and 23/25 hold short image sequences.

Main experiment

There were large and consistent differences in mean muscle–tendon slack angle across the four conditions (P < 0.001). This was observed in the aggregate data and the data for each subject, and even in individual trials (Figs 2 and 3 A; Supplementary Video S1). For all subjects, the average muscle–tendon slack angle was lowest in the contract‐short condition. Muscle–tendon slack angle was lower after contract‐short conditioning (mean 9.9°) than after contract‐long‐weak conditioning (mean 23.6°; mean difference −13.7°, 95% CI −14.9 to −12.5°; P < 0.001), contract‐long conditioning (mean 22.9°; mean difference −13.0°, 95% CI −14.2 to −11.8°; P < 0.001) and stretch conditioning (mean 23.7°; mean difference −13.8°, 95% CI −15.0 to −12.7°; P < 0.001). There were no differences between the contract‐long‐weak and contract‐long conditions (mean difference 0.7°, 95% CI −0.5 to 1.9°; P = 0.25), contract‐long‐weak and stretch conditions (mean difference −0.1°, 95% CI −1.3 to 1.1°; P = 0.84), and contract‐long and stretch conditions (mean difference −0.8°, 95% CI −2.0 to 0.3°; P = 0.17).

Figure 2. Fascicle lengths (mm) as a function of knee angle (degrees).

Figure 2

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Each panel shows data from one subject. The lines represent the fascicle lengths over the imaged range of movement for the contract‐short (red), contract‐long‐weak (black), contract‐long (blue) and stretch (green) conditions. The coloured open circles represent the fascicle slack lengths (on the y‐axis) and muscle–tendon slack angle (on the x‐axis). Note that the ranges of the x‐axis (knee angle) and y‐axis (fascicle length) differ across subjects.

Figure 3. Mean muscle–tendon slack angle (A) and mean fascicle slack length (B) for each condition for each subject.

Figure 3

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The coloured open squares represent means for each condition. *The contract‐short condition differed significantly from all other conditions: P < 0.001.

There were also consistent differences in mean muscle fascicle slack lengths across the four conditions (P < 0.001). In 9/10 subjects the average fascicle length for each condition was shortest in the contract‐short condition (Figs 2 and 3 B; Supplementary Video S1). Fascicle slack length was shorter after contract‐short conditioning (mean 76 mm) than after contract‐long‐weak conditioning (mean 80 mm; mean difference −4 mm, 95% CI −6 to −2 mm; P < 0.001), contract‐long conditioning (mean 81 mm; mean difference −5 mm, 95% CI −7 to −3 mm; P < 0.001) and stretch conditioning (mean 81 mm; mean difference −6 mm, 95% CI −8 to −4 mm; P < 0.001). There were no differences between fascicle slack lengths in the contract‐long‐weak and contract‐long conditions (mean difference 0 mm, 95% CI −3 to 2 mm; P = 0.64), contract‐long‐weak and stretch conditions (mean difference −1 mm, 95% CI −3 to 1 mm; P = 0.20), and contract‐long and stretch conditions (mean difference −1 mm, 95% CI −3 to 1 mm; P = 0.40).

Supplementary experiment

In the main experiment, the contract‐short condition reduced muscle–tendon slack angles and fascicle slack lengths compared to the other conditions. The main experiment did not allow us to determine if this effect was caused by contraction or by holding the muscle at the short length. The supplementary experiment investigated whether the effect was due to contraction or holding the muscle at short lengths.

Muscle–tendon slack angle was larger after hold‐short conditioning (18.0°) than contract‐short conditioning (9.0°; mean difference 9.1°, 95% CI 6.8 to 11.3°; P < 0.001) and was not different from contract‐long conditioning (18.0°; mean difference 0.1°, 95% CI −2.3 to 2.4°; P = 0.95). Fascicle slack length was longer after hold‐short conditioning (84 mm) than after contract‐short conditioning (77 mm; mean difference 7 mm, 95% CI 3 to 10 mm; P < 0.001) and was not different from contract‐long conditioning (85 mm; mean difference −1 mm, 95% CI −5 to 2 mm; P = 0.51).

Discussion

This is the first study to show directly that the contraction history of a muscle affects muscle–tendon and muscle fascicle slack lengths. Contractions of the human vastus lateralis muscle at short muscle–tendon lengths produced a large reduction in the muscle–tendon slack angle (∼13.5°), indicative of a reduction in muscle–tendon slack length compared to conditioning with contractions at long muscle lengths or with slow stretches. In addition, contraction at short lengths reduced fascicle slack length by ∼5 mm, indicating that at least some of the reduction in muscle–tendon slack length was due to a reduction in the slack length of the muscle fascicles. This phenomenon is caused by contraction‐related mechanisms because holding the relaxed muscle at short muscle–tendon lengths did not reduce slack in the muscle–tendon unit or in muscle fascicles.

Previous studies have inferred history‐dependent changes in muscle–tendon and muscle fascicle slack lengths from measurements of changes in joint stiffness, time taken to develop force, or the magnitude and latencies of reflexes. For example, Hagbarth et al. (1995) examined short‐latency stretch reflexes in the human finger flexor muscles. The stretch reflexes were elicited by rapidly stretching the finger flexor muscles from a test position. Stretch reflexes could be delayed (i.e. reflex latencies increased) by first passively stretching the finger flexor muscles and returning the fingers to the test position. The effect of stretch conditioning could be abolished (reflex latencies reduced) by a subsequent isometric contraction of the finger flexor muscles at the test position. A possible mechanism for these findings is that stretch induced slack in intrafusal or extrafusal fibres at the test position and isometric contraction reduced or removed slack in intrafusal or extrafusal fibres at the test position. In addition, Jahnke et al. (1989) investigated stiffness during passive extension movements of the fingers and elbow after conditioning with either isometric contractions of the flexor muscles in a flexed position or a passive stretch of the flexor muscles. Passive extension stiffness was higher following the contraction than following the stretch. This finding suggests isometric contraction at short lengths removed slack from muscle fascicles and muscle–tendon units. Our findings are consistent with those of these previous studies (Jahnke et al. 1989; Hagbarth et al. 1995) and demonstrate directly that contraction at short lengths reduces muscle–tendon and muscle fascicle slack lengths.

The current study also provides an explanation for the findings of studies of proprioception in humans. When blindfolded subjects were asked to perform position‐matching tasks, the elbow was perceived as more extended following contraction of the elbow flexor muscles at short muscle lengths and more flexed following contraction of the elbow extensor muscles at short lengths (Gregory et al. 1988; Winter et al. 2005; Allen et al. 2007; Proske & Gandevia, 2012). The findings of the current study suggest that contraction of muscles at short lengths reduces slack in intrafusal or extrafusal muscle fibres, increasing the sensitivity of intrafusal muscle fibres to stretch.

The reduction in muscle–tendon slack length produced by the contract‐short condition might be expected to cause passive joint torque to rise at shorter muscle–tendon lengths or to increase passive muscle torque at muscle–tendon lengths above slack length, compared to the other conditions. Passive knee torque data were collected incidentally but because the data were not of high quality they are not presented here. The data suggest that passive torques were higher on average after contract‐short conditioning than in the other conditions. It is not possible to determine how much the increase in knee extensor torque was due to an increase in tension of the vastus lateralis muscle.

The history‐dependent behaviours reported here may be another manifestation of the phenomenon of passive force enhancement (Herzog & Leonard, 2002). Passive force enhancement refers to the increase in passive force at a particular muscle length produced by stretching the contracting muscle to that length; it has been demonstrated in single myofibrils from rabbit psoas muscle (Joumaa et al. 2007), single fibres from frog lumbrical muscles (Rassier et al. 2005), cat soleus muscles in situ (Herzog & Leonard, 2002), and human adductor pollicis (Lee & Herzog, 2002) and quadriceps muscles (Hahn et al. 2007; Seiberl et al. 2010) in vivo. Herzog and Leonard (2002) noted passive force enhancement in the cat soleus muscle scaled inversely with initial muscle length: when the contraction began at short muscle lengths (i.e. when the stretch amplitude was large), passive force enhancement was greater than when the contraction began at longer lengths (stretch amplitude was small). This is similar to our observation that the muscle slack length was shorter after isometric contractions at short lengths than after isometric contractions at long lengths. Also, Herzog et al. (2003) observed that passive force enhancement could be abolished by passively shortening and lengthening the muscle. Similarly we observed that the stretch condition, in which the muscle was passively shortened then lengthened, increased muscle slack length compared to the contract‐short condition. Lastly, Herzog and Leonard (2002) observed that the passive force enhancement was persistent – it persisted at least for the 5–10 s that the muscle was held at the target length after the muscle had been allowed to relax. In another study, also on the cat soleus muscle, they showed passive force enhancement persisted at least 25 s after conditioning (Herzog et al. 2003). In the present study, large effects of conditioning were apparent more than 60 s after conditioning.

Mechanisms

History‐dependence of muscle–tendon slack angle could be due to history‐dependent properties of muscle, tendon, or both muscle and tendon. The reduction in fascicle slack length following contraction of the muscle at short lengths strongly suggests that the reduction in muscle–tendon slack length is at least partly muscle‐mediated.

There are at least two hypotheses that could explain history‐dependent muscle behaviours. The first is a titin‐based mechanism. According to this hypothesis, muscle contraction causes a specific site on the titin molecule to bind with actin. If the contraction occurs at short muscle lengths, the titin molecule is under little or no stretch so titin binds at a site on the actin filament that is close to the z‐line. In contrast, if the contraction occurs at stretched muscle lengths the titin molecule is under stretch so it binds on the actin filament further from the z‐line. In this way, the history of muscle contraction regulates the effective length of the free (unbound) part of the titin molecule, and thus also the slack length and stiffness of the passive muscle (Herzog et al. 2012; Schappacher‐Tilp et al. 2015; but see DuVall et al. 2017). The second hypothesis is a cross‐bridge mechanism. According to this hypothesis, a small number of actin–myosin cross‐bridges persist or spontaneously reform when the muscle is relaxed following contraction at short lengths (Campbell & Lakie, 1998), precluding muscle slack (Jahnke et al. 1989).

The current study does not provide direct evidence of either a titin or cross‐bridge mechanism. However, the study findings are consistent with the hypothesis that muscle contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles. The bonds are either long‐lasting or they continue to be re‐formed.

Effect of isometric contraction

One of the manuscript reviewers pointed out that our observation of shorter slack lengths after contract‐short conditioning than hold‐short conditioning appears to be inconsistent with the observation made in previous studies (e.g. Herzog & Leonard, 2002; Lee & Herzog, 2002; Hahn et al. 2007) that conditioning with isometric contractions did not increase passive force over that observed when the muscle was rested at the same length. However, unlike the isometric contractions in those studies, which were performed above muscle slack length, the isometric contraction performed in the contract‐short condition in our study was performed at a very short muscle length – shorter than the slack muscle length observed after contract‐long or stretch conditioning. When, in the contract‐long condition, contractions were performed at long muscle lengths, muscle slack lengths were longer than those observed after contract‐short conditioning and similar to those observed after stretch conditioning. It is possible that if previous studies had conditioned the muscle with an isometric contraction at a muscle length that was shorter than the final muscle length they would have observed passive force enhancement. It is possible that isometric contraction at short lengths causes passive force enhancement at longer lengths – if so, this would imply that it is contraction at short lengths rather than active stretch that is the necessary precursor to the development of passive force enhancement. This prediction is consistent with the findings of Hagbarth et al. (1995) who showed that isometric contractions at short lengths removed the slack created by prior stretch and the findings of Jahnke et al. (1989) who showed that muscle stiffness could be increased by conditioning with isometric contractions at short lengths.

Implications

A practical implication of these findings is that studies or clinical tests which investigate passive properties of muscles or muscle fibres, or phenomena that might be influenced by the existence of slack in muscles (such as position sense, stretch reflexes or twitch properties), should control for the history of the muscle using a pre‐conditioning paradigm to set the muscle to a control state. The same point was made by Proske and colleagues many years ago (Proske et al. 1993).

The observation that the knee can be flexed more than 20° without changing the length of muscle fascicles in the vastus lateralis strongly suggests that the vastus lateralis muscle fascicles fall slack at short muscle lengths. This does not imply that there is no resistance to knee joint rotation over this range. It is likely that resistance to knee flexion is provided by other knee extensor muscles which have shorter slack lengths or by periarticular connective tissues.

Conclusion

The slack length of human muscles and muscle fascicles is not a fixed property, but instead depends on the length and contraction history of the muscle. Contraction at short lengths causes the formation of bonds which reduce the effective length of structures that generate passive tension in muscles.

Additional information

Competing interests

The authors report no competing interests.

Author contributions

P.S., L.W., S.G. and R.H. conceived and designed the research; P.S., A.D. and R.H. performed experiments; P.S. analysed the data; P.S. and R.H. graded the video quality; P.S., M.H. and R.H. graded the fascicle length vs. knee angle curves; M.H. created the algorithm to determine slack lengths; B.B. assisted in the generation of MATLAB scripts for data analysis; P.S. prepared the figures; P.S. prepared the supplementary video; P.S. and R.H. drafted the manuscript; P.S., L.W., A.D., M.H., B.B., S.G. and R.H. edited and revised the manuscript. All authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Funding

The study was supported by the Australian National Health and Medical Research Council (NHMRC; Program Grant APP1055084). R.H. and S.G. are supported by NHMRC research fellowships. A.D. is supported by the Royal Freemasons’ Benevolent Institution Scholarship for Postgraduate Research Studies.

Supporting information

Disclaimer: Supporting information has been peer‐reviewed but not copyedited.

Video S1. Video showing example ultrasound image sequences for contract‐short (top left), contract‐long‐weak (top right), contract‐long (bottom left) and stretch (bottom right) conditions from one subject.

Click here for additional data file. (16.2MB, avi)

Biography

Peter Stubbs graduated with a BSc in Sport and Exercise Science from Auckland University, a Master of Physiotherapy from Sydney University and a PhD in Neurophysiology from Aalborg University. Following his PhD, he volunteered in the Philippines before returning to academia where he conducted post‐doctoral research at Neuroscience Research Australia and Hammel Neurorehabilitation Centre. He has an interest in muscle physiology, neurophysiology and neurorehabilitation.

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Edited by: Scott Powers & Paul Greenhaff

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Associated Data

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Supplementary Materials

Disclaimer: Supporting information has been peer‐reviewed but not copyedited.

Video S1. Video showing example ultrasound image sequences for contract‐short (top left), contract‐long‐weak (top right), contract‐long (bottom left) and stretch (bottom right) conditions from one subject.

Click here for additional data file. (16.2MB, avi)

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