Compliance of the muscle-tendon complex decreases the shortening velocity of myofiber compared with the shortening velocity of the muscle-tendon complex (11,12). This results in increased force generation relative to myofiber activation (16,17) and is felt to occur because the nonrigidity of the muscle-tendon complex allows storage of elastic energy and increased force-generating potential (35). Investigations involving stretch-shortening cycles have indicated that, during walking and vertical jumping, medial gastrocnemius muscle contracts very close to isometrically (unchanging muscle length) during the push-off phase of the movement (13,14,26,27). This allows a greater stretch and storage in compliant tendon and increased force generation (4,13,29,36). Because isometric muscle actions require about one-third as much energy expenditure as concentric muscle actions matched for force levels (38), more economical walking and jumping would be expected when the muscle-tendon complex is more compliant. Indeed, in long distance runners greater compliance of the quadriceps tendon is related to improved running economy at different speeds (2).
Numerous factors potentially influence the compliance of the muscle-tendon complex, but two that have received little interest are length and thickness of tendon and flexibility of involved joints. Theoretically, a longer tendon should have more potential for stretch, storage of elastic energy, and, ultimately, more force generation during the subsequent concentric muscle action (33). Although thinner tendons (smaller cross sections) will store and return less mechanical energy for the same stretch, thinner tendons have more stress, strain, and stretch for any given force, favoring increased elastic storage when force is held constant (6). Comparative physiology research suggests that animals such as the wallaby (5) and turkey (6) that have relatively long tendons are very economical in locomotion, especially while hopping, an activity that induces greater gravity-induced downward forces and thus more stretch-shortening cycle potentiation. In addition, we have recently shown that Achilles tendon length is inversely related to net oxygen uptake of premenopausal women while walking at a moderate speed (33). Because running includes an airborne phase and more gravity-induced downward force, stretch-shortening cycle potentiation should be at least as large if not larger during running as during walking.
Another factor that could affect muscle-tendon compliance and facilitate the elastic stretch during the eccentric phase of biped locomotion is the nonpathological muscle tightness of a joint (19). Less flexible joints have been reported to be inversely related to running oxygen uptake (8,15,23). In addition, we have recently shown that both changes in knee extension and plantarflexion flexibility are inversely related to changes in walking net oxygen uptake (19).
Muscle around the ankle and knee joints contribute >70% of the total mechanical work during running (1,28). In addition, during running, energy stored in the tendon and aponeurosis of the triceps surae and quadriceps are estimated to store 75% of the energy stored in all the tendons active in running (39). This suggests that compliance of these two muscle tendon complexes may be particularly important for generation of stretch-shortening cycle potentiation during running. To our knowledge, no study has investigated the independent relationships of knee/ankle flexibility, quadriceps/patella tendon length, and Achilles tendon length and thickness with walking and running economy. The purpose of this study was to do so in a group of recreational distance runners. We hypothesize that tendon length and thickness, as well as joint flexibility, will be related to walking and running economy.
Twenty-one male recreational distance runners served as a sample of convenience in this study. Subjects all had completed either a 10-km or a marathon within the last 6 months. Subjects were between 24 and 40 yr and reported no health problems on a health history questionnaire. None were taking any medication known to affect body composition, energy expenditure, or exercise performance. The study was approved by the University of Alabama at Birmingham (UAB) institutional review board. All volunteers were screened and briefed about the experimental protocol, and written informed consent was obtained before testing. Protocol required three visits to UAB labs: 1) screening and body composition, 2) magnetic resonance imaging, and 3) walking/running economy and maximum oxygen uptake tests.
Body fat percentage was determined by DXA (DPX-L; Lunar Radiation Corp., Madison, WI). The scans were analyzed using the Adult Software (Version 1.33).
Flexibility, seated resting, and submaximal walking and running oxygen uptake.
Between 4:00 and 5:00 p.m. and after a 3- to 4-h fast, subjects reported to the laboratory. Flexibility was measured using adaptations of methods described by Norkin and White (34). One investigator stabilized and stretched the tendon-muscle complexes while a second investigator measured angles with a goniometer. Flexibility of the ankle plantar flexors was measured with the subject lying supine and the knee extended. Goniometer fulcrum was placed over the lateral aspect of the lateral malleolus. The proximal arm was aligned with the head of fibula, and the distal arm was aligned parallel to the lateral aspect of the fifth metatarsal. Measurement of passive plantar flexor flexibility occurred with the investigator pushing on the plantar surface of the foot toward a dorsiflexion position. The knee was stabilized to prevent movement, and pressure was not placed on the toes throughout measurement of plantar flexor flexibility. Measurement of passive flexibility of the knee extensors occurred with the subjects lying supine and the leg extending off the end (approximately 6 inches superior to knee) of a specially designed table. The center of the fulcrum of the goniometer was placed over the lateral epicondyle of the femur with the proximal arm aligned with the greater trochanter, and the distal arm was aligned with lateral malleolus. Gripping the lower leg just above the ankle, one investigator pushed the lower leg toward a more flexed position while the other investigator maintained hip position. In all flexibility evaluations, the investigator stopped the stretch when a firm end feel was noted (34).
Oxygen uptake (V˙O2) measurement.
Resting as well as walking/running volume of oxygen and carbon dioxide were measured continuously and analyzed using a MAX-II CART metabolic system (Physio-Dyne Instrument Co., Quogue, NY) that was calibrated for volume with a 3-L syringe and known calibration gases before each test. Subjects sat in a chair and rested quietly for 10 min before each resting oxygen uptake test. Expired gases were then measured for 10 min while the subjects continued to sit quietly. The MAX-II CART metabolic system was also used in all exercise tests. About 5 min after the resting oxygen uptake test, subjects walked on the treadmill at 3 mph for 4-5 min while oxygen uptake was measured. As soon as steady state was reached in the walking task (two consecutive minutes with no rise in oxygen uptake), the speed was increased to 6 mph for 4-5 min. When steady state was reached, the speed was increased to 7 mph for 4-5 min until steady state was reached. The walking speed was calibrated before the treadmill test, and calibration was completed during the first minute for each of the two running tasks. After the 7-mph running task, the subject rested for 5-10 min before beginning the maximal oxygen uptake test. Net V˙O2 for the walking and two running tasks was calculated by subtracting the minute values of seated resting V˙O2 from the steady-state 4.84 kph walking V˙O2 (NVOWK), 9.68 kph (6-mph run, NVO6), and 11.29 kph.
Maximum oxygen uptake.
V˙O2max (maximum oxygen uptake) was determined by indirect calorimetry (MAX-II CART) on a treadmill using a protocol previously developed for trained runners (30,37). Briefly, the subjects ran for 1 min at each intensity, which started at 6 mph. Each minute, the subject was offered the choice of increasing speed (0.5 mph) or grade (2.5% grade). The subjects ran until voluntary exhaustion. All subject reached at least two criteria for achieving V˙O2max (plateauing of V˙O2, RER >1.2, and HR within 10 beats of age-predicted maximum).
Both upper and lower leg lengths were measured in a sitting position with foot flat on the floor using methods adapted from Martin et al. (31). Upper leg length was measured from the distal edge of the greater trochanter to the proximal lateral tibial border. Lower leg length was measured from the top of the patella to the proximal surface of the lateral malleolus.
Tendon length/cross section volume.
Measurements of tendon length and area was determined using a three-dimensional volumetric T1-weighted turbo field echo imaging sequence (T1TFE) and T1-weighted turbo spin echo imaging sequences (TSE) using a 1H transmit/receive torso phased-array coil on a 3-T Philips Achieva system. Overall, we collected a series of scout images followed by a set of separate coronal, sagittal, and axial scans from below the patient's foot to above the maximum cross-sectional area of the subject's thigh (i.e., slightly below the patients groin). The complete set of images was broken into four segments or regions corresponding to the various anatomical locations from the subject's foot up to their groin.
Region 1 was defined as slightly below the subject's foot to approximately midcalf. In region 1, we collected a set of 32 coronal images (T1TFE, flip angle = 8°, 32 contiguous slices, slice thickness = 2 mm, repetition time (TR) = 8.068 ms, echo time (TE) = 4.60 ms, echo train length (ETL) = 160, acquisition matrix = 160 × 160, reconstructed matrix = 256 × 256, field of view (FOV) = 250 mm × 250 mm), 32 sagittal images (T1TFE, flip angle = 8°, 32 contiguous slices, slice thickness = 2 mm, TR = 8.068 ms, TE = 4.60 ms, ETL = 160, acquisition matrix = 160 × 160, reconstructed matrix = 256 × 256, FOV = 250 mm × 250 mm), and 48 axial images (TSE, flip angle = 90°, 48 contiguous slices, slice thickness = 5 mm, TR = 800 ms, TE = 15 ms, ETL = 3, acquisition matrix = 153 × 192, reconstructed matrix = 256 × 256, FOV = 160 mm × 160 mm). The coronal and sagittal images were used for tendon length measurements through the subject's ankle and lower calf region. The axial images in this region were used for tendon and muscle cross-sectional area measurements.
Region 2 was defined as an area from midcalf through the patient's knee. In region 2, we collected a series of 54 axial images (TSE, flip angle = 90°, 54 contiguous slices, slice thickness = 5 mm, TR = 800 ms, TE = 15 ms, ETL = 3, acquisition matrix = 153 × 192, reconstructed matrix = 256 × 256, FOV = 160 mm × 160 mm). The axial images in this region were used for muscle cross-sectional area measurements and were combined with the axial images from region 1 to provide a contiguous cross-sectional view of the subject's lower leg (i.e., from below the foot to slightly above the knee).
Region 3 was defined as an area slightly below the subject's knee to approximately midthigh. In region 3, we collected a set of 32 sagittal images (T1TFE, flip angle = 8°, 32 contiguous slices, slice thickness = 2 mm, TR = 8.068 ms, TE = 4.60 ms, ETL = 160, acquisition matrix = 160 × 160, reconstructed matrix = 256 × 256, FOV = 250 mm × 250 mm) and 54 axial images (TSE, flip angle = 90°, 54 contiguous slices, slice thickness = 5 mm, TR = 800 ms, TE = 15 ms, ETL = 3, acquisition matrix = 153 × 192, reconstructed matrix = 256 × 256, FOV = 160 mm × 160 mm). The sagittal images were used for tendon length measurements through the knee and lower thigh regions. The axial images in this region were used for tendon and muscle cross-sectional area measurements.
Finally, region 4 was defined as an area slightly above the subject's knee to above the subject's maximum cross-sectional area of their thigh muscle (i.e., slightly below the subject's groin). In region 4, we collected a series of 42 axial images (TSE, flip angle = 90°, 42 contiguous slices, slice thickness = 5 mm, TR = 800 ms, TE = 15 ms, ETL = 3, acquisition matrix = 153 × 192, reconstructed matrix = 256 × 256, FOV = 200 mm × 200 mm). The axial images in this region were used for tendon cross-sectional area measurements and were combined with the axial images from region 3 to provide a contiguous cross-sectional view. All acquisition parameters were selected to optimize the signal intensity contrast between muscle, fat, and tendon and to reduce the scan time as much as possible.
All were analyzed by importing the resulting DICOM images into the ImageJ software package, and regions of interest and length measurements were manually drawn around the tendons and outlines of the muscles of interest. Quadriceps/patella tendon length was measured from the distal attachment of the inferior rectus femoris on the quadriceps tendon to the tibialis tuberosity (quadriceps tendon + patella height + infrapatella tendon). Achilles tendon length was measured from the distal attachment of the medial gastrocnemius on the Achilles to the superior border of the calcaneus. Test-retest analysis of Achilles tendon length and thickness on five subjects yielded a coefficient of variation of 1.1% and 1.8%, respectively.
Means and SD were calculated for all study variables. Note that age, height, lower leg length, upper leg length, weight, and % fat with walking and running economy because all these variables have the potential to be confounders with the variables of primary interest. Pearson product correlations were calculated between all the five economy measures (NVOWK, NVO6, and NVO7) with anthropometric and flexibility measures. Multiple linear regression was used to identify relationships between economy measures (dependent variables) with tendon length and flexibility measures independent of each other. Age, height, lower leg length, upper leg length, weight, and percent fat were not included in any of the multiple regression models because none of these variables were significantly related to any of the economy measures. Significance was set at a probability of 0.05. All analyses were done using Statistical Package for the Social Sciences, version 10 (SPSS, Chicago, IL).
Means and SD for all variables are shown in Table 1. Subjects were relatively young (31.9 ± 4.7 yr), with a V˙O2max of 54.7 ± 7.5 mL O2·kg−1·min−1.
No significant correlations were found between any of the anthropometric variables and walking, running, or delta oxygen uptake (Table 2). Achilles tendon length was significantly related to NVOWK (r = −0.46, P = 0.02), NVO6 (r = −0.51, P = 0.01), and NVO7 (r = 0.40, P = 0.04) (Table 2). Achilles tendon cross section area was unrelated to any walking or running oxygen uptake. Quadriceps/patella tendon length was significantly related to NVO7 (r = −0.43, P = 0.03). Significance was approached for the correlation between quadriceps/patella tendon length and NVO6 (r = −0.36, P = 0.06), and no relationship was seen between quadriceps/patella tendon length and NVOWK. Knee extension flexibility was unrelated to any economy measure, whereas plantarflexion flexibility was not significantly related to any economy measure except NVO7 (r = 0.38, P = 0.05). However, the correlations between plantarflexion flexibility and NVO6 (r = 0.31, P = 0.09) and NVOWK (r = 0.33, P = 0.08) approached significance.
Multiple linear regression was used to examine the independent relationship of the lengths of the two involved tendons, quadriceps/patella and Achilles, as well as the two involved joints, flexibility of the knee extensors and plantar flexors (Table 3). Variables that either approached significance or were significant correlates with economy measures were combined as independent variables in models of the pertinent individual economy measure. No significant multiple regression model of locomotion economy was found when quadriceps/patella tendon length was included in the regression. Therefore, no models are presented that include quadriceps/patella tendon length. Achilles tendon length was a significant independent correlate (partial r varying from −0.53, P = 0.03 to −0.64, P < 0.01) for measures of running economy whether the adjusting variable was knee or plantar flexor flexibility (Table 3). Flexibility of the knee extensors was independently related to NVO6 (partial r = 0 46, P = 0.05) while flexibility of the Achilles tendon was independently related to NVO6 (partial r = 0.51, P = 0.03) and NVO7 (partial r = 0.53, P = 0.02). Figures 1-3 graphically display the relationship between log of NVO6 and regression model studentized residuals (dependent variables flexibility of plantar flexors and log of lateral gastrocnemius tendon length) as well as partial correlation plots for both Achilles tendon length and flexibility of the plantar flexors.
The main finding of this study is that a longer Achilles tendon and reduced flexibility of the plantar flexors are independently related to running economy (Table 3 and Fig. 1). Although compliance was not measured, these results are suggestive that individuals who have longer less flexible tendons have a more compliant ankle plantar flexor muscle-tendon complex, thus increasing stretch-shortening cycle potentiation and running economy. Although we have previously shown that Achilles tendon length is positively related to walking economy (33), to our knowledge, this is the first study to show this with running economy in humans. Also several investigators have shown that running economy is related to reduced flexibility in several joints (8,15,23), but none have shown that this relationship is independent of tendon length.
During running, the legs absorb energy as the center of mass falls and slows in the first part of the step. Work must be done to elevate the center of mass in the second part of the step (35). Although actin-myosin interaction accounts for much of the work performed, recovery of elastic energy previously stored during the first part of the step allows work to be performed on the center of mass that does not require use of ATP. Available data suggest that only relatively small amounts of elastic energy are recovered from myosin cross-bridges (6). Much more stored elastic energy is recovered from aponeuroses and tendons (6). Muscles with long external tendons have more potential for developing elastic energy recovery. Thinner tendons (smaller cross sections) have more stress and strain for any given force, favoring increased elastic storage (6). Theoretically, relatively short muscles with longer and thinner tendons should be conducive for increased stretch-shortening cycle potentiation (6). Economical running of different animal models including turkeys (6) and wallabies (5) supports this premise. We did not observe any relationship between walking or running economy and Achilles tendon cross section, although we expected a positive relationship between tendon cross section and running net V˙O2 (decreased economy). The relationship did not exist even when adjusting the tendon cross sections for the subject's weight. We do not have an explanation for this. The supporting data for tendon cross section being important for determining tendon strain energy savings are derived when comparing different animal models (6). Perhaps elastic strain energy savings across the tendon thickness ranges observed in this sample of recreational runners are relatively insensitive to tendon thickness differences. It is possible that faster running speeds that increase force and elastic strain on the tendon may be required to observe the expected relationship with running economy.
Several investigators have previously shown that lower limb inflexibility is related to improved running economy (8,15,23), suggesting that elastic energy storage and reuse may be enhanced in a tighter muscle-tendon complex. Of course, it is possible that energy requirements might be reduced in a tighter joint by increasing stability of that joint, thus diminishing the need for energy consuming muscle stabilization. However, several studies support the premise that elastic characteristics are enhanced in stiffer tissues (3,9,25). Whatever the cause, we found that both plantar flexor and knee extensor joint flexibility were both positively and independently related to net V˙O2 during running (inversely related to running economy). We have also found previously that 1-yr changes in plantar flexor and knee extensor flexibility are negatively related to walking economy (19).
A number of factors not included in this study probably also affect running economy. Increased type I/decreased type II muscle fiber type is associated with increased isometric (20,24), walking (20), knee extension (7), and stationary cycling (7) economy. Presumably muscle fiber type would also have some effect on running economy. We have shown that resistance training induced increases in strength are associated with improved walking economy (19), whereas several other studies have shown that resistance training increases mediate increases in work (21), cross-country skiing (18), and running (10,22). Shorter muscle fascicles and greater pennation may affect economy in various activities including running (35). Considering all the factors that can affect running economy, it is somewhat surprising that we were able to explain as much variance in running economy as we did. The coefficient of determination for two of the models indicate that almost 50% of the variance in running economy was explained with Achilles tendon length and either plantar flexor or knee extensor flexibility.
Because combustion of lipid yields less energy per liter of oxygen consumed than CHO (32), a potential confounder exists when using oxygen uptake as a measure of exercise economy. To ensure that this potential confounder did not inappropriately cause relationships between Achilles tendon, length and joint flexibility with running economy respiratory quotation was used as an adjusting variable in multiple regression analysis. Resulting partial correlations were very similar and in no case changed the significance level showing that differences in lipid/CHO oxidation rates did not explain the significant relationships between tendon length and joint flexibility with running economy.
Quadriceps/patella tendon length was only significantly related to running economy for the 7-mph run and the delta walk to the 6-mph run. It was not significantly related to any of the running economy variables after adjusting for joint flexibility, although the adjusted betas were at least supporting a trend toward improved running economy (adjusted betas varying from −0.27 to −0.39 with net oxygen uptake). The sample size is too small to have sufficient power to accept the null, i.e., no independent relationship exists between quadriceps/patella tendon length and running economy. However, taken together, it can be assumed that the relationship for quadriceps/patella tendon length with running economy, if it exists, is at least weaker than the relationships with Achilles tendon length. This would be expected for several reasons. First, pennation angle of the plantarflexion muscles (soleus ∼25%, medial gastrocnemius ∼17%, and lateral gastrocnemius ∼8%) is much greater than pennation angle of the quadriceps (∼5% for all) (40). More highly pennate muscles with shorter fascicles favor greater compliance and improved economy (6,35). Second the almost 85% longer Achilles tendon length compared with the quadriceps/patella tendon length observed (Table 1) should favor greater potential for storing elastic energy.
Consistent with our previous work in a group of premenopausal women (33), we found that Achilles tendon length is related to decreased net oxygen uptake (increased economy) during walking (33). That was not the case for quadriceps/patella tendon length where no relationship was observed. Again indicating that of the two tendons, the Achilles tendon may have a greater potential for providing stretch-shortening cycle potentiation during walking and running than the quadriceps/patella tendon.
It is a little surprising that neither tendon length or joint flexibility was related to delta 6- to 7-mph run although both Achilles and quadriceps/patella tendon length as well as flexibility of the knee extensors were related to delta walk to 6-mph run. The difference in oxygen uptake was relatively small for the delta 6- to 7-mph run (6.6 mL·kg−1·min−1) compared with the delta walk to 6-mph run (22 mL·kg−1·min−1), so the lack of relationship might simply be a truncated delta V˙O2. A more intriguing possible explanation involves changes in biomechanics that occur between walking and running. Because runners are airborne, greater velocities would be achieved in the descent phase requiring higher eccentric forces to stop the rapid descent. This would be expected to have greater potential for stretching elastic tissues and producing more stretch-shortening potentiation, similar to what is observed when a drop jump is compared with a countermeasure jump. Of course, there may other equally plausible explanations for the lack of a relationship between tendon length and joint flexibility with delta 6- to 7-mph run.
In conclusion, lower limb tendon length, especially Achilles tendon length, is associated with improved running economy in male recreational distance runners. In addition, plantarflexion and knee extension flexibility are negatively related to running economy. Taken together, these data support the premise that longer lower limb tendons and less flexible lower limb joints are associated with increased joint compliance while running and improved running economy.
The authors thank David Bryan, Bob Petri, and Paul Zuckerman for help in data acquisition. The UAB Department of Human Studies supplied funding. No external funding was used, and there is no conflict of interest for any of the authors.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2011The American College of Sports Medicine
NET OXYGEN UPTAKE; WALKING ECONOMY; ACHILLES TENDON; STRETCH CYCLE SHORTENING POTENTIATION