Jens Bojsen-Møller

Habitual loading results in tendon hypertrophy and increased stiffness of the human patellar tendon

The purpose of this study was to examine patellar tendon (PT) size and mechanical properties in subjects with a side-to-side strength difference of > or =15% due to sport-induced loading. Seven elite fencers and badminton players were included. Cross-sectional area (CSA) of the PT obtained from MRI and ultrasonography-based measurement of tibial and patellar movement together with PT force during isometric contractions were used to estimate mechanical properties of the PT bilaterally. We found that distal tendon and PT, but not mid-tendon, CSA were greater on the lead extremity compared with the nonlead extremity (distal: 139 +/- 11 vs. 116 +/- 7 mm(2); mid-tendon: 85 +/- 5 vs. 77 +/- 3 mm(2); proximal: 106 +/- 7 vs. 83 +/- 4 mm(2); P < 0.05). Distal tendon CSA was greater than proximal and mid-tendon CSA on both the lead and nonlead extremity (P < 0.05). For a given common force, stress was lower on the lead extremity (52.9 +/- 4.8 MPa) compared with the nonlead extremity (66.0 +/- 8.0 MPa; P < 0.05). PT stiffness was also higher in the lead extremity (4,766 +/- 716 N/mm) compared with the nonlead extremity (3,494 +/- 446 N/mm) (P < 0.05), whereas the modulus did not differ (lead 2.27 +/- 0.27 GPa vs. nonlead 2.16 +/- 0.28 GPa) at a common force. These data show that a habitual loading is associated with a significant increase in PT size and mechanical properties.

Assessment of satellite cell number and activity status in human skeletal muscle biopsies

The primary aim of our study was to validate the assessment of myonuclear and satellite cell number in biopsies from human skeletal muscle. We found that 25 type I and 25 type II fibers are sufficient to estimate the mean number of myonuclei per fiber. In contrast, the assessment of satellite cells improved when more fibers were included. Second, we report that small differences in counting satellite cells using CD56 and Pax7 antibodies can be attributed to the different staining profiles. Third, we provide support for the use of Ki67 in evaluating the proportion of active satellite cells. We observed very few (up to 1.3%) active satellite cells in healthy adult skeletal muscle at rest, but they increased significantly (up to 7‐fold) following muscle activity. This study provides valuable tools to assess the behavior of satellite cells, both in pathological conditions and in response to physiological stimuli. Muscle Nerve 40: 455–465, 2009

Sequenced response of extracellular matrix deadhesion and fibrotic regulators after muscle damage is involved in protection against future injury in human skeletal muscle

The purpose of this study was to test the hypothesis that remodeling of skeletal muscle extracellular matrix (ECM) is involved in protecting human muscle against injury. Biopsies were obtained from medial gastrocnemius muscles after a single bout of electrical stimulation (B) or a repeated bout (RB) 30 d later, or 30 d after a single stimulation bout (RBc). A muscle biopsy was collected from the control leg for comparison with the stimulated leg. Satellite cell content, tenascin C, and muscle regeneration were assessed by immunohistochemistry; real‐time PCR was used to measure mRNA levels of collagens, laminins, heat‐shock proteins (HSPs), inflammation, and related growth factors. The large responses of HSPs, CCL2, and tenascin C detected 48 h after a single bout were attenuated in the RB trial, indicative of protection against injury. Satellite cell content and 12 target genes, including IGF‐1, were elevated 30 d after a single bout. Among those displaying the greatest difference vs. control muscle, ECM laminin‐β1 and collagen types I and III were elevated ~6‐ to 9‐fold (P<0.001). The findings indicate that the sequenced events of load‐induced early deadhesion and later strengthening of skeletal muscle ECM play a role in protecting human muscle against future injury.—Mackey, A. L., Brandstetter, S., Schjerling, P., Bojsen‐Moller, J., Qvortrup, K., Pedersen, M. M., Doessing, S. Kjaer, M., Magnus‐son, S. P., Langberg, H. Sequenced response of extracellular matrix deadhesion and fibrotic regulators after muscle damage is involved in protection against future injury in human skeletal muscle. FASEB J. 25, 1943‐1959 (2011). www.fasebj.org

Effect of habitual exercise on the structural and mechanical properties of human tendon, in vivo, in men and women.

We examined whether long‐term habitual training (a) was associated with differences in structural and mechanical properties in tendon in women and (b) yielded different tendon properties in men and women. Ten male runners, 10 female runners and 10 female non‐runners were tested. Tendon cross‐sectional area (CSA) and length of the patellar and Achilles tendon were determined with MRI. Ultrasonography‐based measurement of tendon elongation and force during isometric contractions provided mechanical properties. Distal patellar and Achilles tendon CSAs were greater than the proximal part in all three groups (P<0.05). Weight‐normalized Achilles tendon CSA were similar in trained (2.78±0.17 mm2/kg3/4) and untrained women (2.60±0.13 mm2/kg3/4), while that in trained men (3.77±0.27 mm2/kg3/4) was greater compared with trained women (P<0.01). Patellar tendon CSA were comparable in trained and untrained women, while that in trained men was greater compared with trained women (P<0.01). Patellar tendon stiffness was greater in male runners (3528±773 N/mm) compared with female runners (2069±666 N/mm) and non‐runners (2477±381 N/mm), (P<0.01), but patellar tendon deformation, stress, strain and modulus were similar. These data indirectly suggest that the ability of Achilles and patellar tendons to adapt in response to habitual loading such as running is attenuated in women.

Evidence of skeletal muscle damage following electrically stimulated isometric muscle contractions in humans.

It is unknown whether muscle damage at the level of the sarcomere can be induced without lengthening contractions. To investigate this, we designed a study where seven young, healthy men underwent 30 min of repeated electrical stimulated contraction of m. gastrocnemius medialis, with the ankle and leg locked in a fixed position. Two muscle biopsies were collected 48 h later: one from the stimulated muscle and one from the contralateral leg as a control. The biopsies were analyzed immunohistochemically for inflammatory cell infiltration and intermediate filament disruption. Ultrastructural changes at the level of the z-lines were investigated by transmission electron microscopy. Blood samples were collected for measurement of creatine kinase activity, and muscle soreness was assessed in the days following stimulation. The biopsies from the stimulated muscle revealed macrophage infiltration and desmin-negative staining in a small percentage of myofibers in five and four individuals, respectively. z-Line disruption was evident at varying magnitudes in all subjects and displayed a trend toward a positive correlation (r = 0.73, P = 0.0663) with the force produced by stimulation. Increased muscle soreness in all subjects, combined with a significant increase in creatine kinase activity (P < 0.05), is indirectly suggestive of muscle damage, and the novel findings of the present study, i.e., 1) macrophages infiltration, 2) lack of desmin staining, and 3) z-line disruption, provide direct evidence of damage at the myofiber and sarcomere levels. These data support the hypothesis that muscle damage at the level of the sarcomere can be induced without lengthening muscle contractions.

Intermuscular force transmission between human plantarflexor muscles in vivo

The exact mechanical function of synergist muscles within a human limb in vivo is not well described. Recent studies indicate the existence of a mechanical interaction between muscle actuators that may have functional significance and further play a role for injury mechanisms. The purpose of the present study was to investigate if intermuscular force transmission occurs within and between human plantarflexor muscles in vivo. Seven subjects performed four types of either active contractile tasks or passive joint manipulations: passive knee extension, voluntary isometric plantarflexion, voluntary isometric hallux flexion, passive hallux extension, and selective percutaneous stimulation of the gastrocnemius medialis (MG). In each experiment plantar- and hallux flexion force and corresponding EMG activity were sampled. During all tasks ultrasonography was applied at proximal and distal sites to assess task-induced tissue displacement (which is assumed to represent loading) for the plantarflexor muscles [MG, soleus (SOL), and flexor hallucis longus (FHL)]. Selective MG stimulation and passive knee extension resulted in displacement of both the MG and SOL muscles. Minimal displacement of the triceps surae muscles was seen during passive hallux extension. Large interindividual differences with respect to deep plantarflexor activation during voluntary contractions were observed. The present results suggest that force may be transmitted between the triceps surae muscles in vivo, while only limited evidence was provided for the occurrence of force transfer between the triceps surae and the deeper-lying FHL.

Measuring mechanical properties of the vastus lateralis tendon-aponeurosis complex in vivo by ultrasound imaging.

The mechanical properties of the human vastus lateralis (VL) tendon‐aponeurosis complex were investigated in eight male subjects. Knee extensor force, knee joint angle, and corresponding longitudinal VL aponeurosis displacement were monitored synchronously during graded (10‐s) maximal isometric knee extension contractions. Displacement observed during isometric conditions may be regarded as an expression of deformation in the tissues distal to the measurement site. Furthermore, aponeurosis displacement was measured during passive knee extension (90–75°°), and used to correct displacement values obtained during active contraction for joint angular motion. The passive trial yielded a highly linear relationship between aponeurosis displacement and joint angular motion (r2 = 0.998 ± 0.002) with a mean correction factor of 0.41 ± 0.10 mm/degree. Maximal knee extensor force was 5834 ± 1341 N with a corresponding VL aponeurosis displacement of 12.7 ± 2.5 mm, while correcting for joint angular motion reduced maximal displacement ∼9% (to 11.6 ± 2.5 mm, P < 0.005) (data presented as means ± SD). Two separate graded contraction trials were performed, and no between‐trial differences were observed in either maximal force or maximal displacement. Between trial coefficient of determination and CV for maximal force and maximal displacement were r2 = 0.97, CV = 2.9% and r2 = 0.92, CV = 4.6%, respectively, indicating intra‐day reproducibility of measurements. These data demonstrate that when applying the newly established ultrasound‐based method of investigating quadriceps connective tissue mechanical properties, maximal isometric contraction is inevitably associated with some joint angular motion that significantly influences the calculations.

NET EXCITATION OF THE MOTOR UNIT POOL VARIES WITH LOAD TYPE DURING FATIGUING CONTRACTIONS

To identify the underlying physiological mechanisms for the difference in the time to failure for two types of fatiguing contractions, 20 subjects performed force and position tasks with the elbow flexor muscles at a comparable net muscle torque for a similar duration. Prior to terminating each task, blood flow was occluded to estimate the relative amount of feedback transmitted by small‐diameter afferents to the spinal cord. Mean arterial pressure at the conclusion of the fatiguing contraction increased similarly for the two tasks (force: 119% ± 14%; position: 114% ± 15%). However, the final values for the electromyographic activity for the elbow flexor muscles (26% ± 14% and 21% ± 11%, respectively; P < 0.05), and the increase in the fluctuations in acceleration and force (225% ± 152% and 154% ± 53%, respectively; P < 0.05) in the sagittal plane, were significantly greater during the position task compared with the force task. These results suggest a different balance in the excitatory and inhibitory inputs to the spinal motor neurons for the two tasks, which has implications for the design of work tasks and exercise prescription in rehabilitation. Muscle Nerve, 2005

Ultrasound-based testing of tendon mechanical properties: a critical evaluation

In the past 20 years, the use of ultrasound-based methods has become a standard approach to measure tendon mechanical properties in vivo. Yet the multitude of methodological approaches adopted by various research groups probably contribute to the large variability of reported values. The technique of obtaining and relating tendon deformation to tensile force in vivo has been applied differently, depending on practical constraints or scientific points of view. Divergence can be seen in 1) methodological considerations, such as the choice of anatomical features to scan and to track, force measurements, or signal synchronization; and 2) in physiological considerations related to the viscoelastic behavior or length measurements of tendons. Hence, the purpose of the present review is to assess and discuss the physiological and technical aspects connected to in vivo testing of tendon mechanical properties. In doing so, our aim is to provide the reader with a qualitative analysis of ultrasound-based techniques. Finally, a list of recommendations is proposed for a number of selected issues.

Mechanical properties of the patellar tendon in elite volleyball players with and without patellar tendinopathy

Background Although differences in mechanical properties between symptomatic and healthy tendons have been observed for the Achilles tendon, the impact of tendinopathy on patellar tendon mechanics is not fully documented. The aim of the present case–control study was to assess the mechanical properties of the tendon and jump performance in elite athletes with and without patellar tendinopathy. Methods We identified 17 male volleyball players with patellar tendinopathy and 18 healthy matched controls from a 5-year prospective cohort study on junior elite volleyball players. Outcome variables included three measures of maximal vertical jump performance and ultrasound-based assessments of patellar tendon cross-sectional area, stiffness and Youngs modulus. Results The proximal cross-sectional area of the patellar tendon was significantly larger in the tendinopathic group (133±11 vs 112±9 mm2, respectively; p<0.001). Pathological tendons presented lower stiffness (2254±280 vs 2826±603 N/mm, respectively; p=0.006) and Youngs modulus (0.99±0.16 vs 1.17±0.25 GPa, respectively; p=0.04) than healthy tendons. However, the difference between the countermovement jump height and the squat jump height (3.4±2.2 vs 1.2±1.5 cm, p=0.005) was significantly higher in the tendinopathic group compared with the control group. Conclusions Patellar tendinopathy is associated with a decrease in the mechanical and material properties of the tendon in elite athletes subjected to a high volume of jumping activity. However, compared with their healthy counterparts, tendinopathic volleyball players have a better ability to utilise the stretch-shortening cycle when jumping.