The evaluation of rate of force development during rapid contractions has recently become quite popular for characterising explosive strength of athletes, elderly individuals and patients. The main aims of this narrative review are to describe the neuromuscular determinants of rate of force development and to discuss various methodological considerations inherent to its evaluation for research and clinical purposes. Rate of force development (1) seems to be mainly determined by the capacity to produce maximal voluntary activation in the early phase of an explosive contraction (first 50–75 ms), particularly as a result of increased motor unit discharge rate; (2) can be improved by both explosive-type and heavy-resistance strength training in different subject populations, mainly through an improvement in rapid muscle activation; (3) is quite difficult to evaluate in a valid and reliable way. Therefore, we provide evidence-based practical recommendations for rational quantification of rate of force development in both laboratory and clinical settings.

/pdf/rate-of-force-development-physiological-and-methodological-5a4kwox9nh.pdf

Rate of force development: physiological and methodological considerations

A randomized-controlled single-blind trial was conducted to investigate the clinical, structural and functional effects of peritendinous corticosteroid injections (CORT), eccentric decline squat training (ECC) and heavy slow resistance training (HSR) in patellar tendinopathy. Thirty-nine male patients were randomized to CORT, ECC or HSR for 12 weeks. We assessed function and symptoms (VISA-p questionnaire), tendon pain during activity (VAS), treatment satisfaction, tendon swelling, tendon vascularization, tendon mechanical properties and collagen crosslink properties. Assessments were made at 0 weeks, 12 weeks and at follow-up (half-year). All groups improved in VISA-p and VAS from 0 to 12 weeks (Po0.05). VISA-p and VAS improvements were maintained at follow-up in ECC and HSR but deteriorated in CORT (Po0.05). In CORT and HSR, tendon swelling decreased ( � 13 � 9% and � 12 � 13%, Po0.05) and so did vascularization ( � 52 � 49% and � 45 � 23%, Po0.01) at 12 weeks. Tendon mechanical properties were similar in healthy and injured tendons and were unaffected by treatment. HSR yielded an elevated collagen network turnover. At the half-year follow-up, treatment satisfaction differed between groups, with HSR being most satisfied. Conclusively, CORT has good short-term but poor long-term clinical effects, in patellar tendinopathy. HSR has good short- and long-term clinical effects accompanied by pathology improvement and increased collagen turnover.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0838.2009.00949.x

Corticosteroid injections, eccentric decline squat training and heavy slow resistance training in patellar tendinopathy

The present article systematically reviews recent literature on the in vivo adaptation of asymptomatic human tendons following increased chronic mechanical loading, and meta-analyzes the loading conditions, intervention outcomes, as well as methodological aspects. The search was performed in the databases PubMed, Web of Knowledge, and Scopus as well as in the reference lists of the eligible articles. A study was included if it conducted (a) a longitudinal exercise intervention (≥8 weeks) on (b) healthy humans (18 to 50 years), (c) investigating the effects on mechanical (i.e., stiffness), material (i.e., Young’s modulus) and/or morphological properties (i.e., cross-sectional area (CSA)) of tendons in vivo, and was reported (d) in English language. Weighted average effect sizes (SMD, random-effects) and heterogeneity (Q and I
 2
 statistics) of the intervention-induced changes of tendon stiffness, Young’s modulus, and CSA were calculated. A subgroup analysis was conducted regarding the applied loading intensity, muscle contraction type, and intervention duration. Further, the methodological study quality and the risk of bias were assessed. The review process yielded 27 studies with 37 separate interventions on either the Achilles or patellar tendon (264 participants). SMD was 0.70 (confidence interval: 0.51, 0.88) for tendon stiffness (N=37), 0.69 (0.36, 1.03) for Young’s modulus (N=17), and 0.24 (0.07, 0.42) for CSA (N=33), with significant overall intervention effects (p<0.05). The heterogeneity analysis (stiffness: I
 2
 =30%; Young’s modulus: I
 2
 =57%; CSA: I
 2
 =21%) indicated that differences in the loading conditions may affect the adaptive responses. The subgroup analysis confirmed that stiffness adaptation significantly (p<0.05) depends on loading intensity (I
 2
 =0%), but not on muscle contraction type. Although not significantly different, SMD was higher for interventions with longer duration (≥12 weeks). The average score of 71±9% in methodological quality assessment indicated an appropriate quality of most studies. The present meta-analysis provides elaborate statistical evidence that tendons are highly responsive to diverse loading regimens. However, the data strongly suggests that loading magnitude in particular plays a key role for tendon adaptation in contrast to muscle contraction type. Furthermore, intervention-induced changes in tendon stiffness seem to be more attributed to adaptations of the material rather than morphological properties.

/pdf/human-tendon-adaptation-in-response-to-mechanical-loading-a-18uz7oc08u.pdf

Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults

The adaptive response of connective tissue to loading requires increased synthesis and turnover of matrix proteins, with special emphasis on collagen. Collagen formation and degradation in the tendon increases with both acute and chronic loading, and data suggest that a gender difference exists, in that females respond less than males with regard to an increase in collagen formation after exercise. It is suggested that estrogen may contribute toward a diminished collagen synthesis response in females. Conversely, the stimulation of collagen synthesis by other growth factors can be shown in both animal and human models where insulin-like growth factor 1 (IGF-I) and transforming growth factor-beta-1 (TGF-beta-1) expression increases to accompany or precede an increase in procollagen expression and collagen synthesis. In humans, it can be demonstrated that an increase in the interstitial concentration of TGF-beta, PGE2, IGF-I plus its binding proteins and interleukin-6 takes place after exercise. The increase in IGF-I expression in tendon includes the isoform that has so far been thought only to exist in skeletal muscle (mechano growth factor). The increase in IGF-I and procollagen expression showed a similar response whether the tendon was stimulated by concentric, isometric or eccentric muscle contraction, suggesting that strain rather that stress/torque determines the collagen-synthesis stimulating response seen with exercise. The adaptation time to chronic loading is longer in tendon tissue compared with contractile elements of skeletal muscle or the heart, and only with very prolonged loading are significant changes in gross dimensions of the tendon observed, suggesting that habitual loading is associated with a robust change in the size and mechanical properties of human tendons. An intimate interplay between mechanical signalling and biochemical changes in the matrix is needed in tendon, such that chemical changes can be converted into adaptations in the morphology, structure and material properties.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0838.2009.00986.x

From mechanical loading to collagen synthesis, structural changes and function in human tendon.

Tendons are often injured and heal poorly. Whether this is caused by a slow tissue turnover is unknown, since existing data provide diverging estimates of tendon protein half-life that range from 2 mo to 200 yr. With the purpose of determining life-long turnover of human tendon tissue, we used the 14C bomb-pulse method. This method takes advantage of the dramatic increase in atmospheric levels of 14C, produced by nuclear bomb tests in 1955–1963, which is reflected in all living organisms. Levels of 14C were measured in 28 forensic samples of Achilles tendon core and 4 skeletal muscle samples (donor birth years 1945–1983) with accelerator mass spectrometry (AMS) and compared to known atmospheric levels to estimate tissue turnover. We found that Achilles tendon tissue retained levels of 14C corresponding to atmospheric levels several decades before tissue sampling, demonstrating a very limited tissue turnover. The tendon concentrations of 14C approximately reflected the atmospheric levels present during the first 17 yr of life, indicating that the tendon core is formed during height growth and is essentially not renewed thereafter. In contrast, 14C levels in muscle indicated continuous turnover. Our observation provides a fundamental premise for understanding tendon function and pathology, and likely explains the poor regenerative capacity of tendon tissue.—Heinemeier, K. M., Schjerling, P., Heinemeier, J., Magnusson, S. P., Kjaer, M. Lack of tissue renewal in human adult Achilles tendon is revealed by nuclear bomb 14C.

Lack of tissue renewal in human adult Achilles tendon is revealed by nuclear bomb (14)C.

AIM To examine if cross-sectional area (CSA) differs along the length of the human patellar tendon (PT), and if there is PT hypertrophy in response to resistance training. METHODS Twelve healthy young men underwent baseline and post-training assessments. Maximal isometric knee extension strength (MVC) was determined unilaterally in both legs. PT CSA was measured at the proximal-, mid- and distal PT level and quadriceps muscle CSA was measured at mid-thigh level using magnetic resonance imaging. Mechanical properties of the patellar tendons were determined using ultrasonography. Subsequently, subjects performed 12 weeks of heavy resistance knee extension training with one leg (Heavy-leg), and light resistance knee extension training with the other leg (Light-leg). RESULTS The MVC increased for heavy-leg (15 +/- 4%, P < 0.05), but not for light-leg (6 +/- 4%). Quadriceps CSA increased in heavy-legs (6 +/- 1%, P < 0.05) while unchanged in light-legs. Proximal PT CSA (104 +/- 4 mm(2)) was smaller than the mid-tendon CSA (118 +/- 3 mm(2)), which again was smaller than distal tendon CSA (127 +/- 2 mm(2), P < 0.05). Light-leg PT CSA increased by 7 +/- 3% (P < 0.05) at the proximal tendon level, but was otherwise unchanged. Heavy-leg PT CSA increased at the proximal and distal tendon levels by 6 +/- 3% and 4 +/- 2% respectively (P < 0.05), but was unchanged at the mid tendon level. PT stiffness increased in heavy-legs (P < 0.05) but was unchanged in light-legs. Modulus remained unchanged in both legs. CONCLUSIONS To our knowledge, this study is the first to report tendon hypertrophy following resistance training. Further, the data show that the human PT CSA varies along the length of the tendon.

Region specific patellar tendon hypertrophy in humans following resistance training

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Region specific patellar tendon hypertrophy in humans following resistance training