{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,9]],"date-time":"2026-09-09T11:16:04Z","timestamp":1788952564082,"version":"build-2803163510"},"reference-count":35,"publisher":"MDPI AG","issue":"3","license":[{"start":{"date-parts":[[2026,9,9]],"date-time":"2026-09-09T00:00:00Z","timestamp":1788912000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["J. Funct. Morphol. Kinesiol.","JFMK"],"abstract":"<jats:p>Background: Achilles tendon viscoelasticity and ankle neuromuscular co-contraction are functionally linked, yet how they jointly adapt during brief high-speed repetitive loading remains unclear. We simultaneously quantified Achilles tendon viscoelasticity (MyotonPRO) and ankle agonist\u2013antagonist co-contraction (surface electromyography, sEMG) under two stance conditions and three repetition counts. Methods: Twenty-seven active male collegiate American football players (23.19 \u00b1 2.63 years) performed rapid heel-raise\/landing cycles in a bilateral ground stance (BGS) and a unilateral box stance (UBS) at 1, 5, and 10 repetitions. Tendon decrement (D), stiffness (S), and tone (F), the co-activation index (CoAct), and the co-contraction index (CCI) were analyzed using two-way (2 \u00d7 3) repeated-measures ANOVA. Results: D decreased significantly as repetitions increased (pooled across stance conditions: 1 rep 0.973 \u2192 10 reps 0.937; F (2, 52) = 4.68, p = 0.014, \u03b7p2 = 0.153), indicating improved elastic-recovery efficiency, whereas S and F remained stable. CoAct was maintained constantly at \u224872% regardless of stance or repetition (all p &gt; 0.05), and the medial gastrocnemius was consistently activated first (agonist-led feedforward priming). Tissue-level elastic adaptation thus occurred without a concurrent change in neuromuscular co-contraction\u2014a tissue\u2013neural dissociation confirmed in both change scores and correlation structure. Conclusions: Acute enhancement of Achilles tendon elasticity during high-speed repetition appears to be a peripheral, tissue-level mechanical phenomenon expressed independently of neuromuscular co-contraction. Combined myotonometric\u2013sEMG assessment reveals functional information not captured by imaging alone.<\/jats:p>","DOI":"10.3390\/jfmk11030361","type":"journal-article","created":{"date-parts":[[2026,9,9]],"date-time":"2026-09-09T10:19:16Z","timestamp":1788949156000},"page":"361","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Acute Effects of High-Speed Repetitive Loading on Achilles Tendon Viscoelasticity and Ankle Agonist\u2013Antagonist Co-Contraction: A Combined MyotonPRO and Surface Electromyography Analysis in Male Collegiate American Football Players"],"prefix":"10.3390","volume":"11","author":[{"given":"Shuho","family":"Kang","sequence":"first","affiliation":[{"name":"Department of Sports Rehabilitation, Busan University of Foreign Studies, Busan 46234, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"methodology","vocabulary":"credit"},{"role":"data-curation","vocabulary":"credit"},{"role":"writing-original-draft","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"},{"role":"formal-analysis","vocabulary":"credit"},{"role":"visualization","vocabulary":"credit"},{"role":"investigation","vocabulary":"credit"},{"role":"resources","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0009-0009-2944-1049","authenticated-orcid":false,"given":"Ja Yeon","family":"Lee","sequence":"additional","affiliation":[{"name":"Department of Sports Rehabilitation, Busan University of Foreign Studies, Busan 46234, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"data-curation","vocabulary":"credit"},{"role":"investigation","vocabulary":"credit"},{"role":"formal-analysis","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-5995-9712","authenticated-orcid":false,"given":"Ji Hwan","family":"Jeong","sequence":"additional","affiliation":[{"name":"Department of Convergence Sports Instruction, Hwashin Cyber University, Busan 47598, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"resources","vocabulary":"credit"},{"role":"project-administration","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-5634-8764","authenticated-orcid":false,"given":"Chae Kwan","family":"Lee","sequence":"additional","affiliation":[{"name":"Department of Sports Rehabilitation, Busan University of Foreign Studies, Busan 46234, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"project-administration","vocabulary":"credit"},{"role":"validation","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-3469-2204","authenticated-orcid":false,"given":"Ilbong","family":"Park","sequence":"additional","affiliation":[{"name":"Department of Sports Rehabilitation, Busan University of Foreign Studies, Busan 46234, Republic of Korea"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"validation","vocabulary":"credit"},{"role":"supervision","vocabulary":"credit"}]}],"member":"1968","published-online":{"date-parts":[[2026,9,9]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"307","DOI":"10.1111\/j.1469-7793.1999.00307.x","article-title":"In vivo human tendon mechanical properties","volume":"521","author":"Maganaris","year":"1999","journal-title":"J. Physiol."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"1087","DOI":"10.1016\/S1095-6433(02)00244-1","article-title":"The integrated function of muscles and tendons during locomotion","volume":"133","author":"Roberts","year":"2002","journal-title":"Comp. Biochem. Physiol. A Mol. Integr. Physiol."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"106","DOI":"10.1097\/00003677-200207000-00003","article-title":"Muscle and tendon interaction during human movements","volume":"30","author":"Fukunaga","year":"2002","journal-title":"Exerc. Sport Sci. Rev."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"4715","DOI":"10.1242\/jeb.01950","article-title":"In vivo mechanical properties of the human Achilles tendon during one-legged hopping","volume":"208","author":"Lichtwark","year":"2005","journal-title":"J. Exp. Biol."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"2743","DOI":"10.1242\/jeb.003814","article-title":"Adaptational responses of the human Achilles tendon by modulation of the applied cyclic strain magnitude","volume":"210","author":"Arampatzis","year":"2007","journal-title":"J. Exp. Biol."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"255","DOI":"10.1016\/j.fcl.2005.01.013","article-title":"Achilles tendon disorders: Etiology and epidemiology","volume":"10","author":"Kannus","year":"2005","journal-title":"Foot Ankle Clin."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"133","DOI":"10.1097\/01.jsm.0000165347.55638.23","article-title":"Cumulative incidence of Achilles tendon rupture and tendinopathy in male former elite athletes","volume":"15","author":"Kujala","year":"2005","journal-title":"Clin. J. Sport Med."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"262","DOI":"10.1038\/nrrheum.2010.43","article-title":"The pathogenesis of tendinopathy: Balancing the response to loading","volume":"6","author":"Magnusson","year":"2010","journal-title":"Nat. Rev. Rheumatol."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"675","DOI":"10.1016\/S0278-5919(03)00004-8","article-title":"Types and epidemiology of tendinopathy","volume":"22","author":"Maffulli","year":"2003","journal-title":"Clin. Sports Med."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"17064","DOI":"10.1038\/s41598-018-34719-7","article-title":"Assessing the elastic properties of skeletal muscle and tendon using shearwave ultrasound elastography and MyotonPRO","volume":"8","author":"Feng","year":"2018","journal-title":"Sci. Rep."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"459","DOI":"10.1016\/S0268-0033(03)00042-1","article-title":"Reliability of a new, hand-held device for assessing skeletal muscle stiffness","volume":"18","author":"Bizzini","year":"2003","journal-title":"Clin. Biomech."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"e31","DOI":"10.1016\/j.archger.2012.03.005","article-title":"Quadriceps muscle tone, elasticity and stiffness in older males: Reliability and symmetry using the MyotonPRO","volume":"55","author":"Aird","year":"2012","journal-title":"Arch. Gerontol. Geriatr."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"532","DOI":"10.1016\/j.apmr.2011.09.014","article-title":"Reliability, validity, and responsiveness of myotonometric measurement of muscle tone, elasticity, and stiffness in patients with stroke","volume":"93","author":"Chuang","year":"2012","journal-title":"Arch. Phys. Med. Rehabil."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"4876","DOI":"10.12659\/MSM.909531","article-title":"Quantifying the stiffness of the Achilles tendon: Intra- and inter-operator reliability and the effect of ankle joint motion","volume":"24","author":"Liu","year":"2018","journal-title":"Med. Sci. Monit."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"e000446","DOI":"10.1136\/bmjsem-2018-000446","article-title":"Objective assessment of stiffness in Achilles tendinopathy: A novel approach using the MyotonPRO","volume":"4","author":"Morgan","year":"2018","journal-title":"BMJ Open Sport Exerc. Med."},{"key":"ref_16","first-page":"135","article-title":"Quantitative assessment of co-contraction at the ankle joint in walking","volume":"25","author":"Falconer","year":"1985","journal-title":"Electromyogr. Clin. Neurophysiol."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"37","DOI":"10.2165\/00007256-199825010-00004","article-title":"Quantification of Quadriceps and Hamstring Antagonist Activity","volume":"25","author":"Kellis","year":"1998","journal-title":"Sports Med."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"113","DOI":"10.1177\/036354658801600205","article-title":"Muscular coactivation: The role of the antagonist musculature in maintaining knee stability","volume":"16","author":"Baratta","year":"1988","journal-title":"Am. J. Sports Med."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"361","DOI":"10.1016\/S1050-6411(00)00027-4","article-title":"Development of recommendations for SEMG sensors and sensor placement procedures","volume":"10","author":"Hermens","year":"2000","journal-title":"J. Electromyogr. Kinesiol."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"195","DOI":"10.1007\/s40846-023-00782-3","article-title":"Normalization of EMG signals: Optimal MVC positions for the lower limb muscle groups in healthy subjects","volume":"43","author":"Avdan","year":"2023","journal-title":"J. Med. Biol. Eng."},{"key":"ref_21","unstructured":"Konrad, P. (2005). The ABC of EMG: A Practical Introduction to Kinesiological Electromyography, Noraxon Inc."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"262","DOI":"10.1007\/s001670000130","article-title":"Dynamic stability after ACL injury: Who can hop?","volume":"8","author":"Rudolph","year":"2000","journal-title":"Knee Surg. Sports Traumatol. Arthrosc."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"16","DOI":"10.1037\/1082-989X.1.1.16","article-title":"The robustness of test statistics to nonnormality and specification error in confirmatory factor analysis","volume":"1","author":"Curran","year":"1996","journal-title":"Psychol. Methods"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"S56","DOI":"10.1007\/s001670050224","article-title":"Effect of cyclic preconditioning on the tensile properties of human quadriceps tendons and patellar ligaments","volume":"6","author":"Schatzmann","year":"1998","journal-title":"Knee Surg. Sports Traumatol. Arthrosc."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"2813","DOI":"10.1016\/j.jbiomech.2009.08.023","article-title":"Dynamic creep and pre-conditioning of the Achilles tendon in-vivo","volume":"42","author":"Hawkins","year":"2009","journal-title":"J. Biomech."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"212","DOI":"10.1186\/2193-1801-2-212","article-title":"Viscoelastic properties of the Achilles tendon in vivo","volume":"2","author":"Peltonen","year":"2013","journal-title":"SpringerPlus"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"1197","DOI":"10.1016\/S0021-9290(00)00064-6","article-title":"Stretch-shortening cycle: A powerful model to study normal and fatigued muscle","volume":"33","author":"Komi","year":"2000","journal-title":"J. Biomech."},{"key":"ref_28","doi-asserted-by":"crossref","unstructured":"Mulla, D.M., and Keir, P.J. (2023). Neuromuscular control: From a biomechanist\u2019s perspective. Front. Sports Act. Living, 5.","DOI":"10.3389\/fspor.2023.1217009"},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"88","DOI":"10.1152\/jn.00084.2018","article-title":"Muscle coactivation: Definitions, mechanisms, and functions","volume":"120","author":"Latash","year":"2018","journal-title":"J. Neurophysiol."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"2396","DOI":"10.1152\/jn.01020.2002","article-title":"Role of cocontraction in arm movement accuracy","volume":"89","author":"Gribble","year":"2003","journal-title":"J. Neurophysiol."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Hirayama, K., Iwanuma, S., Ikeda, N., Yoshikawa, A., Ema, R., and Kawakami, Y. (2017). Plyometric training favors optimizing muscle\u2013tendon behavior during depth jumping. Front. Physiol., 8.","DOI":"10.3389\/fphys.2017.00016"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"e15073","DOI":"10.14814\/phy2.15073","article-title":"Effects of plyometric training on muscle\u2013tendon mechanical properties and behavior of fascicles during jumping","volume":"9","author":"Kubo","year":"2021","journal-title":"Physiol. Rep."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"69","DOI":"10.1016\/j.jsams.2014.11.391","article-title":"Achilles tendinopathy alters stretch\u2013shortening cycle behaviour during a sub-maximal hopping task","volume":"19","author":"Debenham","year":"2016","journal-title":"J. Sci. Med. Sport"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"1023","DOI":"10.1016\/j.jelekin.2010.07.004","article-title":"How should we normalize electromyograms obtained from healthy participants? What we have learned from over 25 years of research","volume":"20","author":"Burden","year":"2010","journal-title":"J. Electromyogr. Kinesiol."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"7","DOI":"10.1186\/s40798-015-0009-9","article-title":"Human tendon adaptation in response to mechanical loading: A systematic review and meta-analysis of exercise intervention studies on healthy adults","volume":"1","author":"Bohm","year":"2015","journal-title":"Sports Med. Open"}],"container-title":["Journal of Functional Morphology and Kinesiology"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2411-5142\/11\/3\/361\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,9]],"date-time":"2026-09-09T10:38:33Z","timestamp":1788950313000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2411-5142\/11\/3\/361"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,9,9]]},"references-count":35,"journal-issue":{"issue":"3","published-online":{"date-parts":[[2026,9]]}},"alternative-id":["jfmk11030361"],"URL":"https:\/\/doi.org\/10.3390\/jfmk11030361","relation":{},"ISSN":["2411-5142"],"issn-type":[{"value":"2411-5142","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,9,9]]}}}