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The Influence of Muscle-Tendon Unit Structure on Rate of Force Development, During the Squat, Countermovement, and Depth Drop Jumps.

Earp, J E; Newton, R U; Cormie, P; Kraemer, W J
Journal of Strength & Conditioning Research:
doi: 10.1097/01.JSC.0000395587.79369.ff
Abstract: PDF Only

Previous research suggests low pennated (P) muscles are beneficial for sprinting and high velocity movements, while highly P muscles are suited for high force development. Long muscles fascicles can be beneficial for both types of movement. Long compliant tendons are better suited for longer stretch shortening cycles (SSC) movements like the countermovement jump (CMJ) while short stiff tendons are suited for short SSC movements such as sprinting. Research on muscle and tendon structure has focused on general performance or isometric rate of force develop (RFD). To our knowledge, no research has looked at RFD during different phases of SSC movements, which is important for sport because the countermovement and time to complete a jump is dependent on play. PURPOSE: To determine if muscle and tendon structure influences how force is produced during static squat jumps (SJ), CMJ, and depth drop jumps (DDJ) off a 25cm box. METHODS: 25 strength & power trained men from a cross section of athletic backgrounds (Age: 23.3 +/- 3.2, Height: 176.1 +/- 7.4 cm, Mass: 86.2 +/- 11.6 kg) had their muscle and tendon structure analysed using ultrasounography. Vastus lateralis (VL) and gastrocnemius (GAS) P and FL, patellar tendon thickness, and achilles tendon (AT) thickness and length were measured. Subjects performed SJ, CMJ, and DDJ and vertical ground reaction force was measured via a force plate. RFD was calculated at time intervals of 0-10, 10-30, 30-50, 50-100, 100-200, and 200-300 ms from the start of force development. Regressions were used to determine if force expression could be predicted by anatomical measures. RESULTS: AT length was an inverse predictor of SJ-RFD late in the propulsion (200-300 ms; r2 = 0.217, [beta] = -0.501, p = 0.013) as well as early initial RFD during CMJ (0-10 ms, r2 = 0.293, [beta] = -0.307, p = 0.020). GAS FL was a positive predictor of performance during early CMJ (0-10, 10-30 ms, r2 = 0.293, 0.213, [beta] = 0.345, 0.623, p = 0.02, 0.008) and an inverse predictor during early DDJ (0-10ms, [beta] = -0.304, p = 0.008). Both VL P and GAS P were also significant predictors of early DDJ RFD (0-10ms, r2 = 0.421, [beta] = 0.421, 0.209, p = 0.008). CONCLUSIONS: Longer AT's have increased elastic properties which can decrease initial RFD during CMJ. Longer GAS FL also possess greater elastic properties but have greater contraction potential, which can benefit CMJ during early propulsion. During DDJ the short GAS FL and high GAS P & VL P predicted greater RFD during the first 10ms, but not during any subsequent times. Extending upon a recent investigation, which showed shorter GAS FL was related to higher DDJ jump height, the current research suggests that short highly angled fascicles are more suited to handle the high eccentric loads encountered during early DDJ. PRACTICAL APPLICATIONS: Interpretation of previous work showing long compliant tendons were better suited for jumping should take into account that during the quick jump situations often observed in sport that tendon length may actually diminish rather than enhance performance, and thus decrease the importance of AT-length for talent identification. Optimal muscle architecture appears to be dependent on both the eccentric load and the phase of jump. While both strength and plyometric training have been shown to increase FL only heavy strength training has been shown to increase P. Thus when a high eccentric load or multiple jumps are required for sport heavy strength training should be used to allow for early force production during jumping.

(C) 2011 National Strength and Conditioning Association