ECCENTRIC (ECC) STRENGTH TRAINING IS BECOMING INCREASINGLY POPULAR AMONG STRENGTH AND CONDITIONING COACHES AND PRACTITIONERS GIVEN THE PROVEN BENEFITS FOR PERFORMANCE IMPROVEMENTS, INJURY PREVENTION AND REHABILITATION. THE PURPOSE OF THIS ARTICLE IS TO UNDERSTAND THE DEVICES THAT ARE AVAILABLE FOR THE TRAINING OF ECC STRENGTH, THE TECHNOLOGY INVOLVED, AND THE ASSOCIATED ADVANTAGES AND DISADVANTAGES. IT IS HOPED THAT WITH THIS KNOWLEDGE THE PRACTITIONER IS BETTER INFORMED AT MATCHING ECC STRENGTH TRAINING NEEDS WITH THE APPROPRIATE TECHNOLOGY. FOR A VIDEO ABSTRACT DESCRIBING THIS ISSUE, SEE VIDEO, SUPPLEMENTAL DIGITAL CONTENT 1, http://links.lww.com/SCJ/A198.

Understanding the impact of friction in sled sprinting allows the quantification of kinetic outputs and the effective loading experienced by the athlete. This study assessed changes in the coefficient of friction (µ) of a sled sprint-training device with changing mass and speed to provide a means of quantifying effective loading for athletes. A common sled equipped with a load cell was towed across an athletics track using a motorised winch under variable sled mass (33.1-99.6 kg) with constant speeds (0.1 and 0.3 m · s), and with constant sled mass (55.6 kg) and varying speeds (0.1-6.0 m · s). Mean force data were analysed, with five trials performed for each condition to assess the reliability of measures. Variables were determined as reliable (ICC > 0.99, CV < 4.3%), with normal-force/friction-force and speed/coefficient of friction relationships well fitted with linear (R = 0.994-0.995) and quadratic regressions (R = 0.999), respectively (P < 0.001). The linearity of composite friction values determined at two speeds, and the range in values from the quadratic fit (µ = 0.35-0.47) suggested µ and effective loading were dependent on instantaneous speed on athletics track surfaces. This research provides a proof-of-concept for the assessment of friction characteristics during sled towing, with a practical example of its application in determining effective loading and sled-sprinting kinetics. The results clarify effects of friction during sled sprinting and improve the accuracy of loading applications in practice and transparency of reporting in research.

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This study determined the acute changes in spatio-temporal and impulse variables when wearable resistance (WR) of 2% body mass was attached distally to the thighs during 50 m maximal sprint-running. Fifteen sub-elite male sprinters performed sprints with and without WR over 50 m of in-ground force platforms in a randomised order. A paired t-test was used to determine statistical differences (p < .05), with effect sizes (ES) calculated between conditions over steps: 1-4, 5-14, and 15-23. WR resulted in small increased 10 and 50 m sprint times (1.0%, ES = 0.31, 0.9%, ES = 0.44, respectively, p > .05) compared to the unloaded sprint condition. For spatio-temporal variables, the WR condition resulted in moderate ES changes in step frequency (−2.8%, ES = −0.53, steps 5-14, p > .05), and contact time (2.5%, ES = 0.57, steps 5-14, and 3.2%, ES = 0.51, average of 23 steps, p > .05), while step length was unaffected during all step phases of the sprint (ES = 0.02-0.07, p > .05). Regarding kinetics, during steps 5-14, WR resulted in a moderate decrease (−4.8%, ES = −0.73, p < .05) in net anteriorposterior impulses and a moderate decrease in vertical stiffness (−5.7%, ES = −0.57, p > .05). For athletes seeking to overload step frequency and develop anterior-posterior impulse during mid to late accelerated sprinting, WR enables the application of a sprint-specific form of resistance training to be completed without decreasing step length.

Arm action is critical for optimising sprinting performance. This study aimed to examine overground sprinting performance and step characteristics during unloaded and 2% body mass (BM) forearm wearable resistance loaded sprinting. Fourteen collegiate male track sprinters performed unloaded and forearm loaded sprints over thirty metres of in-ground force plates.Step kinematics and relative kinetics were compared between the unloaded and forearm loaded conditions over four acceleration phases (i.e. steps 1-4, 5-8, 9-12 and 13-16). Affixing 2% BM loads to the forearms did not significantly alter 0-30-m sprint times (p > 0.05; −1.38 to −1.75%; ES = −0.38 to −0.54). Sprinting with forearm loads resulted in significant (p ≤ 0.05) increases in relative propulsive impulse (5.48%; ES = 1.09) and step length (4.01%; ES = 1.04) over the 1st acceleration phase. Relative vertical impulse was the only variable to change over the middle two acceleration phases (3.94-4.18%; ES = 0.77-1.00). Over the last acceleration phase stride frequency was lower (−4.86%, ES = −0.92), yet both flight time (7.70%; ES = 0.79) and vertical impulse (4.12%; ES = 0.89) increased. These findings provide interesting programming implications for coaches who wish to improve the determinants of sprinting via dedicated and specific arm loaded training. Sprinting with forearm loads may be used to develop longer stride lengths by generating greater horizontal propulsion during early acceleration and promote alterations to step frequency and flight time imposed through greater vertical loading demands over the later phases of accelerated sprinting.

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Lower-limb wearable resistance (WR) provides a specific and targeted overload to the musculature involved in sprint running, however, it is unknown if greater impact forces occur with the additional limb mass. This study compared the contact times and ground reaction force waveforms between sprint running with no load and 2% body mass (BM) shank-positioned WR over 30 m. Fifteen male university-level sprint specialists completed two maximum effort sprints with each condition in a randomised order. Sprint running with shank WR resulted in trivial changes to contact times at 5 m, 10 m, and 20 m (effect size [ES] = < 0.20, p > 0.05) and a small, significant increase to contact time at 30 m by 1.94% (ES = 0.25, p = 0.03). Significant differences in ground reaction force between unloaded and shank loaded sprint running were limited to the anterior-posterior direction and occurred between 20−30% of ground contact at 10 m, 20 m, and 30 m. Shank WR did not result in greater magnitudes of horizontal or vertical forces during the initial impact portion of ground contact. Practitioners can prescribe shank WR training with loads ≤ 2% BM without concern for increased risk of injurious impact forces.

The assessment of horizontal force during overground sprinting is increasingly prevalent in practice and research, stemming from advances in technology and access to simplified yet valid field methods. As researchers search out optimal means of targeting the development of horizontal force, there is considerable interest in the effectiveness of external resistance. Increasing attention in research provides more information surrounding the biomechanics of sprinting in general and insight into the potential methods of developing determinant capacities. However, there is a general lack of consensus on the assessment and computation of horizontal force under resistance, which has resulted in a confusing narrative surrounding the practical applicability of loading parameters for performance enhancement. As such, the aim of this commentary was twofold: to provide a clear narrative of the assessment and computation of horizontal force in resisted sprinting and to clarify and discuss the impact of methodological approaches to subsequent training implementation. Horizontal force computation during resisted sleds, a common sprint-training apparatus in the field, is used as a test case to illustrate the risks associated with substandard methodological practices and improperly accounting for the effects of friction. A practical and operational synthesis is provided to help guide researchers and practitioners in selecting appropriate resistance methods. Finally, an outline of future challenges is presented to aid the development of these approaches.