Validity and Reliability of Force-Time Characteristics Using a Portable Load Cell for the Isometric Midthigh Pull.

ABSTRACT: Pichardo, AW, Neville, J, Tinwala, F, Cronin, JB, and Brown, SR. Validity and reliability of force-time characteristics using a portable load cell for the isometric midthigh pull. J Strength Cond Res 38(1): 185-191, 2024-Many practitioners use the isometric midthigh pull (IMTP) to assess maximal strength in a safe, time-effective manner. However, expensive, stationary force plates are not always practical in a large team setting. Therefore, the purpose of this study was to establish the validity and between-session reliability of peak force, rate of force development (RFD), and impulse during an IMTP using 2 experimental protocols: a traditional fixed bar with a force plate (BarFP) and a flexible chain measured with a force plate (ChainFP) and a load cell (ChainLC). After a familiarization session, 13 resistance-trained men performed 3 trials of the BarFP condition and 3 trials of the chain-based conditions. The identical procedures were replicated twice more, with a week between each testing session. The main findings were (a) no RFD or impulse measures were found to achieve acceptable reliability across all methodological approaches and testing occasions; (b) peak force was reliable across all methods, with coefficient of variation ranging from 4.6 to 8.3%, intraclass correlation coefficient ranging from 0.94 to 0.98, and the least variability associated with the ChainLC condition; and (c) the ChainFP method was found to significantly underrepresent peak force by 4.8% (p < 0.05), with no significant differences between the ChainLC and BarFP methods. Therefore, the ChainLC would seem a valid, reliable, portable, and cost-effective alternative to force plates when assessing maximal isometric strength in the IMTP.

Lower-limb wearable resistance overloads joint angular velocity during early acceleration sprint running.

Lower-limb wearable resistance (WR) facilitates targeted resistance-based training during sports-specific movement tasks. The purpose of this study was to determine the effect of two different WR placements (thigh and shank) on joint kinematics during the acceleration phase of sprint running. Eighteen participants completed maximal effort sprints while unloaded and with 2% body mass thigh- or shank-placed WR. The main findings were as follows: 1) the increase to 10 m sprint time was small with thigh WR (effect size [ES] = 0.24), and with shank WR, the increase was also small but significant (ES = 0.33); 2) significant differences in peak joint angles between the unloaded and WR conditions were small (ES = 0.23-0.38), limited to the hip and knee joints, and <2° on average; 3) aside from peak hip flexion angles, no clear trends were observed in individual difference scores; and, 4) thigh and shank WR produced similar reductions in average hip flexion and extension angular velocities. The significant overload to hip flexion and extension velocity with both thigh- and shank-placed WR may be beneficial to target the flexion and extension actions associated with fast sprint running.

Thigh loaded wearable resistance increases sagittal plane rotational work of the thigh resulting in slower 50-m sprint times.

This study determined the acute changes in rotational work with thigh attached wearable resistance (WR) of 2% body mass during 50-m sprint-running. Fourteen athletes completed sprints with, and without, WR in a randomised order. Sprint times were measured via timing gates at 10-m and 50-m. Rotational kinematics were obtained over three phases (steps 1-2, 3-6 and 7-10) via inertial measurement unit attached to the left thigh. Quantification of thigh angular displacement and peak thigh angular velocity was subsequently derived to measure rotational work. The WR condition was found to increase sprint times at 10-m (1.4%, effect size [ES] 0.38, p 0.06) and 50-m (1.9%, ES 0.55, p 0.04). The WR condition resulted in trivial to small increases in angular displacement of the thigh during all phases (0.6-3.4%, ES 0.04-0.26, p 0.09-0.91). A significant decrease in angular velocity of the thigh was found in all step phases (-2.5% to -8.0%, ES 0.17-0.51, p < 0.001-0.04), except extension in step phase 1 with the WR. Rotational work was increased (9.8-18.8%, ES 0.35-0.53, p < 0.001) with WR in all phases of the sprint. Thigh attached WR provides a means to significantly increase rotational work specific to sprinting.

Waveform analysis of shank loaded wearable resistance during sprint running acceleration.

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 randomized 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% and 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.

Changes to horizontal force-velocity and impulse measures during sprint running acceleration with thigh and shank wearable resistance.

This study determined the effects of two wearable resistance (WR) placements (i.e. thigh and shank) on horizontal force-velocity and impulse measures during sprint running acceleration. Eleven male athletes performed 50 m sprints either unloaded or with WR of 2% body mass attached to the thigh or shank. In-ground force platforms were used to measure ground reaction forces and determine dependent variables of interest. The main findings were: 1) increases in sprint times and reductions in maximum velocity were trivial to small when using thigh WR (0.00-1.93%) and small to moderate with shank WR (1.56-3.33%); 2) athletes maintained or significantly increased horizontal force-velocity mechanical variables with WR (effect size = 0.32-1.23), except for theoretical maximal velocity with thigh WR, and peak power, theoretical maximal velocity and maximal ratio of force with shank WR; 3) greater increases to braking and vertical impulses were observed with shank WR (2.72-26.3% compared to unloaded) than with thigh WR (2.17-12.1% compared to unloaded) when considering the entire acceleration phase; and, 4) no clear trends were observed in many of the individual responses. These findings highlight the velocity-specific nature of this resistance training method and provide insight into what mechanical components are overloaded by lower-limb WR.

Multijoint Musculoarticular Stiffness Derived From a Perturbation Is Highly Variable.

ABSTRACT: Schofield, M, Tinwala, F, Cronin, J, Hébert-Losier, K, and Uthoff, A. Multijoint musculoarticular stiffness derived from a perturbation is highly variable. J Strength Cond Res 35(9): 2498-2503, 2021-Testing musculoarticular stiffness may provide insights into multijoint elastic properties. Yet, most research has focused on quantifying stiffness, or elastic potential, at a single joint. The purpose of this study was to quantify the test-retest reliability of musculoarticular stiffness derived from the perturbation technique across the bench pull, bench press, and squat movements. Eight resistance-trained men performed bench pull, bench press, and squat repetition maximums, after which a perturbation protocol was tested over multiple days. During the 3 movements, a brief perturbation was applied to the bar. The resulting sinusoidal wave was measured by an underbench force plate and a linear position transducer attached to the bar. From the sinusoidal wave, stiffness was derived and found to be unreliable across movements and days (change in mean: -35.1 to 15.8%; coefficient of variation: 7.1-111%; intraclass correlation: -0.58 to -0.89). Squat data were removed from the analysis entirely because of the inability to consistently determine the perturbations on the force plate. Practitioners need to be aware that musculoarticular stiffness as measured using the perturbation technique on the movements performed in this study has considerable limitations in terms of reproducibility.

Effects of forearm wearable resistance on acceleration mechanics in collegiate track sprinters.

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.

Thigh positioned wearable resistance affects step frequency not step length during 50†m sprint-running.

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 anterior-posterior 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.

Force-velocity profile changes with forearm wearable resistance during standing start sprinting.

Abstract Horizontal force-velocity (F-V) profiling is a strategy to assess athletes' individual performance capabilities during sprinting. This study investigated the acute changes in F-V profiles during sprinting of fourteen collegiate male sprinters with a mean 100-m sprint time of 11.40 ± 0.39 s, from a split-stance starting position. The subjects sprinted 30-m with, and without, wearable resistance (WR) equivalent to 2% body mass, attached to their forearms. Sprinting time at 5, 10, 20, and 30-m was assessed using laser technology. External horizontal F-V relationships were calculated via velocity-time signals. Maximal theoretical velocity (V 0), theoretical relative and absolute horizontal force (F 0), and horizontal power (P max) were determined from the F-V relationship. Paired t-tests were used to determine statistical differences (p ≤ 0.05) in variables across conditions with Cohen's d as effect sizes (ES) calculated to assess practical changes. Sprint times at 10-m and beyond were significantly increased (1.9-3.3%, p 0.01-0.03, ES 0.46-0.60) with WR compared to unloaded sprinting. The only significant change in F-V with the WR condition was found in relative P max system (-6.1%, p 0.01, ES 0.66). A small decrease was reported in V 0 (-1.0%, p 0.11, ES 0.27), with small to medium ES decreases reported in F 0 (-4.8% to -6.1%, p 0.07-0.21, ES 0.25-0.51) and P max (-4.3% to -4.6%, p 0.06-0.08, ES 0.32-0.45). The greater changes to F 0 and P max suggest that forearm WR may be a possible training tool for athletes who wish to focus on force and power adaptation during sprint acceleration from a standing start.

Assessing Horizontal Force Production in Resisted Sprinting: Computation and Practical Interpretation.

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.

Kinematic and kinetic differences in block and split-stance standing starts during 30 m sprint-running.

This study aimed to understand the kinematic and kinetic differences between two sprint starts: block and split-stance standing. Fourteen sub-elite male sprinters (100 m time: 11.40 ± 0.39 s) performed block and split-stance standing starts sprints over 30 m of in-ground force platforms in a randomised order. Independent t-tests and repeated measures mixed model analysis of variance were used to analyse the between-condition variables across conditions, and over four step phases. Block start sprints resulted in significantly (p < .05) faster 5 m (5.0%, effect size [ES] = 0.89) and 10 m (3.5%, ES = 0.82) times, but no significant differences were found at 20 and 30 m. No significant differences were found in any kinematic measure between starting positions. However, block starts resulted in significantly (p < .001) greater propulsive impulses (6.8%, ES = 1.35) and net anterior-posterior impulses (6.5%, ES = 1.12) during steps 1-4, compared to the standing start. Block starts enable athletes to produce a greater amount of net anterior-posterior impulse during early accelerated sprinting, resulting in faster times up to 10 m. When seeking to improve initial acceleration performance, practitioners may wish to train athletes from a block start to improve horizontal force production.

Determining friction and effective loading for sled sprinting.

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 (µk) 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-1), and with constant sled mass (55.6 kg) and varying speeds (0.1-6.0 m · s-1). 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 (R2 = 0.994-0.995) and quadratic regressions (R2 = 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 (µk = 0.35-0.47) suggested µk 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.