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Edward C. Frederick, Jeremy J. Determan, Saunders N. Whittlesey and Joseph Hamill

Seven top amateur or professional skateboarders (BW = 713 N ± 83 N) performed Ollie maneuvers onto and off an elevated wooden platform (45.7 cm high). We recorded ground reaction force (GRF) data for three Ollie Up (OU) and Ollie Down (OD) trials per participant. The vertical GRF (VGRF) during the OU has a characteristic propulsive peak (M = 2.22 body weight [BW] ± 0.22) resulting from rapidly rotating the tail of the board into the ground to propel the skater and board up and forward. The anterior-posterior (A-P) GRF also shows a pronounced peak (M = 0.05 ± 0.01 BW) corresponding with this propulsive VGRF peak. The initial phase of landing in the OD shows an impact peak in VGRF rising during the first 30 to 80 ms to a mean of 4.74 ± 0.46 BW. These impact peaks are higher than expected given the relatively short drop of 45.7 cm and crouched body position. But we observed that our participants intentionally affected a firm landing to stabilize the landing position; and the Ollie off the platform raised the center of mass, also contributing to higher forces.

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Nicholas Caplan and Trevor N. Gardner

The aim of the present study was to determine the effect of varying the height of the foot stretcher on the mechanical effectiveness of rowing. Ten male university level rowers rowed maximally for 3 minutes 30 seconds on a modified Concept 2 rowing ergometer. Each participant completed one trial at three foot stretcher heights. Position 1 was the original Concept 2 stretcher position, with Position 2 being located 5 cm and Position 3 being 10 cm above the original position and in the same orientation. Pull force and velocity were measured, and mean power generated by the rowers was calculated for each stroke. It was shown that in all three stretcher positions, mean power per stroke decreased as a function of time during the trial, confirming the fatiguing effects of the task. Although mean power per stroke did not differ significantly between stretcher positions at the start of the trial, p = 0.082, a significant difference was observed between the original stretcher position and Positions 2 and 3 at the end of the trial, p < 0.05. The lowest decline in mean power occurred in the highest stretcher position. It is suggested that this improvement in effectiveness is due to a reduction in the active downward vertical forces applied to the foot stretchers which does not contribute to forward propulsion, and thus a reduction in energy waste during each stroke. It was hypothesized that further raising the stretchers will continue to lead to an improvement in effectiveness until the optimum stretcher height is reached, above which effectiveness will be reduced.

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Jeffrey M. McBride, Tony R. Larkin, Andrea M. Dayne, Tracie L. Haines and Tyler J. Kirby

Purpose:

The purpose of this investigation was to determine the effect of stable and unstable conditions on one repetition maximum strength and muscle activity during dynamic squatting using absolute and relative loading.

Methods:

Ten recreationally weight-trained males participated in this study (age = 24.1 ± 2.0 y, height = 178.0 ± 5.6 cm, body mass = 83.7 ± 13.4 kg, 1RM/body mass = 1.53 ± 0.31), which involved two laboratory sessions separated by 1 wk. Linear position transducers were used to track bar displacement while subjects stood on a force plate for all trials. Vastus lateralis (VL), biceps femoris (BF) and erector spinae (L1) muscle activity (average integrated EMG [IEMG]) was also recorded during all trials. During the frst session subjects complete a one repetition maximum test in a stable dynamic squat (S1RM = 128.0 ± 31.4 kg) and an unstable dynamic squat (U1RM = 83.8 ± 17.3 kg) in a randomized order with a 30-min rest period between conditions. The second session consisted of the performance of three trials each for 12 different conditions (unstable and stable squats using three different absolute loads [six conditions] and unstable and stable squats using three different relative loads [six conditions]).

Results:

Results revealed a statistically significant difference between S1RM and U1RM values (P < .05). The stable trials resulted in the same or a significantly higher value for VL, BF and L1 muscle activity in comparison with the unstable trials for all twelve conditions.

Conclusions:

Unstable squatting is of equal or less (depending on the loading condition) benefit to improving or maximizing muscle activity during resistance exercise.

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Caroline Lisee, Tom Birchmeier, Arthur Yan, Brent Geers, Kaitlin O’Hagan, Callum Davis and Christopher Kuenze

ACL injury associated with common sport-related tasks. Kinetic variables, such as peak vertical ground reaction force (vGRF), and linear loading rates provide key insights into the characteristics of forces acting on the body as well as an individual’s response to these forces during functional tasks

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Javin C. Pierce, Malcolm H. Pope, Per Renstrom, Robert J. Johnson, Janet Dufek and Charles Dillman

A method for measuring the forces between the shoe and ski and upon the pole has been developed. Instrumented skis and poles are used with a portable data acquisition system that is carried by the skier in the field. Elite, top-level collegiate, and citizen skiers were used as subjects. Skiers performed the diagonal stride, and a marathon skate. Axial force levels at the forefoot were found to reach 164%, and 120% of body weight in the diagonal skate strides, respectively.

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Mehmet Uygur, Goran Prebeg and Slobodan Jaric

We compared two standard methods routinely used to assess the grip force (GF; perpendicular force that hand exerts upon the hand-held object) in the studies of coordination of GF and load force (LF; tangential force) in manipulation tasks. A variety of static tasks were tested, and GF-LF coupling (i.e., the maximum cross-correlation between the forces) was assessed. GF was calculated either as the minimum value of the two opposing GF components acting upon the hand-held object (GFmin) or as their average value (GFavg). Although both methods revealed high GF-LF correlation coefficients, most of the data revealed the higher values for GFavg than for GFmin. Therefore, we conclude that the CNS is more likely to take into account GFavg than GFmin when controlling static manipulative actions as well as that GFavg should be the variable of choice in kinetic analyses of static manipulation tasks.

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Steven J. Obst, Lee Barber, Ashton Miller and Rod S. Barrett

Estimates of in vivo Achilles tendon (AT) force are needed to measure tendon mechanical properties as a function of the measured net ankle joint torque, and to understand AT function using musculoskeletal modeling approaches. The AT moment arm is required to convert the measured external ankle

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Paavo V. Komi

To understand cross-country (X-C) siding it is important to record and identity forces of skis and poles separately and together. They both contribute to the forward progression, but their functional significance may be more complex than that of the ground reaction forces in running and walking. This report presents two methods to record forces on skis and poles during normal X-C skiing. A long force-platform system with four rows of 6-m long plates is placed under the snow track for recording of Fz and Fy forces of each ski and pole separately. This system is suitable especially for the study of diagonal technique under more strict experimental conditions. The second system consists of small lightweight Fz and Fy component force plates which are installed under the boot and binding. These plates can be easily changed from one ski to another, and telemetric recording allows free skiing over long distances and with different skiing techniques, including skating. The presentation emphasizes the integrated use of either system together with simultaneous cinematographic and electromyographic recordings.

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Talyene G.C. Corrêa, Stephanie V.S. Donato, Kauê C.A. Lima, Ronaldo V. Pereira, Mehmet Uygur and Paulo Barbosa de Freitas

The ability to produce muscle force rapidly (i.e., neuromuscular quickness) is an important feature of the human motor repertoire. Self-initiated rapid movements are clearly necessary in some sports actions and, occasionally, in daily activities (e.g., catching a fly, deviating from a moving object

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Marco J. Konings, Jordan Parkinson, Inge Zijdewind and Florentina J. Hettinga

and by relating these to neuromuscular adjustments in the knee extensors and perceived exertion. We hypothesized that the presence of a virtual opponent would invite a change in pacing and evoke an improvement in performance, leading to a greater decline in voluntary muscle force after a 4-km TT than