(8) POSITIONAL COMPARISON OF COUNTERMOVEMENT JUMP STRATEGIES IN AMERICAN FOOTBALL ATHLETES

PURPOSE: Understanding positional differences within a given sport can assist in the design of training programs to better suit the needs of athletes. In American football differences in the technical demands of each position favor certain anthropometric and physical attributes. A common assessment in understanding positional differences is the countermovement jump (CMJ) as the kinetic and kinematics demands of the task are shared across sporting movements involving the lower body. Thus, understanding the different strategies that are used to achieve a maximal jump performance would be of interest to practitioners both from a program design and longitudinal monitoring perspective. Thus, the purpose of this investigation was to examine strategy differences between positional groups in American football. 

Methods:  95 Division I collegiate football athletes (age 20.18 ± 0.76 years, height 183.94 ± 5.63 cm, body mass 105.98 ± 24.68 kg) were tested at the beginning of the summer training period as a part of their athlete monitoring program. Participants were assigned to one of three groups (lineman = 32, mid skill = 29, skill = 24). Each participant performed three CMJ trials using a portable force platform with their arm swing restricted by placing a dowel across their upper back. Participants were instructed to go to a self-selected countermovement depth and stance. Ground reaction force data was sampled at 1000 Hz and was then exported for post-processing using a customized Excel spreadsheet. Force-time data was then integrated to determine take-off velocity to determine jump height using the impulse-momentum theorem. Variables of interest for this investigation were jump height, time to take off, reactive strength index modified (RSIm), mean net braking, force at zero velocity, and net propulsive forces as well as phase durations. A one-way analysis of variance was performed on all variables of interest. If significant, a Fisher’s least significant difference post hoc analysis was performed. An apriori alpha level of 0.05 was used for all analyses. 

Results:  Significant differences were present across all three groups for jump height (p< 0.01) and RSIm (p< 0.001) with the skill group having the highest values and the linemen having the lowest. Significant differences were seen for mean net propulsive force with the lineman producing greater net force over both the mid skill (p< 0.05) and skill groups (p< 0.01). Similarly, the lineman had longer durations in the propulsive phase (p< 0.01), braking phase (p< 0.01), and time to take off (p< 0.01) than the other two groups. No differences were seen in mean net braking force, force at zero velocity, and countermovement depth. CONCLUSION: This investigation demonstrates that different strategies are used between positional groups during the CMJ as mean net braking force and force at zero velocity were not different between groups, while the propulsive phase metrics demonstrated differences between groups. PRACTICAL APPLICATION: The differences between groups can aid in the design of training programs, as no differences between groups in mean braking force and force at zero velocity demonstrates potentially an inability of the lineman group to transfer to the propulsive phase quickly. This would point to the need to enhance elastic qualities during portions of the training year. While the lower force values in the mid skill and skill groups would indicate a need to enhance force production. 

Acknowledgements: None