Success in collegiate American football programs depends upon the ability of coaches to identify and recruit athletes with a high degree of football playing ability. Many previous studies (7,9,10,16,17,31,33) have examined a number of performance variables that may help identify players with a high degree of playing ability (i.e., starters from nonstarters). For example, Sawyer et al. (31) reported that football playing ability was significantly related to several vertical jumping performance variables; however, the majority of the anthropometric, speed, and strength measures (39 of 45) were not significantly correlated to playing ability. Black and Roundy (9) compared anthropometric and performance predictor variables of starters and nonstarters and reported that 32 of the 80 performance variable comparisons significantly favored the starters; however, unlike Sawyer et al's. (31) results, strength measures (i.e., 10 of 16 player positions for the bench press) were significantly higher for the starters. Burke et al. (10) and Fry and Kraemer (16) also reported conflicting findings when examining bench press and back squat strength as predictors of football playing level.
Although the majority of previous studies examining muscular function between starters and nonstarters have used dynamic measures of performance, isometric strength testing may provide additional controlled and reliable (11,43) assessments to help discriminate between playing levels. For example, the analysis of the isometric torque—time curve provides several variables other than peak torque (PT) that collectively may be defined as rapid torque variables, which include the rate of torque development (RTD), time to peak RTD (TTRTDpeak), contractile impulse, and torque values produced at specific time intervals. Given some very important football-related activities such as running (22,24,36,40), jumping (13,24,36), and accelerating (12,23,36) include movement durations that last <250 milliseconds, the ability to develop torque rapidly is often considered a critical requirement for successful performance in collegiate football (21,25). Thus, because of the relatively long durations that are required to attain maximal strength (>300 milliseconds) (1,3,35), rapid torque characteristics may provide a more sensitive representation of muscle function (1,3,28) when identifying players with a high degree of football playing ability.
Traditionally, studies examining the relationship between maximal and rapid torque characteristics and performance have used the leg extensors (8,29,37,38). These variables, however, are rarely assessed for the leg flexors, which may provide a fundamental role in running (14,26,27,41) and running, and agility or cutting (4,20) movements commonly performed in the sport of football.
Based on these conflicting findings and limited research investigating maximal and rapid isometric torque characteristics as a tool to predict football playing ability, further investigation is warranted. We hypothesized that rapid torque characteristics may more effectively discriminate between playing level than maximal torque measurements due to its strong relationship with many football-related activities. Thus, the purpose of this study was to examine the efficacy of maximal and rapid torque isometric characteristics of the leg flexors and extensors to discriminate between starters and nonstarters in elite Division I American collegiate football players.
Experimental Approach to the Problem
This study was designed to investigate the discriminative ability of maximal and rapid isometric torque characteristics between starters and nonstarters in elite Division I collegiate American football players. Many previous studies (5,6,9,10,16) have demonstrated that dynamic measures of performance (i.e., vertical jump, power, speed, squat, and bench press strength) may discriminate between playing levels; however, we are aware of no studies that have examined maximal and rapid isometric torque characteristics to differentiate between playing levels in this population. Thus, to test the hypothesis that laboratory-based isometric strength testing may translate to on-the-field performance, Division I collegiate football starters and nonstarters performed isometric maximal voluntary contraction (MVCs) of the leg extensors and flexors.
Sixteen starters (mean ± SD: age = 20.81 ± 1.28 years; mass = 108.69 ± 22.16 kg; and height = 184.53 ± 6.58 cm) and 15 nonstarters (age = 20.40 ± 1.68 years; mass = 104.60 ± 22.44 kg; and height = 182.27 ± 10.52 cm) volunteered for this investigation. Player position (classified as linemen or nonlinemen and excluding kickers and holders) was similarly represented among starters (linemen = 7; nonlineman = 9) and nonstarters (linemen = 5; nonlineman = 10). All the participants were Division I collegiate football players. All testing was conducted during the offseason period prior to summer conditioning in the late afternoon to early evening. This study was approved by the University Institutional Review Board, and all the participants completed and signed an informed consent document. None of the participants reported any current or ongoing neuromuscular diseases or musculoskeletal injuries.
Maximal isometric strength testing was performed on the right leg using a Biodex System 4 isokinetic dynamometer (Biodex Medical Systems, Inc., Shirley, NY, USA). All the participants were seated with restraining straps over the trunk, pelvis, and thigh, and the input axis of the dynamometer was aligned with the axis of rotation of the knee. All isometric torque assessments were performed at a leg flexion angle of 60° below the horizontal plane. Prior to maximal isometric strength testing, the participants performed a 5-minute warm-up on a cycle ergometer (Monark Exercise 828E, Vansbro, Sweden) at a self-selected low-intensity workload, followed by 3 submaximal isokinetic leg extension and leg flexion muscle actions at 60°·s−1 at approximately 75% of their perceived maximal effort. Following the submaximal contractions, each participant performed 2 isometric MVCs with the leg flexors and extensors with 1 minute of recovery between each contraction and 3 minutes of recovery between muscle groups. The order of testing was randomized to control for any potential effects of fatigue. The participants were verbally instructed for the leg flexion and extension MVCs to ‘pull' or ‘push,' “as hard and fast as possible” for a total of 3–4 seconds (1,3).
The torque signal was sampled at 2 kHz with a Biopac data acquisition system (MP150WSW, Biopac Systems, Inc.; Santa Barbara, CA, USA) and smoothed using a zero phase—shift 100-point moving average and corrected for passive torque so that the baseline torque value was 0 N·m. Signals were recorded and stored on a personal computer (Dell OptiPlex, GX270, Dell Inc., Round Rock, TX, USA) and subsequently processed off-line with custom written software (Labview 8.5, National Instruments, Austin, TX, USA). All subsequent analyses were performed on the filtered torque signal. Isometric MVC PT was determined as the highest 0.5-second epoch during the entire 3- to 4-second MVC (Figure 1). All rapid torque variables were subsequently determined from the trial with the highest isometric MVC from the onset of contraction. The RTD was calculated as the slope of the torque-time curve ([INCREMENT]torque/[INCREMENT]time) over the time intervals of 0–30 (RTD30), 0–50 (RTD50), 0–100 (RTD100), and 100–200 (RTD100–200) milliseconds (Figure 1). Also, peak RTD (RTDpeak) was calculated as the highest slope value for any 50-millisecond epoch that occurred over the initial 200 milliseconds of the torque-time curve, and TTRTDpeak was calculated as the time (milliseconds) that occurred from the onset of contraction until RTDpeak occurred. Similarly, the contractile impulse was calculated as the area under the torque-time curve (∫Torque dt) for the same time intervals (i.e., IMPULSE30, IMPULSE50, IMPULSE100, IMPULSE100–200) used to calculate RTD (Figure 1). Absolute rapid torque variables were determined as the average torque (N·m) value of 10 consecutive data points (5 milliseconds) at 30 (TORQUE30), 50 (TORQUE50), 100 (TORQUE100), and 200 (TORQUE200) milliseconds from the onset of contraction. The onset of contraction was determined as the point when the torque signal reached a threshold of 4 N·m for the leg flexors and 7 N·m for the leg extensors (∼2% of mean MVC values) (1).
Based on the procedures described by Weir (39), test-retest reliability from our laboratory was examined during leg extension isometric MVCs from 10 male participants measured 48–72 hours apart. For all the variables reported in this study, the intraclass correlation coefficients (ICC) and standard error of measurement (SEM) values expressed as a percentage of the mean ranged from 0.57 to 0.86 and from 7.30 to 29.6%, respectively. In addition, there were no significant (p > 0.05) differences between testing sessions for all variables.
Independent samples t-tests were used to analyze differences in age, body mass, and height between starters and nonstarters. Separate 2-way mixed factorial analyses of variance (ANOVAs) (muscle × playing level) were used to analyze the PT, TTRTDpeak, RTD, impulse, and absolute rapid torque data. When appropriate, follow-up analyses included independent samples t-tests and paired samples t-tests. PASW software version 18.0 (SPSS Inc, Chicago, IL, USA) was used for all statistical analyses. An alpha value of p ≤ 0.050 was considered statistically significant for all comparisons. The effect size (ES) statistics were calculated by dividing the difference in the performance variable between groups by the pooled SD (19).
All participants' demographic values are presented in Table 1. There were no differences in age (p = 0.446), body mass (p = 0.614), or height (p = 0.475) between groups.
Peak Torque and Rate of Torque Development
The results for MVC PT and RTDpeak indicated that there was no interaction (p = 0.094–0.114; muscle × playing level) and no main effect for playing level (p = 0.347–0.674), but there was a main effect for muscle (p < 0.001). Peak torque (PT) and RTDpeak were greater for the leg extensors than for the leg flexors. For TTRTDpeak and RTD30, there was a significant interaction (p = 0.038–0.043; muscle × playing level). TTRTDpeak was shorter (p = 0.020) and RTD30 was greater (p = 0.019) for the starters when compared to the nonstarters for the leg flexors but was similar between groups for the leg extensors (p = 0.291–0.829). The TTRTDpeak was also shorter and RTD30 was greater for the leg extensors than for the leg flexors for both the starters and nonstarters (p = 0.022). For RTD50, RTD100, and RTD100–200, there were no interactions (p = 0.061–0.179; muscle × playing level) and no main effects for playing level (p = 0.399–0.810), but there was a main effect for muscle (p < 0.001). RTD50, RTD100, and RTD100–200 were greater for the leg extensors than for the leg flexors. The mean values for all PT and RTD variables by playing level are presented in Tables 2 and 3.
The results for IMPULSE30 and IMPULSE50 indicated that there was a significant interaction (p = 0.032–0.045; muscle × playing level). IMPULSE30 and IMPULSE50 were greater (p = 0.021–0.025) for the starters when compared to the nonstarters for the leg flexors but were similar between groups for the leg extensors (p = 0.210–0.304). The IMPULSE30 and IMPULSE50 were also greater for the leg extensors than for the leg flexors for both groups (p < 0.001). The 2-way ANOVA for IMPULSE100 indicated a significant interaction (p = 0.047; muscle × playing level); however, the post hoc analyses revealed no differences between groups for the leg flexors (p = 0.055) or the leg extensors (p = 0.258). IMPULSE100 was also greater for the leg extensors than for the leg flexors for both groups (p < 0.001). The 2-way ANOVA for IMPULSE100-200 indicated a significant interaction (p = 0.034; muscle × playing level); however, the post hoc analyses revealed no differences between groups for the leg flexors (p = 0.328) or the leg extensors (p = 0.104). IMPULSE100–200 was greater for the leg extensors than the leg flexors for both groups (p < 0.001). The mean values for all impulse variables by playing level are presented in Tables 2 and 3.
The results for TORQUE30 indicated that there was a significant interaction (p = 0.043; muscle × playing level). The TORQUE30 was greater (p = 0.020) for the starters when compared to the nonstarters for the leg flexors but was similar between groups for the leg extensors (p = 0.295). TORQUE30 was also greater for the leg extensors than for the leg flexors for both groups (p < 0.001). For TORQUE50, there was no interaction (p = 0.059; muscle × playing level) and no main effect for playing level (p = 0.857), but there was a main effect for muscle (p < 0.001). TORQUE50 was greater for the leg extensors than for the leg flexors. The 2-way ANOVAs for TORQUE100 and TORQUE200 indicated that there was a significant interaction (p = 0.039–0.048; muscle × playing level); however, the post hoc analyses revealed no differences between groups for the leg flexors (p = 0.213–0.332) or the leg extensors (p = 0.154–0.108). The TORQUE100 and TORQUE200 were greater for the leg extensors than the leg flexors for both groups (p < 0.001). The mean values for all torque variables by playing level are presented in Tables 2 and 3.
The primary findings of the present study indicated that leg flexion rapid torque characteristics (Table 2) were the only variables that effectively discriminated between starters and nonstarters (Figure 2) in elite Division I collegiate American football players. In this study, early rapid torque characteristics included time intervals at 30 and 50 milliseconds from the onset of contraction (2). These variables also demonstrated the largest ESs between groups ranging from 0.71 to 0.82, which has been considered to be a large and practically important effect (19). A likely explanation of these findings is the relationship between early rapid force production and the athletic skills that may be important for successful performance in football-related activities. For example, several critical football-related tasks such as running, accelerating, and cutting may involve relatively short foot strike contact times (≤100 milliseconds) (3,22,32,40). Previous authors have reported that running speed is significantly related to the force applied to the running surface (40) and also that faster runners may achieve peak ground reaction force earlier than slower runners (23). In addition, the initial peak of the vertical ground reaction force occurs at 35 millliseconds of the ground contact phase (15), which collectively emphasizes the importance of early rapid torque characteristics in football-related activities.
The maximum rate of force or torque development (i.e., RTDpeak) has been previously assessed as a discriminatory variable for sport and sport related tasks with conflicting results. For example, Wilson and Murphy (42) reported that maximal rate of force development (RFD) during an isometric squat test was unable to discriminate between subjects of differing levels in cycling performance. In contrast, Pryor et al. (30) revealed that maximum concentric RFD was able to discriminate among good and poor performers in bench press throws and the seated shot put; however, isometric RFD testing was unable to discriminate between performers. Strass et al. (34) found that maximal RFD was a significant predictor of sprint swimming ability in a 50-m swim test; however, this finding was only effective in 1 of the 3 positions where maximal RFD was assessed. Our findings add to these conflicting results and demonstrated that isometric RTDpeak was unable to discriminate between playing levels. It is possible that players are becoming stronger, faster, and more powerful than in past decades (33), which may lead to reduced variation among players in various performance measures (18), thus making measures of RTDpeak less sensitive in discriminating between playing levels. However, the time taken to achieve RTDpeak (TTRTDpeak) for the leg flexors was significantly different among playing levels, with the starters and nonstarters achieving their RTDpeak at 41.84 and 56.20 milliseconds, respectively. These findings may indicate that the TTRTDpeak may be a more sensitive indicator of playing level in elite collegiate football players and highlights the importance of rapid torque characteristics occurring within the first 50 milliseconds from the onset of contraction.
Previous authors who have investigated the influence of maximal strength on talent identification in American football have reported contrasting findings (7,9,10,16,31). For example, Fry and Kraemer (16) reported that bench press strength but not back squat strength was able to more effectively discriminate among playing divisions in collegiate football players, whereas Burke et al. (10) reported that back squat strength was an effective measure in identifying playing level. In the current study, maximal isometric strength and the later rapid torque characteristics (≥100 milliseconds) for both muscle groups were unable to discriminate among playing levels in this population. It is possible that starters and nonstarters at elite Division I programs have similar anthropometric, strength, and power characteristics (18), making maximal isometric strength measures a less sensitive measure to discriminate between playing levels. Similarly, later rapid torque characteristics (>100 milliseconds) were unable to differentiate between playing levels which may be because of their strong relationship with MVC strength (2).
An interesting finding of this study was the ability of leg flexion but not leg extension rapid torque characteristics to discriminate among playing levels. Previous authors have suggested that the leg flexors are important in many football-related tasks such as sprinting (4,14,26,27,41), agility (4), and cutting maneuvers (20). For example, Mero and Komi (26) reported higher biceps femoris muscle activation during the push-off phase of sprint running compared with the rectus femoris, and Anderson et al. (4) reported that the peak concentric hamstring force at 60°·s−1 and eccentric hamstring force at 90°·s−1 were better predictors of 40-yd dash and agility, respectively, than were quadriceps strength measures. Delecluse (14) has also suggested that “the hamstrings are even singled out as contributing most to producing the highest levels of speed” (p. 149), which is often considered an important component of football playing ability (31).
This study was designed to determine if leg flexion and leg extension maximal and rapid isometric torque characteristics could discriminate between starters and nonstarters in elite Division I collegiate football players. The results of the present study indicated that only leg flexion early rapid torque characteristics (≤50 milliseconds) were able to discriminate between different playing levels. These findings demonstrate that rapid torque variables may serve as additional performance measures to help coaches recruit and identify top-level players and provide support for the utility of these laboratory measures to translate to “on-the-field performance.” In addition, coaches and practitioners may also consider using these findings when designing strength and conditioning programs which may be aimed at maximizing early rapid muscle torque characteristics, particularly for the leg flexors.
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Keywords:© 2013 National Strength and Conditioning Association
talent identification; rate of torque development; quadriceps; hamstrings