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ABSTRACTNineteen professional football players (Age: 26±5 years; Height: 1.84±0.08 m; Mass: 83.4±8.9 kg) completed three x 30 m maximal acceleration sprints from a standing start before completing the Yo-Yo intermittent recovery test level 1. Three x 30 m maximal acceleration sprints were then repeated post-fatigue. Light gates recorded sprint times from 0-5 m, 0-10 m, 0-15 m and 0-30 m. Force platforms collected ground reaction force of the first stance phase of the sprint run. Differences between pre- and post-fatigue were observed in the sprint times over 0-15 m (P = 0.015; CI [0.007, 0.110]) and 0-30 m (P = 0.004; CI [0.056, 0.234]). Peak medial-lateral ground reaction force was lower (P = 0.045; CI [-0.146, -0.005]) post- than pre-fatigue. The ratio of force were significantly different between pre- and post-fatigue for the medial-lateral and anterior-posterior comparison (P = 0.017; CI [-0.063, -0.010]), and the medial-lateral and vertical comparison (P = 0.012; CI [-0.036, -0.007]). Football players altered their sprint mechanics to reduce medial-lateral loading and orient the force in an increased anteroposterior and vertical direction in order to maintain 0-10 m sprint performance. Practitioners should observe medial-lateral force contributions and improve sprint technical efficacy.

ABSTRACTPurpose:To assess traditional and novel isokinetic strength characteristics of the eccentric knee flexor (eccKF) and concentric knee extensor (conKE) musculature, 26 professional football players completed bilateral conKE and eccKF contractions at angular velocities of 180, 270, and 60° · s−1. Method:Peak torque (PT), angle of peak torque (APT), angle-specific torque (AST) analysed every 10° between 40° and 70°, functional range (FR), and dynamic control ratios (DCR) calculated from both the PT (DCRPT) and AST data (DCRAST) were analysed. Results: The PT, APT, and FR data elicited significant contraction*angular velocity interactions (P < 0.001). Significant main effects for contraction*angular velocity*angle and contraction*angular velocity*limb*angle interactions (P ≤ 0.001) were identified for AST data. The DCRPTdata elicited a significant main effect for angular velocity (P < 0.001) and limb (P = 0.018), whereas the DCRASTdata were significantly different across angles (P < 0.001) and elicited a significant (P = 0.002) limb*angle interaction. Conclusion: Traditional analysis variables utilised for isokinetic strength assessments in football may not be appropriate and/or sensitive enough to identify injury risk. Practitioners should utilise the novel metrics proposed in the current study and conduct assessments across a range of joint angles and angular velocities.

This study examined the effect of stimulation frequency (140, 200, 230 and 260 Hz) on isometric force, work loop (WL) power and the fatigue resistance of extensor digitorum longus (EDL) muscle (n=32), isolated from 8- to 10-week-old CD-1 female mice. Stimulation frequency had significant effects on isometric properties of isolated mouse EDL, whereby increasing stimulation frequency evoked increased isometric force, quicker activation and prolonged relaxation (P<0.047) up to 230 Hz and above; thereafter, force and activation did not differ (P>0.137). Increasing stimulation frequency increased maximal WL power output (P<0.001; 140 Hz, 71.3±3.5; 200 Hz, 105.4±4.1; 230 Hz, 115.5±4.1; 260 Hz, 121.1±4.1 W kg−1), but resulted in significantly quicker rates of fatigue during consecutive WLs (P<0.004). WL shapes indicate impaired muscle relaxation at the end of shortening and subsequent increased negative work appeared to contribute to fatigue at 230 and 260 Hz, but not at lower stimulation frequencies. Cumulative work was unaffected by stimulation frequency, except at the start of the fatigue protocol, where 230 and 260 Hz produced more work than 140 Hz (P<0.039). We demonstrate that stimulation frequency affects force, power and fatigue, but these effects are not uniform between different assessments of contractile performance. Therefore, future work examining the contractile properties of isolated skeletal muscle should consider increasing the stimulation frequency beyond that needed for maximal force when examining maximal power but should utilise a sub-maximal stimulation frequency for fatigue assessments to avoid a high degree of negative work atypical of in vivo function.