(10) SEX BASED COMPARISONS OF ABSOLUTE AND NORMALIZED ISOMETRIC AND ISOKINETIC LEG EXTENSION STRENGTH AND POWER

Purpose: The purpose of this study was to compare absolute and normalized isometric and isokinetic leg extension strength and power between adult males and females.

Methods: Twenty-eight college-aged males and females participated in this study (n=14 males, mean±95% confidence interval, age=23±2yrs; n=14 females, age=24±1yrs). Panoramic ultrasound images quantified quadriceps femoris muscle cross-sectional area (CSA). Participants completed maximal voluntary isometric contractions (MVICs) of the leg extensors, followed by maximal voluntary isokinetic leg extension muscle actions at angular velocities of 60, 120, 180, 240, and 300°∙sec^-1. Peak torque (PT) was taken during MVICs and each isokinetic muscle action, while mean power (MP) was taken during each isokinetic muscle action. For each participant, PT and MP were normalized to CSA and MVIC PT separately. Independent samples t-tests compared absolute CSA, MVIC PT, and MVIC PT normalized to CSA between sexes. Mixed factorial analyses of variance compared absolute and normalized isokinetic PT and MP between sexes and across velocity. An alpha level of 0.05 was considered statistically significant for all tests.

Results: CSA, absolute MVIC PT, and MVIC PT normalized to CSA were greater in males compared to females (p≤0.050). For absolute isokinetic PT, isokinetic PT normalized to CSA, and isokinetic PT normalized to MVIC PT, the males were stronger than the females at each velocity (p≤0.002). In both males and females, absolute isokinetic PT, isokinetic PT normalized to CSA, and isokinetic PT normalized to MVIC PT decreased from 60-300°∙sec^-1 (p≤0.026). For absolute isokinetic MP and isokinetic MP normalized to CSA, the males were stronger than the females at each velocity (p≤0.026). When isokinetic MP was normalized to MVIC PT, the males were stronger than the females from 120-300°∙sec^-1 (p≤0.049), with no differences at 60°∙sec^-1 (p=0.498). For the males, absolute isokinetic MP increased from 60-180°∙sec^-1 (p< 0.001), then plateaued from 180-300°∙sec^-1 (p≥0.287). For the females, absolute isokinetic MP increased from 60-120°∙sec^-1 (p< 0.001), then plateaued from 120-300°∙sec^-1 (p≥0.090). For the males, CSA and MVIC PT normalized isokinetic MP increased from 60-180°∙sec^-1 (p< 0.001), then plateaued from 180-300°∙sec^-1 (p≥0.249). For the females, CSA and MVIC PT normalized isokinetic MP increased from 60-180°∙sec^-1 (p≤0.010), plateaued from 180-240°∙sec^-1 (p=1.000), then decreased from 240-300°∙sec^-1 (p≤0.024).

Conclusions: Although the males were stronger than the females for absolute and normalized isokinetic strength and power, the nature of the power-velocity relationship changed with normalization for females. Specifically, when normalized, isokinetic MP increased to a higher angular velocity compared to absolute MP (180 vs. 120°∙sec^-1) for the females. Thus, it appears that power output at higher angular velocities may be at least partially dependent on muscle size and muscle strength, particularly for females. Additionally, other factors outside of CSA and maximal strength may influence sex-specific differences in muscle strength and power. PRACTICAL APPLICATIONS: Females may benefit from prioritizing strength and hypertrophy training when the goal is to increase power output across the velocity spectrum. Future studies should consider holistically examining the underlying physiological factors affecting muscle strength and power differences between males and females.

Acknowledgements: None