Velocity-dependent power loss in the knee extensors of young and old men Geoffrey A. Power, Brian H. Dalton, Charles L. Rice (FACSM) and Anthony A. Vandervoort (FACSM) Canadian Centre for Activity and Aging, School of Kinesiology, The University of Western Ontario, London, ON Abstract Methods Results ↓40% ↓15% Raw Fatigue Data Young Old Velocity (deg·s-1) Contraction Number Neuromuscular fatigue, defined as any exercise induced reduction in the ability to generate force or power is influenced by such factors as age, muscle group and contraction type. During isometric tasks old adults incur less muscle fatigue than young. This apparent ability to resist fatigue is often abolished when dynamic tasks are performed. However, few studies have investigated the age-related fatigue response to velocity-dependent contractions. The PURPOSE here was to investigate the contributions of fatigue-related declines in shortening velocity in 10 young (~25 y) and 10 old (~75 y) men. METHODS Neuromuscular measures were collected from the knee extensors at baseline and during a bout of thirty maximal velocity-dependent shortening contractions. Measures included: electrically evoked twitch torque, maximal voluntary isometric contractions (MVCs), electromyography of the vastus lateralis and medialis, voluntary activation, maximal velocity-dependent contractions and ratings of perceived exertion (RPE). RESULTS absolute measures of twitch torque, MVC and velocity-dependent power were 37%, 32%, and 46% higher in the young men compared to the old, respectively and both groups were capable of high activation (>95%) of the knee extensors. Similar to young men, the old had a reduction in velocity-dependent power throughout the fatigue task. The young progressively decreased to 82% of baseline power at task termination, while the old decreased to 81% of baseline power by the twentieth contraction and was further reduced to 67% at task termination. RPE increased with contraction number similarly between groups. CONCLUSION this study highlights the novel contributions of fatigue-related declines in shortening velocity to power reduction and found that slower shortening velocities in older adults may be an important factor in the greater fatigability observed during velocity-dependent tasks, which relate to tasks of daily living. Participant Characteristics: Neuromuscular Properties: Age (y) Height (cm) Mass (kg) Young (10) 24.5 ± 2.7 177.1 ± 8.3 79.5 ± 9.8 Old (9) 73.2 ± 3.5 175.6 ± 4.1 84.1 ± 9.4 Pt (Nm) MVC Velocity (deg·s-1) Power (W) Young 50.3 ± 10.5 311.5 ± 65.3 481.4 ± 26.3 506.3 ± 116.7 Old 29.6 ± 7.3* 208.0 ± 34.9* 383.2 ± 28.6* 279.7 ± 53.8* 900 Graphic design by Andrew Davidson Hypothesis Introduction and Rationale Purpose Experimental Set-up: Relative Baseline Differences % Difference from Young O Twitch Torque & Half Relaxation Time % Baseline † † * † † * Velocity-Dependent Power Baseline / Fatigue Neuromuscular Measures: Peak twitch (Pt), twitch half-relaxation time Isometric maximum voluntary contraction (MVC) Voluntary activation (VA) = (1 - superimposed twitch/control twitch) x 100 Fatigue is any exercise-induced reduction in strength or power and is task- and measure-dependent. Old adults incur less muscle fatigue than young during isometric tasks (Kent-Braun 2009). This fatigue resistance is often abolished when dynamic tasks are performed. Old adults have reduced isometric contractile speed and dynamic shortening velocity. This may limit the older adult’s ability to resist fatigue when the neuromuscular system is stressed with contractions involving a velocity component. Velocity-dependent contractions involve a constant load (%MVC) and variable velocity, thus resembling daily tasks where the load is fixed and velocity is variable. Few studies have investigated age-related fatigue during maximal velocity-dependent contractions (McNeil et al. 2007). * * † * Y O Y O Y O Y O * † * † Y % Baseline † Fatigue Protocol: 30 maximal velocity-dependent contractions (20% MVC) Peak twitch torque was reduced similarly Half relaxation time slowed more in old than young † O † Tabular data (Mean ± SD) graphical data (Mean ± SE) Significant (p < 0.05) for time † for age * Maximum Voluntary Contraction & RPE % Baseline MVC decreased similarly Voluntary activation remained high(>90%) for both RPE was slightly elevated in old at task termination Contraction Number † Old had a greater slowing in shortening velocity Velocity-dependent power loss was ~25% greater in old O Y † † † Conclusions † Old had a greater fatigue-related decline in shortening velocity leading to greater power loss. Slower shortening velocities in older adults may be an important factor in the greater fatigability observed during velocity-dependent tasks. The greater reduction in velocity-dependent power may adversely affect daily tasks requiring repetitive dynamic contractions Supported by: To investigate changes in voluntary and electrically-evoked neuromuscular properties and explore the contributions of fatigue-related declines in velocity-dependent power of the knee extensors in young and old men. Y O Y O Old men will experience a larger reduction in shortening velocity resulting in a greater loss of velocity-dependent power. Old were 33% weaker, 20% slower and 45% less powerful