Comparison of reactive knee stiffening strategies between sexes

Date
2011
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University of Delaware
Abstract
Context: Our understanding of joint stability is limited regarding knee muscle stiffening strategies in response to unanticipated external loads. Knee stiffness is suggested to differ between sexes, potentially explaining the gender bias related to knee injuries. Greater insight may be provided by investigating strategies of both sexes to reactively stiffening the knee against sudden perturbations. Purpose: The purpose of this study was to examine stiffness and neuromuscular strategies between males and females when having to volitionally react to an unanticipated perturbation at the knee from pre-contracted and relaxed states. Participants: Stiffness values were obtained from 40 participants (20 males, 20 females), who were recreationally active with healthy knees. To minimize hormonal influence on the stiffness values, females were tested between days 0 and 12 of the menstrual cycle. Method: A posttest-only comparison group design was used with the independent variables being sex, contraction state prior to the perturbation (relaxed, pre-contracted to 30% quadriceps MVIC) and response (reaction or no reaction). Participants were seated on a custom-built stiffness assessment device (SPAD) and were fitted with an 8-channel Konigsberg EMG transmitter. The dominant limb knee stiffness was measured at 30o of knee flexion under four conditions: relaxed non-reactive; 30% quadriceps maximum voluntary isometric contraction (QMVIC) non-reactive; relaxed reactive; and 30% quadriceps MVIC reactive. For each condition the SPAD imparted a randomly-timed knee flexion perturbation (Excursion = 40o, Velocity = 75o/s, Acceleration = 1000o/s2) upon the subjects’ knee. During the non-reactive trials, the participants were instructed “not to interfere with the move.” During the reactive trials, the participants were instructed to rapidly and forcefully resist knee flexion as soon as perturbation motion was detected. Main Outcome Measures: Knee stiffness measurements were calculated as ? torque (Nm) / ? position (degrees). Stiffness values were body mass-normalized. Muscle activation strategy was determined by MVIC amplitude normalized EMG (%/ms), peak (%), onset time, EMG areas, and time to peak (TTP) at various time points after the knee perturbation. Results: Reacting to the perturbation exhibited greater stiffness values than not reacting (p<.05). Males demonstrated significantly (p<0.05) higher shorter range (4o) stiffness values (stiffness = 0.073±0.003 Nm/deg/kg) than females (stiffness = 0.064±0.003 Nm/deg/kg). No gender differences were observed for total stiffness. Contraction and reaction interactions were observed for both 4o and total stiffness (4o relaxed, no reaction = 0.057±0.002 Nm/deg/kg, 4o relaxed, reaction = 0.063±0.002 Nm/deg/kg, p<0.05; total pre-contracted, no reaction = 0.006±0.001 Nm/deg/kg, total pre-contracted, reaction = 0.048±0.002 Nm/deg/kg, p<0.05; total relaxed, no reaction = 0.001±0.000 Nm/deg/kg, total relaxed, reaction = 0.054±0.002 Nm/deg/kg). Conclusions: Upon differing instructions, all individuals can greatly modify knee stiffness levels in response to unanticipated loads, which may aid in the design of specific exercises in prevention and rehabilitation programs for knee injuries. Increased male stiffness over the initial short-range of 4o suggests a potential tissue level biomechanical difference in muscle fiber/collagen architecture or resting tone. The lack of significant sex differences in total stiffness (40o) suggests females have an equal capacity to resist large excursions from unanticipated external knee loads through stiffness regulation if body mass is taken into consideration.
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