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David H Perrin

Publications and source records attributed to David H Perrin.

18 recordsLinked to original sources
PubMed2006

Absolute serum hormone levels predict the magnitude of change in anterior knee laxity across the menstrual cycle.

This study aimed to determine whether absolute sex hormone concentrations predict the magnitude of knee joint laxity changes across the menstrual cycle. Twenty-two females (18-30 years, body mass index <or=30), who reported normal menstrual cycles for the previous 6 months were tested daily across one complete menstrual cycle for serum levels of estradiol (E=pg/mL), progesterone (P=ng/mL), and testosterone (T=ng/dL), and knee joint laxity (K(Lax)=mm displacement at 134N) measured with a standard knee arthrometer. The change in K(Lax) across the cycle (maximum-minimum), and minimum (early follicular) and peak (postovulatory) hormone concentrations were recorded for each subject. A stepwise linear regression determined if the minimum, peak, or absolute change in hormone concentrations would predict the magnitude of change in K(Lax) across the cycle. K(Lax) changed on average 3.2+/-1.1 mm across the menstrual cycle (range, 1.5-5.3 mm). Minimum levels of E (39.9+/-11.8 pg/mL) and P (0.61+/-0.27 ng/mL), coupled with peak concentrations of E (199.6+/-54.9 pg/mL) and T (22.5+/-10.5 ng/dL) explained 57.6% of the change in K(Lax) across the cycle. Greater absolute changes in K(Lax) were observed in response to peak E and T levels when minimum E concentrations were lower and minimum P concentrations were higher in the early follicular phase. The absolute minimum concentrations of E and P in the early follicular phase appear to be important factors in determining the sensitivity of the knee joint's response to changing hormone levels.

Adolescent↗
PubMed2006

Nonweight-bearing anterior knee laxity is related to anterior tibial translation during transition from nonweight bearing to weight bearing.

We examined the relationship between anterior knee laxity (AKL), evaluated while the knee was nonweight bearing, and anterior translation of the tibia relative to the femur (ATT), evaluated when the knee transitioned from nonweight-bearing to weight-bearing conditions in response to an applied compressive load at the foot. Twenty subjects with normal knees (10 M, 10 F; 25.2 +/- 4.1 years, 169.8 +/- 11.5 cm, 71.6 +/- 16.9 kg) underwent measurements of AKL and ATT of the right knee on 2 days. AKL was measured at 133N with the KT-2000. ATT was measured with the Vermont Knee Laxity Device and electromagnetic position sensors attached to the patella and the anteromedial aspect of the proximal tibia. Three trials for each measure were averaged and analyzed. Measurement consistency was high for both AKL (ICC = 0.97; SEM = 0.44 mm) and ATT (ICC = 0.88; SEM = 0.84 mm). Linear regression revealed that AKL predicted 35.5% of the variance in ATT (p = 0.006), with a prediction equation of Y(ATT) = 3.20 + 0.543(X(AKL)). Our findings suggest that increased AKL is associated with increased ATT as the knee transitions from nonweight-bearing to weight-bearing conditions. The potential for increased knee joint laxity to disrupt normal knee biomechanics during activities such as landing from a jump, or the foot strike phase of gait deserves further study.

Adult↗
PubMed2006

Energy absorption as a predictor of leg impedance in highly trained females.

Although leg spring stiffness represents active muscular recruitment of the lower extremity during dynamic tasks such as hopping and running, the joint-specific characteristics comprising the damping portion of this measure, leg impedance, are uncertain. The purpose of this investigation was to assess the relationship between leg impedance and energy absorption at the ankle, knee, and hip during early (impact) and late (stabilization) phases of landing. Twenty highly trained female dancers (age = 20.3 +/- 1.4 years, height = 163.7 +/- 6.0 cm, mass = 62.1 +/- 8.1 kg) were instrumented for biomechanical analysis. Subjects performed three sets of double-leg landings from under preferred, stiff, and soft landing conditions. A stepwise linear regression analysis revealed that ankle and knee energy absorption at impact, and knee and hip energy absorption during the stabilization phases of landing explained 75.5% of the variance in leg impedance. The primary predictor of leg impedance was knee energy absorption during the stabilization phase, independently accounting for 55% of the variance. Future validation studies applying this regression model to other groups of individuals are warranted.

Adult↗
PubMed2005

Females recruit quadriceps faster than males at multiple knee flexion angles following a weight-bearing rotary perturbation.

OBJECTIVE: To compare the effect of knee angle on muscle response times and neuromuscular recruitment patterns between sexes following a perturbation in single leg stance at 10 degrees, 20 degrees, and 30 degrees. We hypothesized that response times would be faster at lesser knee flexion angles and that females would recruit their quadriceps faster than males at all angles. DESIGN: A repeated-measures design. SETTING: Motion analysis laboratory. PARTICIPANTS: Twenty (10 female; 10 male) healthy, recreationally active volunteers. INTERVENTIONS: A rotary perturbation in single leg stance. OUTCOME MEASUREMENTS: Response times of the medial and lateral quadriceps, hamstrings, and gastrocnemius. RESULTS: There was a trend toward faster response times for all muscles closer toward extension. A consistent neuromuscular recruitment pattern for both males and females was evident for each knee angle tested. Females, however, contracted their quadriceps faster than males at all knee flexion angles. CONCLUSIONS: Small changes in knee angle near extension do not alter muscle response times and hence neuromuscular recruitment patterns in males and females. Regardless of knee flexion angle, following a perturbation in single leg stance, females contract their quadriceps faster than males. CLINICAL RELEVANCE: Earlier contraction of the quadriceps in females may increase anterior tibial translation and hence anterior cruciate ligament strain, thereby heightening injury risk.

Adult↗
PubMed2004

Low levels of anterior tibial loading enhance knee extensor reflex response characteristics.

We examined whether neuromuscular reflexes were altered with anterior loads applied to the tibio-femoral joint. A ligament testing device was modified by attaching a reflex hammer to a steel mounted frame to illicit a patellar tendon tap, while anterior directed loads displaced the tibia on the femur. Five trials were acquired while anterior-directed loads (20, 50, 100 N; counterbalanced) were applied to the posterior tibia between 20 N pre (20 N(Pre)) and post (20 N(Post)) baseline conditions on two different days. Surface electromyography (sEMG) recorded mean quadriceps (Q) and hamstring (H) reflex time (R(Time)=ms) and reflex amplitude (R(Amp)=%MVIC). A load cell on the anterior tibia measured the timing (KE(Time)=ms) and amplitude (KE(Amp)=N) of the knee extension force, and was used to calculate electromechanical delay (EMD=ms) and peak knee extension moment (KE(Mom)=Nm/kg). Data from 19 recreationally active subjects revealed good to excellent response consistency between test days and between baseline conditions for R(Time), R(Amp), KE(Time) and KE(Amp). With anterior tibial loading, R(Time) was faster at 50 N vs. 20 N(Post), and R(Amp) was greater at 20 N(Pre) vs. 20 N(Post) (Q and H) and at 50 N vs. 100 N (Q only). KE(Mom) was greater at 20 N(Pre) and 50 N vs. 20 N(Post), and EMD was shorter at 50 N vs. 20 N, 20 N(Pre) and 20 N(Post). These results suggest that knee extensor reflex responses are enhanced with low (50 N) but not moderate (100 N) anterior loading of the knee.

Adult↗
PubMed2004

Knee ligament behavior following a controlled loading protocol does not differ by menstrual cycle day.

BACKGROUND: Females experience a disproportionate number of anterior cruciate ligament injuries compared to males. Increased estradiol concentration has been suggested to alter ligament properties and strength. Determining whether the knee responds differently to an external load at various hormonal levels may be helpful in further explaining the gender disparity. METHODS: Estradiol, progesterone and testosterone were quantified at menses, near ovulation and at the mid-luteal phase. With one knee serving as the control limb and the other as the experimental limb, displacement at 134N and stiffness between 90 and 134N were recorded with a knee ligament arthrometer on both knees before and after a loading protocol. The protocol consisted of three, 3-min, posterior to anterior normalized loads directed to the posterior calf with a ligament testing device. FINDINGS: The loading protocol produced a measurable increase in displacement but not stiffness. Neither displacement nor stiffness measures however were affected by day of the menstrual cycle. No consistent relationships between hormonal concentrations and displacement or stiffness were evident. INTERPRETATION: Following a controlled, static external load, displacement and stiffness were not affected differently by day of the menstrual cycle.

Adolescent↗
PubMed2004

Kinematic analysis of functional lower body perturbations.

BACKGROUND: Sudden changes in direction on a single weight-bearing-limb are commonly associated with injury to the lower extremity. The purposes of this study were to assess the between day reliability of hip, knee, and ankle kinematic displacements achieved with internal and external femur-on-weight-bearing-tibia rotation perturbations and to determine the effect of these perturbations on three dimensional hip, knee and ankle kinematics. METHODS: Twenty recreationally active, healthy college students with no history of significant orthopedic injury (10 male, 10 female) were subjected to a forward and either internal or external rotary perturbation of the trunk and thigh on the weight-bearing-tibia while three dimensional kinematics were simultaneously collected. The protocol was repeated 24-48 h later to assess reliability. FINDINGS: External perturbations resulted in significant internal rotation (IR) of the tibia on the femur (mean 7.3 (SD 3.9 degrees)) and IR of the femur on the pelvis (mean 6.8 (SD 5.4 degrees)) (P<0.05). Internal perturbations resulted in significant external rotation (ER) of the tibia on the femur (mean 6.8 (5.9 degrees)) and ER of the femur on the pelvis (mean 10.7 (SD 96.1 degrees)) (P<0.05). Additionally the external perturbation results in a significantly greater knee valgus (mean 3.6 (SD 2.2 degrees)) position while the internal perturbation results in a significantly greater knee varus position (mean 2.3 (SD 3.5 degrees)) (P<0.05). External perturbation hip and knee total joint displacements revealed moderate to strong reliability (Intraclass Correlation Coefficient(2,k)=0.67-0.94) while internal perturbations revealed slightly higher Intraclass Correlation Coefficients(2,k)(0.80-0.96). INTERPRETATION: The lower extremity perturbation device provides a consistent external and internal perturbation of the femur on the weight-bearing-tibia. The observed transverse and frontal plane kinematics are similar to motions observed during cross-over and side-stepping tasks.

Adult↗
PubMed2004

Knee joint laxity affects muscle activation patterns in the healthy knee.

This study investigated the effects of anterior knee joint laxity on muscle activation patterns prior to and following a lower extremity perturbation. Participants were subjected to a forward and either internal (IR) or external (ER) rotation perturbation of the trunk and thigh on the weight-bearing shank. Pre-activity (%MVIC) before the perturbation, and reflex time (ms) and mean reflex amplitude (%MVIC) following the perturbation were recorded via surface electromyography (sEMG) in the medial and lateral gastrocnemius, hamstring and quadriceps muscles. Twenty-one NCAA DI intercollegiate female athletes with below average anterior knee laxity (3-5 mm) were compared to 21 with above average anterior knee laxity (7-14 mm) as measured by a standard knee arthrometer. Groups differed in reflex timing by muscle (P = 0.013), with females with above average knee laxity (KT((>7 mm))) demonstrating a 16 ms greater delay in biceps femoris reflex timing compared to females with below average knee laxity (KT((<5 mm))). Groups also differed in muscle activation amplitude by response, muscle and direction of rotation (i.e. a 4-way interaction; P = 0.027). The magnitude of change from pre to post perturbation was significantly less in KT((>7 mm)) vs. KT((<5 mm)) for the medial (MG) and lateral (LG) gastrocnemius muscles, primarily due to higher levels of muscle preactivity while awaiting the perturbation (MG = 20% vs. 12% MVIC, P = 0.05; LG = 33% vs. 21% MVIC, P = 0.11). Further, KT((>7 mm)) demonstrated higher activation levels in the biceps femoris than KT((<5 mm)) (47% vs. 27% MVIC; P = 0.025) regardless of response (pre vs. post perturbation) or direction of rotation. These findings suggest females with increased knee laxity may be less sensitive to joint displacement or loading (delayed reflex), and are more reliant on active control of the gastrocnemius and biceps femoris muscles to potentially compensate for reduced passive joint stability.

Adult↗
PubMed2004

Serial administration of clinical concussion assessments and learning effects in healthy young athletes.

OBJECTIVE: To determine if serial administration of the Standardized Assessment of Concussion (SAC) and Balance Error Scoring System (BESS) would elicit a learning effect in young athletes and to determine the intratester reliability of scoring the BESS. DESIGN: A prospective study of 50 healthy young athletes who were assigned to either the control or practice group. All subjects were administered the assessments on 2 occasions, 60 days apart. In addition, subjects in the practice group received serial administration of the assessments on 3 occasions in the week following the initial assessment. SETTING: University Sports Medicine/Athletic Training Research Laboratory. SUBJECTS: Fifty uninjured young athletes between 9 and 14 years of age. MAIN OUTCOME MEASURED: Scores on 2 clinical concussion assessments, the SAC and the BESS. RESULTS: We found a significant learning effect upon serial BESS testing in the practice group. BESS error scores were significantly lower than baseline (15.0 +/- 4.6) on days 5 (11.3 +/- 5.33), 7 (12.4 +/- 6.2), and 60 (12.6 +/- 6.2). We also found a significant learning effect upon the day 60 BESS administration across all subjects. We did not find a practice or learning effect with serial SAC test administration. The intratester reliability of the investigator's ability to score repeated observations of the same BESS test ranged from 0.87 to 0.98. CONCLUSIONS: Our results demonstrated that serial administration of the BESS elicited a learning effect, which was more prominent during the tandem conditions. Clinicians utilizing the BESS as a measure of postural stability should be aware of the potential for improvement with repeated testing. Clinicians should not expect improvement with the SAC, as scores remained relatively stable across all trials.

Adolescent↗
PubMed2004

Relationship between sex hormones and anterior knee laxity across the menstrual cycle.

PURPOSE: To comprehensively quantify through daily, serial measures changes in knee laxity as a function of changing sex-hormone levels across one complete menstrual cycle. METHODS: Twenty-five females, 18 - 30 yr, body mass index < or = 30, who reported normal menstrual cycles (28-32 d) over the past 6 months participated. Participants were tested daily across one complete menstrual cycle; 5-7 cc of venous blood were withdrawn to assay serum levels of estradiol, progesterone, and testosterone. Knee laxity was measured as the amount of anterior tibial displacement at 133 N, using a standard knee arthrometer. To evaluate the relationship of knee laxity to changes in sex hormone concentrations, a multiple linear regression model with the possibility of a time delay was performed on each individual subject and the group as a whole. RESULTS: Individual regression equations revealed an average of 63% of the variance in knee laxity was explained by the three hormones and their interactions. All three hormones significantly contributed to the prediction equation, and the amount of variance explained was substantially greater when a time delay was considered. On average, knee laxity changed approximately 3, 4, and 4.5 d after changes in estradiol, progesterone, and testosterone, respectively. When females were analyzed as a group, only 8% of the variance in knee laxity was explained by sex-hormones levels. CONCLUSION: Changes in sex hormones mediate changes in knee laxity across the menstrual cycle. However, the strength of this relationship, the relative contribution of each hormone, and the associated time delay are highly variable between women. This individual variability is consistent with the variability in menstrual cycle characteristics among women.

Adolescent↗
PubMed2003

Reliability and validity of the Biodex system 3 pro isokinetic dynamometer velocity, torque and position measurements.

This study quantitatively assessed the mechanical reliability and validity of position, torque and velocity measurements of the Biodex System 3 isokinetic dynamometer. Trial-to-trial and day-to-day reliability were assessed during three trials on two separate days. To assess instrument validity, measurement of each variable using the Biodex System 3 dynamometer was compared to a criterion measure of position, torque and velocity. Position was assessed at 5 degrees increments across the available range of motion of the dynamometer. Torque measures were assessed isometrically by hanging six different calibrated weights from the lever arm. Velocity was assessed (30 degrees/s to 500 degrees/s) across a 70 degrees arc of motion by manually accelerating the weighted lever arm. With the exception of a systematic decrease in velocity at speeds of 300 degrees/s and higher, the Biodex System 3 performed with acceptable mechanical reliability and validity on all variables tested.

Humans↗
PubMed2002

Acute Orthotic Intervention Does Not Affect Muscular Response Times and Activation Patterns at the Knee.

OBJECTIVE: To evaluate the short-term effect of a semirigid foot orthotic device on response times and activation patterns of knee musculature in individuals with hyperpronation after a lower extremity perturbation in a single-leg, weight-bearing stance. DESIGN AND SETTING: We used a lower extremity perturbation device designed to produce a forward and either internal or external rotation of the trunk and femur on the weight-bearing tibia to evoke a reflex response. Subjects were tested both with and without orthotic devices. SUBJECTS: Seventeen (13 male, 4 female) volunteers (age, 20.6 +/- 1.8 years; height, 181.0 +/- 8.1 cm; weight, 87.4 +/- 19.5 kg; navicular drop, 12.1 +/- 1.8 mm) with a navicular drop greater than 10 mm volunteered for this study. MEASUREMENTS: Long latency reflex times were recorded via surface electromyography for the medial and lateral hamstrings, gastrocnemius, and quadriceps muscles. RESULTS: A dependent-sample t test revealed a significant decrease in navicular drop with orthotic intervention (P <.0001). With that confirmed, separate repeated-measures analyses of variance with 2 within factors (orthotic condition and muscle) revealed no significant difference in muscle response time between orthotic and nonorthotic conditions for either internal or external rotation perturbation. Although we found a main effect for muscle for both internal (P <.0001) and external (P <.0001) rotation, indicating a preferred muscle activation order, this activation order did not differ between orthotic and nonorthotic conditions (internal rotation P =.674, external rotation P =.829). CONCLUSIONS: Our findings suggest that a short-term application of a semirigid orthotic device does not alter muscle response times or activation patterns of the muscles that stabilize the knee. Further research is needed to determine whether changes in activation patterns may occur over time since mechanical adaptations occur with long-term wear.

Journal Article↗
PubMed2002

Chronic Ankle Instability Does Not Affect Lower Extremity Functional Performance.

OBJECTIVE: To determine if functional performance is impaired in individuals with self-reported chronic ankle instability. DESIGN AND SETTING: We used a between-groups design to assess 3 functional variables. All data were collected at a Division III college and a military academy. Before testing, all subjects performed a 5-minute warm-up, followed by a series of stretches for the lower extremity muscles. Subjects then performed cocontraction, shuttle run, and agility hop tests in a counterbalanced fashion. Three trials for each functional test were completed and averaged for analysis. SUBJECTS: Twenty men with a history of at least 1 significant ankle sprain and episodes of at least 1 repeated ankle injury or feelings of instability or "giving way" were compared with 20 men with no prior history of ankle injury. Subjects were matched by age, height, weight, and activity level. MEASUREMENTS: Time to completion was measured in seconds for the cocontraction and the shuttle run tests. The agility hop test was measured on an error point scale. RESULTS: Using 3 separate, independent, 2-tailed t tests, we found no significant difference between groups for the cocontraction (P =.452), shuttle run (P =.680), or agility hop (P =.902) tests. CONCLUSIONS: Chronic ankle instability is a subjectively reported phenomenon defined as the tendency to "give way" during normal activity. Although athletes commonly complain of subjective symptoms associated with chronic ankle instability, our findings suggest that these symptoms do not negatively influence actual functional performance. Future researchers should evaluate other, more demanding functional-performance tests to further substantiate these findings.

Journal Article↗
PubMed2002

Strength, Functional Outcome, and Postural Stability After Anterior Cruciate Ligament Reconstruction.

OBJECTIVE: To compare postural stability, single-leg hop, and isokinetic strength measurements in subjects after anterior cruciate ligament (ACL) reconstruction with an age- and activity-matched control group. DESIGN AND SETTING: Subjects reported to a sports medicine/athletic training research laboratory for testing. Subjects reported for one testing session for a total test time of 1 hour. SUBJECTS: Twenty subjects with ACL reconstructions (ACLRs) and 20 age- and activity-matched controls were selected to participate in this study. An arthroscopically assisted central one-third bone-patellar tendon procedure was used to repair the ACLs. MEASUREMENTS: We measured concentric and eccentric peak torque (Nm) measurements of the knee extensors and flexors at 120 degrees and 240 degrees /second on an isokinetic dynamometer. Unilateral and bilateral dynamic postural stability was measured as a stability index in the anterior-posterior and medial-lateral planes with the Biodex Stability System. We tested single-leg hop for distance to measure objective function. RESULTS: We found no significant difference between the ACLR and control subjects for stability index or knee-flexion peak torque scores. On the single-leg hop for distance, the ACLR subjects hopped significantly shorter distances with the involved limb than the uninvolved limb. Furthermore, the ACLR subjects' single-leg hop distance was significantly less when the involved limb was compared with the control-group matched involved limb, and the ACLR subjects performed significantly better when the uninvolved limb was compared with the control-group matched uninvolved limb. The ACLR subjects produced significantly greater torque in the uninvolved leg than in the involved leg. In addition, the peak torque was significantly less for the involved limb in the ACLR group when compared with the matched involved limb of the control group. CONCLUSIONS: After ACLR (mean = 18 +/- 10 months), single-leg hop-for-distance scores and quadriceps strength were not within normal limits when compared with the contralateral limb. Our results suggest that bilateral and single-limb postural stability in the ACLR group was not significantly different than the control group at an average follow-up of 18 months after surgery.

Journal Article↗
PubMed2002

Mechanisms and management of stress fractures in physically active persons.

OBJECTIVE: To describe the anatomy of bone and the physiology of bone remodeling as a basis for the proper management of stress fractures in physically active people. DATA SOURCES: We searched PubMed for the years 1965 through 2000 using the key words stress fracture, bone remodeling, epidemiology, and rehabilitation. DATA SYNTHESIS: Bone undergoes a normal remodeling process in physically active persons. Increased stress leads to an acceleration of this remodeling process, a subsequent weakening of bone, and a higher susceptibility to stress fracture. When a stress fracture is suspected, appropriate management of the injury should begin immediately. Effective management includes a cyclic process of activity and rest that is based on the remodeling process of bone. CONCLUSIONS/RECOMMENDATIONS: Bone continuously remodels itself to withstand the stresses involved with physical activity. Stress fractures occur as the result of increased remodeling and a subsequent weakening of the outer surface ofthe bone. Once a stress fracture is suspected, a cyclic management program that incorporates the physiology of bone remodeling should be initiated. The cyclic program should allow the physically active person to remove the source of the stress to the bone, maintain fitness, promote a safe return to activity, and permit the bone to heal properly.

Journal Article↗
PubMedDate not supplied

The independent and interactive effects of navicular drop and quadriceps angle on neuromuscular responses to a weight-bearing perturbation.

CONTEXT: Little is known about the effects of static alignment on neuromuscular control of the knee during dynamic motion. OBJECTIVE: To evaluate the isolated and combined effects of quadriceps angle (QA) and navicular drop (ND) on neuromuscular responses to a weight-bearing perturbation. DESIGN: Mixed-model, repeated-measures design. SETTING: Sports medicine and athletic training research laboratory. PATIENTS OR OTHER PARTICIPANTS: Seventy-nine National Collegiate Athletic Association Division I collegiate female athletes, classified with below-average ND and QA (LND-LQA); below-average ND and above-average QA (LND-HQA); above-average ND and below-average QA (HND-LQA); or above-average ND and QA (HND-HQA). INTERVENTION(S): A lower extremity perturbation device produced a forward and either internal or external rotation of the trunk and femur on the weight-bearing tibia to evoke a reflex response. MAIN OUTCOME MEASURE(S): Neuromuscular responses were examined in the quadriceps, hamstrings, and gastrocnemius muscles: preperturbation amplitude 50 milliseconds before the perturbation, reflex time, and postperturbation amplitude 150 milliseconds immediately postperturbation. RESULTS: Navicular drop had the greatest effect on preperturbation amplitude of the lateral hamstrings and postperturbation amplitude of all muscles, with greater activation amplitude noted in subjects in the HND classifications. Quadriceps angle primarily affected reflex time of the quadriceps; in subjects with LQA, reflex time was faster for internal rotation than external rotation perturbations. The interaction between ND and QA had the greatest effect on reflex time of the lateral hamstrings. For internal rotation perturbations, subjects in the LND classifications had faster reflex times in the lateral hamstrings if they had HQA values rather than LQA values. With external rotation perturbations, HND-LQA subjects had slower reflex times than those in all other alignment classifications. CONCLUSIONS: Navicular drop and QA have both independent and interactive effects on neuromuscular responses to a weight-bearing, rotational perturbation. These interactive effects highlight the importance of considering the entire lower extremity posture rather than a single alignment characteristic, given the potential for one alignment factor to compensate for or interact with another.

Journal Article↗
PubMedDate not supplied

Fatigue, vertical leg stiffness, and stiffness control strategies in males and females.

CONTEXT: Fatigue appears to influence musculoskeletal injury rates during athletic activities, but whether males and females respond differently to fatigue is unknown. OBJECTIVE: To determine the influence of fatigue on vertical leg stiffness (K (VERT)) and muscle activation and joint movement strategies and whether healthy males and females respond similarly to fatigue. DESIGN: Repeated-measures design with all data collected during a single laboratory session. SETTING: Laboratory. PATIENTS OR OTHER PARTICIPANTS: Physically active males (n = 11) and females (n = 10). INTERVENTION(S): Subjects performed hopping protocols at 2 frequencies before and after fatigue, which was induced by repeated squatting at submaximal loads. MAIN OUTCOME MEASURE(S): We measured K (VERT) with a forceplate and peak muscle activity of the quadriceps, hamstrings, gastrocnemius, soleus, and anterior tibialis muscles with surface electromyography. Sagittal-plane kinematics at the knee and ankle were recorded with an electrogoniometer. RESULTS: After fatigue, K (VERT) was unchanged for all subjects. However, both males and females demonstrated reduced peak hamstrings ( P = .002) and anterior tibialis ( P = .001) activation, coupled with increased gastrocnemius ( P = .005) and soleus ( P = .001) peak activity, as well as increased quadriceps-hamstrings ( P = .005) and gastrocnemius/soleus-anterior tibialis coactivation ratios ( P = .03) after fatigue. Overall, females demonstrated greater quadriceps-hamstrings coactivation ratios than males, regardless of the fatigue condition ( P = .026). Only females showed increased knee flexion at initial contact after fatigue during hopping ( P = .03). CONCLUSIONS: Although K (VERT) was unaffected, the peak muscle activation and joint movement strategies used to modulate K (VERT) were affected after fatigue. Once fatigued, both males and females used an ankle-dominant strategy, with greater reliance on the ankle musculature and less on the knee musculature. Also, once fatigued, all subjects used an antagonist inhibition strategy by minimizing antagonist coactivation. Overall, females used a more quadriceps-dominant strategy than males, showing greater quadriceps activity and a larger quadriceps-hamstrings coactivation ratio. Changes in muscle activation and coactivation ratios because of fatigue and sex are suggested to alter knee joint stability and increase anterior cruciate ligament injury risk.

Journal Article↗