The quadriceps function in patellofemoral disorders

Summary The influence of quadriceps contraction on patellar alignment was investigated in tangential views of 31 patients with patellofemoral complaints and compared with 25 controls. The congruence angle decreased significantly following quadriceps contraction in patients with patellar subluxation in contrast to those with chondromalacia and normals. Electromyography (EMG) of the vastus lateralis (VL) and vastus medialis obliquus (VMO) during maximal quadriceps contraction was obtained prior to and after 3 months of isometric quadriceps exercises in 15 knees with patellar subluxation and in 11 with idiopathic chondromalacia of the patella. The EMG after 3 months revealed an increase in the activity of both VL and VMO only in patients with patellar subluxation, but the muscular balance of the patella remained unchanged. Only seven patients were improved after 3 months. A more selective training of the VMO is proposed.

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Zusammenfassung Der Einfluß der Quadricepsmuskelkontraktion auf die Führung der Patella wurde bei 31 Patienten mit Beschwerden im Femoropatellargelenk (Subluxation der Patella, idiopathische Chondromalacia patellae) untersucht. Als Kontrolle dienten 25 beschwerdefreie Personen. Röntgenologisch zeigte sich bei Patienten mit Subluxation der Patella der Patellakongruenzwinkel im Vergleich zu Normalpersonen und Patienten mit Chondromalacia patellae abgeflacht. Der Unterschied war statistisch signifikant. Die elektromyographische Aktivität des M. vastus lateralis and des M. vastus medialis obliquue stieg nach 3monatigem intensiven Muskeltraining le diglich bei Patienten mit Subluxation der Patella an, wobei jedoch die muskuläre Balance unverändert blieb. Das Beschwerdebild war jedoch nur bei 7 Patienten gebessert. Es wird daher ein selektives Training des M. vastus medialis obliquus empfohlen.

     

Cutaneous stimulation from patella tape causes a differential increase in vasti muscle activity in people with patellofemoral pain.

Patella taping reduces pain in individuals with patellofemoral pain (PFP), although the mechanism remains unclear. One possibility is that patella taping modifies vasti muscle activity via stimulation of cutaneous afferents. The aim of this study was to investigate the effect of stretching the skin over the patella on vasti muscle activity in people with PFP. Electromyographic activity (EMG) of individual motor units in vastus medialis obliquus (VMO) was recorded via a needle electrode and from surface electrodes placed over VMO and vastus lateralis (VL). A tape was applied to the skin directly over the patella and stretch was applied via the tape in three directions, while subjects maintained a gentle isometric knee extension effort at constant force. Recordings were made from five separate motor units in each direction. Stretch applied to the skin over the patella increased VMO surface EMG and was greatest with lateral stretch. There was no change in VL surface EMG activity. While there was no net increase in motor unit firing rate, it was increased in the majority of motor units during lateral stretch. Application of stretch to the skin over VMO via the tape can increase VMO activity, suggesting that cutaneous stimulation may be one mechanism by which patella taping produces a clinical effect.     

Isometric contractions in the patellofemoral pain syndrome

Summary The role played by the vastus lateralis and vastus medialis obliquus in their alignment of the patella during maximal isometric quadriceps contractions was studied by electromyography in 28 patients with unilateral symptoms of the patellofemoral pain syndrome. Eleven knees were classified as subluxation of the patella and 17 as idiopathic chondromalacia patellae. The muscular activity pattern was the same in the two groups but decreased as compared with the contralateral nonsymptomatic knees. However, none of the groups revealed differences in the activity of vastus lateralis and vastus medialis obliquus suggesting muscular imbalance.     

Isometric contractions in the patellofemoral pain syndrome. An electromyographic study

The role played by the vastus lateralis and vastus medialis obliquus in their alignment of the patella during maximal isometric quadriceps contractions was studied by electromyography in 28 patients with unilateral symptoms of the patellofemoral pain syndrome. Eleven knees were classified as subluxation of the patella and 17 as idiopathic chondromalacia patellae. The muscular activity pattern was the same in the two groups but decreased as compared with the contralateral nonsymptomatic knees. However, none of the groups revealed differences in the activity of vastus lateralis and vastus medialis obliquus suggesting muscular imbalance.     

Ultrasound evaluation of the quadriceps muscles in pronated foot posture

ObjectiveThe main goal of this study was to investigate the size of all portions of the quadriceps muscles in individuals with pronated foot posture compared to normal foot posture using ultrasound imaging.DesignTwenty nine females with pronated foot posture and 29 age-, body weight-, body height-matched females with normal foot posture were recruited from university communities. The muscle thicknesses of the rectus femoris (RF), vastus medialis (VM), vastus medialis oblique (VMO), vastus lateralis (VL) and vastus intermedius (VI) were measured using ultrasound imaging.ResultsThe thicknesses of the RF and VMO were significantly smaller in individuals with pronated foot posture compared to normal foot posture (p < 0.05). No significant differences were observed in the VM, VL, and VI muscle thicknesses in both groups.ConclusionBased on the results of the present study, it seems that besides the foot and lower leg muscles, an integrated assessment of proximal knee muscles, especially quadriceps, is required in individuals with pronated foot posture.     

The relative timing of VMO and VL in the aetiology of anterior knee pain: a systematic review and meta-analysis

Background  

Anterior knee pain (AKP) is a common musculoskeletal complaint. It has been suggested that one factor that may contribute to the presence of AKP is a delay in the recruitment of the vastus medialis oblique muscle (VMO) relative to the vastus lateralis muscle (VL). There is however little consensus within the literature regarding the existence or nature of any such delay in the recruitment of the VMO within the AKP population. The purpose of this systematic review and meta-analysis was to examine the relative timing of onset of the VMO and VL in those with AKP in comparison to the asymptomatic population.     

Age-related changes in electromyographic quadriceps activity during stair descent.

PURPOSE: To determine the effects of age on (i) the onset of vastus medialis obliquus (VMO) activity relative to that of vastus lateralis (VL) and (ii) the onset of quadriceps activity in terminal swing relative to heel-strike, during stair descent. METHODS: Sixty-six healthy men and women with no known knee pathology were recruited from the community and the University of Melbourne. Thirty-three older people aged over 50 (mean (SD) 68 (8) years) and 33 younger people aged under 40 (mean (SD) 25 (4) years) participated. Surface electromyography (EMG) was used to record muscle activity during stair descent. Muscle onset was identified from individual EMG traces with a computer algorithm and validated visually. RESULTS: Both younger and older participants demonstrated a relatively synchronous onset of VMO and VL (i.e. within 10ms of each other) with no differences between the age groups evident (P=0.262). However older participants activated their quadriceps significantly earlier than the younger group (P<0.001) even when differences in body mass and speed of stair descent were accounted for. CONCLUSION: Age-related changes in quadriceps activity are apparent. The clinical implications of such changes are unknown, but increases in joint loading are possible. Earlier quadriceps activation may compensate for balance and strength impairments in older people during challenging activities.     

Simultaneous feedforward recruitment of the vasti in untrained postural tasks can be restored by physical therapy.

BACKGROUND AND PURPOSE: Physical therapy rehabilitation strategies are commonly directed at the alteration of muscle recruitment in functional movements. The aim of this study was to investigate whether feedforward strategies of the vasti in people with patellofemoral pain syndrome can be changed by a physical therapy treatment program in a randomised, double blind, placebo controlled trial. SUBJECTS: Forty (25 female, 15 male) subjects aged 40 yrs or less (27.2+/-7.8 yrs). METHODS: Subjects were allocated to either a placebo treatment or a physical therapy intervention program. The postural challenge used as the outcome measure was not included in the training program. Electromyography (EMG) onsets of vastus medialis obliquus (VMO), vastus lateralis (VL), tibialis anterior and soleus were assessed before and after the six week standardised treatment programs. RESULTS: At baseline the EMG onset of VL occurred prior to that of VMO in both subject groups. Following physical therapy intervention there was a significant change in the time of onset of EMG of VMO compared to VL with the onsets occurring simultaneously. This change was associated with a reduction in symptoms. In contrast, following placebo intervention the EMG onset of VL still occurred prior to that of VMO. CONCLUSION AND DISCUSSION: The results indicate that the feedforward strategy used by the central nervous system to control the patella can be restored. Importantly, the data suggest that this intervention produced a change that was transferred to a task that was not specifically included in the training program. Furthermore, the change in motor control was associated with clinical improvement in symptoms.     

Muscle Architectural and Force-Velocity Curve Adaptations following 10 Weeks of Training with Weightlifting Catching and Pulling Derivatives

The aims of this study were to examine the muscle architectural, rapid force production, and force-velocity curve adaptations following 10 weeks of resistance training with either submaximal weightlifting catching (CATCH) or pulling (PULL) derivatives or pulling derivatives with phase-specific loading (OL). 27 resistance-trained men were randomly assigned to the CATCH, PULL, or OL groups and completed pre- and post-intervention ultrasound, countermovement jump (CMJ), and isometric mid-thigh pull (IMTP). Vastus lateralis and biceps femoris muscle thickness, pennation angle, and fascicle length, CMJ force at peak power, velocity at peak power, and peak power, and IMTP peak force and force at 100-, 150-, 200-, and 250 ms were assessed. There were no significant or meaningful differences in muscle architecture measures for any group (p > 0.05). The PULL group displayed small-moderate (g = 0.25-0.81) improvements in all CMJ variables while the CATCH group displayed trivial effects (g = 0.00-0.21). In addition, the OL group displayed trivial and small effects for CMJ force (g = -0.12-0.04) and velocity variables (g = 0.32-0.46), respectively. The OL group displayed moderate (g = 0.48-0.73) improvements in all IMTP variables while to PULL group displayed small-moderate (g = 0.47-0.55) improvements. The CATCH group displayed trivial-small (g = -0.39-0.15) decreases in IMTP performance. The PULL and OL groups displayed visible shifts in their force-velocity curves; however, these changes were not significant (p > 0.05). Performing weightlifting pulling derivatives with either submaximal or phase-specific loading may enhance rapid and peak force production characteristics. Strength and conditioning practitioners should load pulling derivatives based on the goals of each specific phase, but also allow their athletes ample exposure to achieve each goal. Key points

• There were no significant or practically meaningful changes in vastus lateralis or biceps femoris muscle thickness, pennation angle, or fascicle length for any group.
• The PULL group produced the greatest CMJ force at peak power, velocity at peak power, and peak power adaptations.
• The PULL and OL groups produced similar benefits in rapid force production; however, peak force adaptations favored the OL group.
• Despite visible shifts in the force-velocity curves of the PULL and OL groups, none of the changes were statistically significant.

Key words: Weightlifting, Olympic weightlifting, countermovement jump, isometric-mid thigh pull, force-velocity profile, rate of force development     

Effect of patellar taping on vasti onset timing, knee kinematics, and kinetics in asymptomatic individuals with a delayed onset of vastus medialis oblique.

This randomized within-subject study investigated the effects of patellar tape on the onset of electromyographic (EMG) activity of vastus medialis obliquus (VMO) relative to vastus lateralis (VL), knee kinematics, and kinetics in 12 currently asymptomatic individuals with a VMO timing deficit and a history of patellofemoral pain. Participants were required to complete stair stepping and normal-pace and fast-pace walking tasks under three experimental conditions; no tape, control tape and therapeutic tape. EMG onsets of VMO and VL were measured by surface electrodes, stance phase knee flexion by the PEAK movement analysis system and vertical ground reaction force by a force plate. A two-way repeated measures analysis of variance showed that neither therapeutic tape nor control tape had any effect on the EMG VMO-VL onset timing difference. Therapeutic tape, but not control tape, led to significant increases in stance phase knee flexion. The first peak vertical ground reaction force was lowered by both control and therapeutic tape but only during fast walking. The results suggest that tape induced effects on neuromotor control of the vasti seen in other studies are related to reductions in pain rather than the presence of a baseline timing deficit. However, this cannot explain the improvements in stance phase knee flexion observed with tape.     

Anatomy of the junction of the vastus lateralis tendon and the patella

Forty-one knees from adult cadavera (twenty female and twenty-one male) were dissected to study the relationship between the longitudinal axis of the patella and the angles of insertion into it of the vastus lateralis and vastus lateralis obliquus muscles. The mean and variance in the angles of insertion of the vastus lateralis obliquus tendon were found to be significantly different between men and women (p less than 0.05 and p less than 0.01, respectively). Three distinct anatomical patterns in the insertion of the vastus lateralis obliquus muscle were delineated. The vastus lateralis muscle, particularly the vastus lateralis obliquus, creates an important lateral force-vector on the patella.     

Electromyographic investigation of unstable patella before and after its realignment operation

Background:

Patellar dislocations are either due to superolateral contracture of the soft tissue or imbalance of the power between the vastus medialis (VM) and the vastus lateralis (VL). The imbalance of muscle power as an etiology of patellar dislocation has not been studied. Hence, we studied the recurrent, habitual and permanent dislocations of the patella with an electromyogram (EMG) of the vastus medialis, vastus lateralis, and pes anserinus, before and after realignment operations, to document the muscle imbalance and effectiveness of the realignment operation.

Materials and Methods:

An electromyographic investigation was carried out on the vastus medialis and vastus lateralis in nine recurrent, 20 habitual, and 13 permanent dislocations of the patella, before and after their realignment operations. Pes anserinus transposition, which acted as a medial stabilizer of the patella, was also investigated with an EMG study, to understand its role on patellar stability at 0°, 30°, 60°, 90°, 120°, 150°, and full flexion of the knee. The age of the patients varied from nine to 30 (mean 15) years. There were 24 males and 18 females. Twenty-six patellar dislocations were on the right and 16 were on the left side.

Results:

Electromyographic pictures reveal subnormal activity of the vastus medialis in all types of dislocations and similar activities of the vastus lateralis in permanent and habitual dislocations recorded pre operatively, which recovered to almost normal values postoperatively, at the mean one-year follow-up. Pes anserinus, which was used for medial stabilization of the patella after its realignment, maintained normal EMG activity before and after the operation.

Conclusion:

This study is significant for understanding the imbalance of muscle activities in patients with an unstable patella, which can be rectified without recurrence after pes anserinus transposition.     

Fatigue rates of vastus medialis oblique and vastus lateralis during static and dynamic knee extension

Fatigue rates of the vastus medialis oblique and vastus lateralis muscles were compared from static and dynamic exercise. Based upon clinical observations, it was expected that the vastus medialis oblique would demonstrate greater fatigability than the vastus lateralis. However, based upon physiological and morphological considerations, it was expected that the vastus lateralis would exhibit greater fatigability than the vastus medialis oblique. In the static exercise condition, nine subjects--having no history of knee problems--maintained an isometric knee extension torque at 30 and 60% of their maximum value until exhaustion, during which time electromyography (EMG) data were collected every 10 s. The knee angle was 20 degrees flexion. In the dynamic exercise condition, a different group of seven subjects performed exercise sets consisting of eight cycles of concentric-eccentric knee extension, with a resistance equal to 40% of the maximum isometric value. The range of motion was restricted to the terminal 30 degrees of knee extension. Each set of eight repetitions was followed by a 1 s 50% maximum isometric knee extension, during which time EMG data were collected. The raw EMG data were numerically processed to extract the median frequency of the power density spectrum, which has been shown to reflect the metabolic processes associated with fatigue. Linear regression generated a slope coefficient representing the rate of change of the median frequency, with respect to contraction duration for each subject, muscle, and condition. Analyses of variance, with repeated measures from both exercise conditions suggest that "short-arc" quadriceps exercise did not selectively fatigue either the vastus medialis oblique or vastus lateralis, thereby supporting neither clinical nor physiological expected outcomes. Therefore, the clinical contention that short-arc quadriceps exercises selectively strengthen the vastus medialis oblique is questioned.     

The Effect of Whole-Body Vibration Frequency and Amplitude on the Myoelectric Activity of Vastus Medialis and Vastus Lateralis

The aim of this study was to evaluate the efficiency of training protocols for whole body vibration (WBV) training through the modulation of the frequency and amplitude of vibration. Despite the large number of studies regarding effects of such training, there is still lack of knowledge regarding optimum training protocols. The study analyzed the influence of whole-body vibration parameters (i.e., the frequency and amplitude) on the myoelectric activity of vastus lateralis and vastus medialis in 29 females with the use of electromyography (EMG). The first and second of the eight consecutive trials were performed without vibrations; the remaining six trials were performed in a randomized order on a platform vibrating at different amplitude (2mm and 4mm) and frequency (20 Hz, 40 Hz and 60 Hz) combinations. The results revealed significantly higher EMG amplitude of both muscles during the vibration as compared with the non- vibrated trials (trial 1 and 2). Furthermore, the EMG activity significantly increased both with the amplitude and frequency, being the highest when the frequency and amplitude of reached 60 Hz and 4 mm, respectively. The study aims to determine the optimal vibration parameters in the aspect of purposeful stimulation of chosen leg muscles. Based on the results of the presented investigation, sports trainers and physiotherapists may be able to optimize training programs involving vibration platforms.

Key points

• The observed vibration effect significantly increases both with the amplitude and frequency.
• Certain frequency/amplitude combinations of mechanical vibrations cause the same level of myoelectric muscle activity.

Key words: EMG, vibration training, whole body vibration (WBV), muscle activity     

Muscle performance about the knee joint in patients who had distal femoral replacement after resection of a bone tumor. An objective study with use of gait analysis

BACKGROUND: The treatment of a malignant bone tumor in the distal aspect of the femur often requires great sacrifice of bone and muscle. The extent of quadriceps removal has been reported to influence the long-term functional efficiency of a patient's gait. The objective of the present study was to determine gait function as it relates to the residual quadriceps strength and to the specific component or components of the quadriceps removed in patients treated with total knee replacement because of a malignant bone tumor in the distal aspect of the femur. METHODS: Sixteen patients were evaluated after implantation of a modular hinged cementless knee prosthesis. The patients were assigned to two groups on the basis of the different components of the quadriceps muscle that were resected. Group 1 consisted of five patients who had removal of the vastus medialis and the vastus intermedius and two who had removal of the vastus medialis only. Group 2 consisted of nine patients who had removal of the vastus lateralis and the vastus intermedius. Residual muscular strength about the treated knee was measured by voluntary maximum contraction isometric testing. Foot-ground reaction forces, kinematic and kinetic findings, and electromyographic activity during free-speed walking were recorded. RESULTS: The kinematic study showed that the patients in Group 1 tended to have a stiff-knee gait during stance, whereas those in Group 2 (in which the vastus medialis was spared) had a more regular flexion-extension knee pattern. Electromyographic findings showed that a higher percentage of patients in Group 1 had reduced or absent rectus femoris activity during the loading response. Compared with the contralateral side, knee-extension strength in the treated limb was decreased in both groups. However, there were no significant differences between the groups with respect to the pattern of strength loss. CONCLUSIONS: Good gait function can be achieved in patients with a distal femoral tumor that is treated with distal femoral resection, partial excision of the quadriceps, and total knee arthroplasty with insertion of a hinged prosthesis. Patients in whom the vastus lateralis and vastus intermedius were removed had better gait performance and a more physiological knee-loading pattern than did patients in whom the vastus medialis was removed.     

Knee flexion during stair ambulation is altered in individuals with patellofemoral pain.

Reduced knee flexion is a logical gait adaptation for individuals with patellofemoral pain (PFP) to lessen the patellofemoral joint reaction force and minimise pain during stair ambulation. This gait adaptation may be related to the co-ordination of individual vasti components. PURPOSE: This study investigated the amount of stance-phase knee flexion in individuals with (n=48) and without (n=18) PFP using a cross-sectional design. The relationship between stance-phase knee flexion and onset timing of individual vasti activity was also examined. METHOD: Stance-phase knee flexion was measured in 2-dimensions using a PEAK movement analysis system during stair ascent and descent. Individuals with PFP were separated into those with synchronous onset of the EMG activity of vastus medialis obliquus (VMO) and vastus lateralis (VL), and those where the onset of VMO EMG activity was delayed relative to the VL. RESULTS: The amount of knee flexion at heel-strike and peak was less in the individuals with PFP compared with the healthy controls. In addition, there were trends towards individuals with PFP who had a delayed EMG onset of VL having reduced knee flexion during stair descent compared with PFP individuals with simultaneous vasti onsets and the control participants. CONCLUSION: These results indicate that the amount of stance-phase knee flexion is lower in individuals with PFP and that this may be related to onset timing of the vasti.     

Electromyographic analysis of core trunk, hip, and thigh muscles during 9 rehabilitation exercises

STUDY DESIGN: Prospective, single-group, repeated-measures design. OBJECTIVE: To identify exercises that could be used for strength development and the exercises that would be more appropriate for endurance or stabilization training. BACKGROUND: The exercises analyzed are often used in rehabilitation programs for the spine, hip, and knee. They are active exercises using body weight for resistance; thus a clinician is unable to determine the amount of resistance being applied to a muscle group. Electromyographic (EMG) analysis can provide a measure of muscle activation so that the clinician can have a better idea about the effect the exercise may have on the muscle for strength, endurance, or stabilization. METHODS AND MEASURES: Surface EMG analysis was carried out in 19 males and 11 females while performing the following 9 exercises: active hip abduction, bridge, unilateral-bridge, side-bridge, prone-bridge on the elbows and toes, quadruped arm/lower extremity lift, lateral step-up, standing lunge, and using the Dynamic Edge. The rectus abdominis, external oblique abdominis, longissimus thoracis, lumbar multifidus, gluteus maximus, gluteus medius, vastus medialis obliquus, and hamstring muscles were studied. RESULTS: In healthy subjects, the lateral step-up and the lunge exercises produced EMG levels greater than 45% maximum voluntary isometric contraction (MVIC) in the vastus medialis obliquus, which suggests that they may be beneficial for strengthening that muscle. The side-bridge exercise could be used for strengthening the gluteus medius and the external oblique abdominis muscles, and the quadruped arm/lower extremity lift exercise may help strengthen the gluteus maximus muscle. All the other exercises produced EMG levels less than 45% MVIC, so they may be more beneficial for training endurance or stabilization in healthy subjects. CONCLUSION: Our results suggest these exercises could be used for a core rehabilitation or performance enhancement program. Depending on the individual needs of a patient or athlete, some of the exercises may be more beneficial than others for achieving strength.     

Effectiveness of goosestep training or its modification on treating patellar malalignment syndrome: clinical, radiographic, and electromyographic studies

Introduction  Imbalanced soft tissue tension between medial and lateral para-patellar structures is the most common pathogenic cause of patellar malalignment syndrome. Despite the fact that an effective technique has not been developed, this disorder is usually treated conservatively. This prospective study tried to identify an effective alternative treatment for patellar malalignment syndrome. Materials and methods  Forty female patients (age, 20–40 years) with bilateral patellar malalignment syndrome were randomly divided into two groups. Forty knees in 20 patients were treated with goosestep training without forcefully striking the ground, and 40 knees in other 20 patients were treated with goosestep training with forcefully striking the ground. Results  After 1 month, significant clinical improvement was observed in 72.5% of all patients (P < 0.001). Congruence or lateral patellofemoral angles and VMO/VL (vastus medialis obliquus/vastus lateralis) ratio significantly improved within 1 or 3 months in both groups (P < 0.001 for each comparison). Conclusion  Goosestep training with or without forcefully striking the ground is a feasible method for treating patellar malalignment syndrome. The main advantages include its convenience, simplicity, lack of cost, and demonstrated effectiveness.     

Low-tech rehabilitation of bilateral patellofemoral knee pain in a runner: a case study

Patellofemoral pain is a common ailment within both the running and general populations. Many of the structures of the anterior knee that comprise the patellofemoral joint can be the source of chronic pain and inflammation that is associated with this condition. Much of the evidence in the literature points to a delay in activation of the vastus medialis oblique muscle as compared to the vastus lateralis, vastus medialis weakness, and ultimately faulty patellar tracking as the chief causative factors in the development of patellofemoral pain. This is a single case study of a 51-year-old recreational runner with an 18-month history of bilateral patellofemoral knee pain. Treatment included the use of low-tech in office rehabilitation strategies known to affect those causative factors responsible for patellofemoral pain. Evidence based treatment modalities were utilized in combination, which included patellar mobilization, spinal manipulation, proprioceptive and strength training, and semi-rigid orthotic use, to effect vastus medialis oblique vs. vastus lateralis activation, vastus medialis strength, and patellar movement. The patient responded very well to a 12 week course of treatment and resumed recreational running with minimal to no pain at the six month, one and two year follow-ups.     

Quadriceps and hamstrings muscle control in athletic males and females

The aim of this study was to investigate whether athletic males and females have differences in their quadriceps and hamstrings muscle control strategies. Twenty‐four people (12 males, 12 females) active in level I activities volunteered to participate. The subjects' muscle control strategies were studied using electromyography and a target matching protocol that assesses one's ability to produce force with control. The degree of control exhibited in the subjects' quadriceps and hamstrings muscle activation strategies was evaluated by calculating specificity indices for each muscle using circular statistics. Females displayed significantly lower specificity than males in their vastus medialis (p < 0.001), rectus femoris (p = 0.044), and lateral hamstrings (p = 0.001) muscle activity patterns, but similar specificity in their vastus lateralis and medial hamstrings activity patterns. Females also used a significantly higher magnitude of vastus lateralis (p < 0.001) and vastus medialis (p < 0.001) muscle activity than males to achieve the same relative force level. These findings indicate athletic males and females have differences in their knee muscle control strategies. © 2008 Orthopaedic Society. Published by Wiley Periodicals, Inc. J Orthop Res 26:800–808, 2008