Teddy W. Worrell

EMG analysis of lower extremity muscle recruitment patterns during an unloaded squat

During an unloaded squat, hamstring and quadriceps co-contraction has been documented and explained via a co-contraction hypothesis. This hypothesis suggests that the hamstrings provide a stabilizing force at the knee by producing a posteriorly-directed force on the tibia to counteract the anterior tibial force imparted by the quadriceps. Research support for this hypothesis, however, is equivocal. Therefore, the purposes of this study were 1) to determine muscle recruitment patterns of the gluteus maximus, hamstrings, quadriceps, and gastrocnemius during an unloaded squat exercise via EMG and 2) to describe the amount of hamstring-quadriceps co-contraction during an unloaded squat. Surface electrodes were used to monitor the EMG activity of six muscles of 41 healthy subjects during an unloaded squat. Each subject performed three 4-s maximal voluntary isometric contractions (MVIC) for each of the six muscles. Electrogoniometers were applied to the knee and hip to monitor joint angles, and each subject performed three series of four complete squats in cadence with a metronome (50 beats.min-1). Each squat consisted of a 1.2-s eccentric, hold, and concentric phase. A two-way repeated measures ANOVA (6 muscles x 7 arcs) was used to compare normalized EMG (percent MVIC) values during each arc of motion (0-30 degrees, 30-60 degrees, 60-90 degrees, hold, 90-60 degrees, 60-30 degrees, 30-0 degrees) of the squat. Tukey post-hoc analyses were used to quantify and interpret the significant two-way interactions. Results revealed minimal hamstring activity (4-12% MVIC) as compared with quadriceps activity (VMO: 22-68%, VL: 21-63% of MVIC) during an unloaded squat in healthy subjects. This low level of hamstring EMG activity was interpreted to reflect the low demand placed on the hamstring muscles to counter anterior shear forces acting at the proximal tibia.

Effect of pelvic position and stretching method on hamstring muscle flexibility

Hamstring muscle strain represents a significant injury to the athlete participating in sporting activities. Lack of hamstring flexibility has been correlated to hamstring muscle injury. There is, however, conflict concerning the most efficient hamstring stretching technique. The purpose of this study was to compare static stretch (SS) and proprioceptive neuromuscular facilitation (PNF) hamstring stretching techniques while maintaining the pelvis in two testing positions: anterior pelvic tilt (APT) or posterior pelvic tilt (PPT). Two groups of 10 subjects were randomly assigned to either an APT or PPT position. Each subject then performed eight sessions using PNF on one leg and SS on the other leg while maintaining the pelvis in the assigned position. Hamstring flexibility was assessed with the hip positioned at 90 degrees while actively extending the knee, i.e., active knee extension test (AKET). A two-way ANOVA comparing stretching technique and pelvic position revealed that the APT group significantly increased hamstring flexibility (P = 0.0375). There was not a significant difference between SS or PNF stretching technique in the APT position. There was not a significant increase in hamstring flexibility in the PPT group with either stretching technique (P > 0.05). The results suggest that APT position was more important than stretching method for increasing hamstring muscle flexibility.

An analysis of supraspinatus Emg activity and shoulder isometric force development

Injury to the shoulder represents a significant disability to the athlete participating in throwing and other overhead sporting activities. There are few scientifically based rehabilitation protocols regarding the most efficient exercises for the rotator cuff musculature. There is contradiction concerning the most efficient position for supraspinatus muscle electromyographic (EMG) activity. Because the supraspinatus is the most frequently injured musculotendinous structure of the shoulder, the most efficient position for testing and rehabilitation of this structure should be identified. The purposes of this study were to compare two test positions for supraspinatus EMG activity and maximal voluntary isometric contraction (MVIC) force development of the shoulder. Twenty-two subjects performed MVIC in the two recommended testing positions. A paired t-test revealed the prone position produced significantly greater (P = 0.04) EMG activity than the standing test position. An additional paired t-test revealed the standing position produced significantly greater (P = 0.0001) force than the prone position. Since the supraspinatus EMG activity was lowest in the position that produced the greatest shoulder MVIC force, we hypothesize that muscle substitution occurred. We recommend the prone position for supraspinatus rehabilitation and testing.

Effect of hip position on gravity effect torque.

Hip position has been hypothesized to influence gravity effect torque (GET) at the knee during isokinetic testing; however, no data exist to support or refute this hypothesis. Therefore, the purposes of this study were 1) to determine if a significant difference exists between GET in seated and supine positions, 2) to determine the effect of the supine and seated GET on isokinetic peak torque values, and 3) to determine the relationship between hamstring flexibility and GET. Gravity effect torque was recorded in supine and seated positions. Peak torque values in flexion and extension were obtained on a isokinetic dynamometer at 1.047 and 5.235 rads.s-1 (60 and 300 degrees.s-1, respectively). Hamstring flexibility was assessed by the active knee extension test (AKET). The mean seated GET value was 5.64 Nm higher than the mean supine GET value (F(1,82) = 97.85, P = 0.0001). Significant correlations existed between hamstring flexibility and GET values measured in the seated and supine positions (r = 0.45, P = 0.0001, and r = 0.30, P = 0.0058, respectively). Significant differences in peak torque values occurred for three of the four isokinetic conditions when using different GET values (P-value 0.0002-0.0049). Although mean differences in peak torque values were only 2.43-4.23 Nm, these differences may translate to significant errors in the isokinetic measurement of the injured population undergoing rehabilitation. Furthermore, every attempt should be made to improve the validity of isokinetic testing. Therefore, we recommend the supine position for GET determination.