Hamstring stiffness was calculated using the following equation, where k = stiffness, m = system mass (mass of shank and foot segment31 + applied mass), and f = damped frequency of oscillation.
Subjects performed 5 trials with 1 minute of rest between the trials to reduce the likelihood of fatigue, and the average of these trials was used for statistical analysis. All stiffness values were normalized to body mass.
Electromechanical delay, RFP, and T50% were calculated from a maximal voluntary isometric hamstring contraction.27 Subjects were positioned prone with their right knee in 30 degrees of flexion similar to the hamstring stiffness protocol. Their foot was secured to a sled that prevented knee flexion from occurring, and a load cell (Honeywell Sensotec, Columbus, Ohio) was attached to the sled at the posterior aspect of the calcaneus. Subjects were instructed to perform a hamstring contraction as quickly and forcefully as possible in response to a light stimulus activated randomly to avoid anticipation by the subject. Each contraction lasted approximately 3 seconds, and 1 minute of rest was provided between the trials. Five trials were recorded, and the average was used for data analysis.
Vertical ground reaction force, EMG, accelerometer, and load cell data were sampled at 1000 Hz using The Motion Monitor motion capture software (Innovative Sports Training, Chicago, Illinois). Data filtering, processing, and onset identification parameters have been previously published.27 The blood sample from 1 subject in the OC group was misplaced, thus hormonal concentrations were not available for this subject, and statistical analyses were conducted on the remaining 14 subjects in the OC group. Differences in dependent variables were evaluated via separate 2 (group) × 2 (phase) repeated measures analysis of variance. Significant main and/or interaction effects were evaluated post hoc via Tukey Honestly Significant Difference test. Statistical significance was established a priori as α ≤ 0.05.
Fifteen subjects were enrolled in each group, and no differences in subject descriptive statistics were noted (Table 1). Statistical information for reproductive hormones can be found in Table 2 and neuromechanical variables in Table 3.
As expected, we observed a significant group by phase interaction for the blood concentration of estradiol-β-17 (F 1,27 = 19.68; P < 0.001), free testosterone (F 1,27 = 10.32; P = 0.003), and progesterone (F 1,27 = 9.55; P = 0.005). The Tukey post hoc revealed that each hormone was greater at ovulation in the non-OC group compared with the OC group. No significant group by phase interaction was present for resting knee angle (F 1,28 = 1.49; P = 0.23) nor angular displacement (F 1,28 = 2.09; P = 0.16) as a result of the perturbation during the active hamstring stiffness assessment. We observed no significant interactions for hamstring stiffness (F 1,28 = 0.34; P = 0.56), lower extremity stiffness (F 1,28 = 2.58; P = 0.12), EMD (F 1,28 = 0.18; P = 0.67), T50% (F 1,28 = 0.97; P = 0.33), or RFP (F 1,28 = 0.33; P = 0.57). No group or phase main effects were observed for any neuromechanical variables (P > 0.05).
The findings of this investigation were contrary to our original hypotheses and suggest that muscle properties are not influenced by hormonal fluctuation across the MC or by OC use. No differences were observed in neuromechanical variables between samples of women who did and did not regularly use OC, and these variables did not change across the MC. These results are in agreement with Elliott et al,32 who demonstrated that muscle strength did not differ between OC users and nonusers. Similarly, Kubo et al33 demonstrated that muscle stiffness did not differ between menses and ovulation, suggesting that these select hormones at 2 time points do not have an appreciable effect on muscle properties.
A number of factors may explain the lack of the significant effect of OC use on muscle properties. Shultz et al34 reported a delayed effect of reproductive hormones on knee laxity across the MC. If these hormones have similar effects on skeletal muscle, our testing intervals may not have captured the appropriate time frame during the MC when these effects are exhibited; however, it is not known whether such a delay exists in muscle. Second, other sex steroid hormones (estradiol and progesterone) can have actions on androgen receptors that normally uptake testosterone and implement the physiological effects seen in tissues. We observed an increase in free testosterone at ovulation, which was expected,35 without appreciable effects on muscle neuromechanics. If sex steroid hormones are limiting testosterone's ability to interact with androgen receptors (through competitive binding and blocking), then they would mitigate the effect on the elevated testosterone observed. Additionally, Eiling et al7 demonstrated a decreased lower extremity stiffness at ovulation using a single-leg hopping task. The lesser relative musculotendinous loading associated with the double-leg hopping used in this investigation, in comparison with the single-leg hopping used by Eiling et al,7 may have minimized the differences in lower extremity stiffness between the groups, thus masking any influence of OC use. Finally, the effect of these hormones on muscle may be too minute for the current testing protocol to quantify.
Additionally, the literature is inconsistent regarding the influence of hormonal fluctuations throughout the MC on muscle properties. Differences in muscle activation patterns,8 strength,10,36 endurance/fatiguability,10,36 stiffness,7 and extensibility9 have been reported across the MC as functions of fluctuating hormonal concentrations. However, other authors have reported a lack of changes in strength,11,13,32,37,38 endurance/fatiguability,37,38 and stiffness.9,33 The same is true of OC use in that some investigations have demonstrated that OC use limits changes in these characteristics across the MC,10,36 whereas others have demonstrated no effect of OC use.32 Collectively, these investigations encompass a wide variety of discrepancies in the experimental design, including (1) muscles being tested (eg, upper vs lower extremities), (2) sample characteristics (eg, age and activity status), (3) testing points within the MC (eg, menses, ovulation, and mid luteal), (4) the method of identifying points in the MC (eg, ovulation kit, estimation based on length), and (5) outcome instruments used (eg, KT-1000, hopping). Yet, no consistent pattern emerges that predicts the likelihood of identifying significant influences of either hormonal fluctuations or OC use. Our results in combination with those of Elliott et al32 and Kubo et al33 suggest that reproductive hormone concentrations and OC use do not influence muscle properties in manners thought to alter ACL loading and injury risk, thus OC use does not seem to have a prophylactic effect on dynamic joint stability.
Researchers agree that the preovulatory phase of the MC has a greater than expected number of ACL injuries.2 However, fewer studies have investigated the influence of OC use on injury risk. Consensus does not exist, and some investigations suggest a decreased risk of injury with OC use15 and others reporting no difference.14,39 Estrogen fluctuation throughout the MC may influence muscle, but these changes seem too small to be clinically relevant. However, OC use has been demonstrated to improve the structural strength of the ACL in the animal model,40 and thus the prophylactic effect may be limited to ligamentous tissues.
Neuromechanical variables and blood hormone concentrations were assessed at 2 points in the MC. More testing sessions may have revealed larger group differences in hormonal concentrations and muscle stiffness, thus maximizing the potential to observe an influence of OC use. Additionally, as seen in ligament, it is possible that only some women are responsive to hormonal fluctuations (ie, responders vs nonresponders).34,41
Another limitation is that OC use was limited to monophasic doses. A variety of common OC alternatives exist, which vary in cycle length and delivery methods that were not considered in this study. All OC types include a placebo interval during which no hormones are delivered, allowing menstruation to occur. Oral contraceptive regimens vary in cycle length, yet our study was limited to subjects using OC with 25-day to 32-day cycles. Extended periods of elevated estrogen concentrations associated with longer cycle OCs may have differing effects on muscle properties.7,42
Only females with self-reported normal MCs were included in this study. Oral contraceptive is commonly prescribed for women with irregular MCs to stabilize hormone levels and cycle length. Larger hormonal surges in women with irregular MCs may have a larger effect on muscle properties compared with normally menstruating women. Finally, our study was powered to detect clinically meaningful changes in lower extremity and hamstring stiffness. Sample size may not be adequate to detect changes in other variables included in this study. However, these variables were associated with low effect sizes (Table 3), which indicate that large increases in sample size would be needed to reach statistical significance. Also, EMD was associated with reliability below clinically acceptable levels, and these results should be interpreted with caution.
In conclusion, we found no effect of MC phase or OC use on muscle properties across the time points studied. These results indicate that the use of OCs does not affect muscle properties in manners thought to reduce ACL injury risk.
The authors acknowledge Megan Kimsey and Lauren Engstrom for their assistance with data collection for this project.
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Keywords:© 2011 Lippincott Williams & Wilkins, Inc.
hormone; estrogen; menstrual cycle; anterior cruciate ligament