Leg stiffness (Kleg) describes the collective ability of the entire leg musculature to resist lengthening when subjected to a given force. For example, 2 athletes perform a task that involves the stretch-shortening cycle (SSC), such as the drop jump. If their leg musculature experience identical forces during the eccentric phase of the jump, the athlete who exhibits less flexion of the ankle, knee, and hip joints during this phase will have demonstrate greater resistance to lengthening and thus greater Kleg. Research has shown that a certain amount of Kleg is required to successfully use the SSC during plyometric tasks. However, excessive Kleg may increase the risk of sustaining skeletal injuries, whereas insufficient Kleg may increase the incidence of soft tissue injuries.
Insufficient Kleg, in addition to potentially harmful Kleg recruitment strategies (mainly modulated through muscle activation), have been previously observed in women when performing plyometric tasks. It has been suggested that the aforementioned factors may contribute to the greater prevalence of both anterior cruciate ligament (ACL) injuries and tendon-related injuries reported for this group. However, through appropriate training program design, it is suggested that Kleg recruitment in women can be positively altered, which may decrease their susceptibility to the aforementioned common injuries.
Previous research showed that there were significant differences in Kleg between men and women during bilateral hopping performance (2). Although normalizing Kleg to body mass resulted in comparable Kleg between these 2 groups (10,11), the mechanisms by which women recruited Kleg (to be termed “Kleg recruitment strategies”) during this task differed markedly compared with their male counterparts. Gender differences in Kleg recruitment strategies during SSC tasks may be primarily attributed to differences in both muscle recruitment patterns and muscle tendon unit (MTU) mechanical properties.
In terms of muscle recruitment, women demonstrated 46% greater quadriceps activity, 37% greater soleus activity and greater quadriceps:hamstrings (Q:H) coactivation ratios (Q:H = 2.01 versus 1.54) than men during bilateral hopping at both 2.3 and 3.0 Hz (11). A similar pattern of muscle recruitment in women was also reported in a more recent study, which examined gender differences in Kleg control strategies during bilateral hopping, after the performance of fatiguing closed-chain exercise (submaximal squatting) (10). Once again, when compared with men, women demonstrated greater Q:H coactivation and 45% greater quadriceps activity during the 50-millisecond time period immediately before and immediately after ground contact (10).
Furthermore, during the 50-milliseconds post–ground contact, peak quadriceps activation amplitude was approximately 4 times greater than peak hamstrings activation amplitude in the female subjects, which was twice that observed in men during the same period of time (10). Large quadriceps forces increase anterior tibial translation and thus ACL strain, which is augmented when the hamstrings are unable to counteract these large quadriceps forces (1). Therefore, despite the gender similarities in normalized Kleg reported (10,11), the quadriceps dominant Kleg recruitment strategy used by women may increase their risk of ACL injury sustainment.
During ramped isometric maximal voluntary contractions of the plantar flexors and knee extensors, the stiffness of the tendon and aponeurosis of gastrocnemius medialis and the patellar tendon was significantly less in women than in men (6,9). This reduced tendon stiffness (i.e., increased tendon compliance) in women may partially explain the increased muscle recruitment seen for the quadriceps and soleus in previous works (10,11). Increased muscle activation in response to a more compliant tendon would act to maintain the stiffness of the overall MTU (and thus the joint) by reducing the lengthening of the muscle fascicles during the eccentric phase of an SSC task. However, using a compensation strategy such as this to maintain overall MTU stiffness may lead to a greater amount of tendon-related injuries because of the potential for excessive (and most likely, repeated) lengthening of the tendon portion of the MTU occurring in this instance.
In addition to lower tendon stiffness, active hamstrings MTU stiffness was also found to be significantly lower in women (3). Based on the research presented earlier (10,11), it is unlikely that the greater hamstrings compliance seen in women is compensated for by increased activation of this muscle group. Therefore, untrained women may not be capable of effectively using the hamstrings during the eccentric phase of the SSC, which may explain the quadriceps dominant strategy used by women during such tasks. This lack of hamstrings utilization may also increase the likelihood of anterior tibial translation and ACL injuries in women, as mentioned earlier (1).
To improve Kleg recruitment strategies in women, it is suggested that an appropriately designed (Table) resistance training program should be adhered to for at least 8 weeks before performing any plyometric tasks. The rationale for this performance duration is based upon previous research that has shown increases in tendon stiffness to occur only after performing a minimum of 8 weeks of resistance training (5). The initial 4 weeks of resistance training should consist of a hypertrophy-based volume load (i.e., 3 sets of 10 reps performed at ≥75% of 1 repetition maximum [1RM]), whereas the second 4 weeks of resistance training should consist of a strength-based volume load (i.e., 3 sets of 6 reps performed at ≥85% 1RM). During each of the 4-week training phases, resistance exercises such as the squat, deadlift, and calf raise (including their derivatives to account for variability in technical proficiency, personal preference, and also equipment availability) should be performed with the aforementioned volume loads because these exercise-volume load combinations have been shown to effectively increase tendon stiffness.
Resistance exercises specifically tailored to improve hamstrings strength during eccentric muscle actions, such as Nordic hamstring lowers and razor curls (Nordic hamstring lowers, Supplemental Digital Content 1, http://links.lww.com/SCJ/A63; and razor curls, Supplemental Digital Content 2, http://links.lww.com/SCJ/A64) should be performed. Research suggests that these exercises may be superior compared with more traditional resistance exercises in improving functional hamstrings strength and have been linked to reducing ACL injury risk in women (7,8).
It is recommended that a further 4 weeks of training, consisting of a combination of the previously described strength training phase and progressive low-intensity plyometric training, should be performed after the initial 8-week training period. For example, the plyometric program should progress from the performance of bilateral exercises, such as the countermovement jump, to unilateral exercises, such as hopping. Please note, however, that all plyometric exercises should be monitored for appropriate landing technique (Figure). This type of progressive low-intensity plyometric training, which can be performed within 15 minutes at the beginning of the resistance training sessions, has been proven to significantly improve landing mechanics in women when performed over a 4-week period (4). Furthermore, plyometric training programs have been shown to increase hamstrings torque and hamstring strength in female athletes while maintaining quadriceps strength, thus improving Q:H strength ratios (12). Therefore, it is hypothesized that such training may help to decrease the prevalence of ACL injuries in women, which has been reported to be partly caused by inefficient Kleg recruitment strategies.
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