Analysis of variance revealed statistically significant mean (±SD) differences between men and women for peak force production [F (1,176) = 60.3;P < 0.0001] (Fig. 2) and peak rate of force production [F (1,176) = 28.0;P < 0.0001] (Fig. 3). Significant interaction effects were demonstrated between groups and time for peak force production [F (4,176) = 2.68;P < 0.05] (Table 4) and peak rate of force production [F (4,176) = 4.0;P < 0.01] (Table 5). Planned comparisons revealed significant between-day differences for peak force production (Table 6) and peak rate of force production (Table 7) (P < 0.016).
Our results reveal that muscle force generating capabilities of physically active men exceed that of women both before and after muscle microinjury. Although our study is not the first to report myokinetic differences between men and women, it is the first to report differences both before and after muscle microinjury. Our findings of a significant gender difference in both the magnitude and rate of force production are consistent with other studies (5,8,9,17,20,21,31). Neural, mechanical, and hormonal factors have been implicated as causative agents (5,8,9,17,20,21,31).
Using the biceps brachii muscle, Bell and Jacobs (5) found women to have deficits with both the magnitude and rate of force production compared with men. Similarly, Huston and Wojtys (17) found women take longer to generate peak hamstring torque than men. Researchers attribute these female deficits to fewer fast twitch motor units (5), and/or an inability to quickly recruit available motor units (5,17).
From a mechanical standpoint, researchers have inferred that gender-linked differences in connective tissue elasticity account for the muscle force deficits in women (8,20,21,30). Komi and Karlsson (21) found women to take twice as long to generate 70% of MVC than men. Komi and Karlsson (21) suggest that this delay is due to women requiring more time to stretch the series and parallel elastic components of the musculotendinous unit.
Hormonal fluctuations during the female reproductive cycle are reported to have direct effects on connective tissue stiffness (33). Wojtys et al. (33) found an association between menstrual cycle and anterior cruciate ligament injuries in female athletes. They reported that during the ovulatory stage of the menstrual cycle women produced higher concentrations of estrogen, progesterone, and relaxin; hormones known to drastically diminish collagen proliferation and stiffness. The common element of soft tissues is collagen, and variability in its biochemical composition may account for differences in its mechanical properties.
Effect of Microinjury
Several studies have reported strength deficits after muscle microinjury (6,10,23,29); however, no study has investigated the importance of gender on muscle force deficits before and after muscle microinjury. Microinjury was induced in the nondominant arm of each subject using a concentric/eccentric exercise bout targeting the biceps brachii muscle. Eccentric exercise protocols have been used extensively to induce muscle microinjury (2,6,10,15,16,22,23,26,29). Muscle microinjury has also been reported as delayed-onset muscle soreness (DOMS) (2,10,14–16,22,26) and/or exercise induced muscle injury (13). Irrespective of syntax, the underlying pathophysiology and symptomatology for each appear to be commensurate (2,14).
To validate our protocol, we assessed pain perception (6,10,16,22) and dysfunction (6,16,23,29) before and after exercise. Pain and dysfunction are two important signs of inflammation secondary to microtrauma (2,6,14). In our subjects, a significant increase in pain perception and dysfunction was observed acutely (24 h postinjury) and persisted several days thereafter (96 h). On average, women perceived more pain and dysfunction than men. These results indicate that our protocol was effective in producing a muscle microinjury.
Our study found muscle microinjury to cause significant myokinetic deficits in both men and women. Myokinetic deficits were most pronounced acutely, between 24 and 48 h postinjury, followed by a near complete recovery at day 4 (96 h postinjury). For peak force production, men had a 16% decline at 24 h postinjury, followed by a 9% (48 h), 7% (72 h), and full return by day 4 (96 h postinjury). Women demonstrated a 15% decline at 24 h postinjury, followed by a 12% (48 h), 8% (72 h), and 7% decline at day 4 (96 h postinjury). The acute deficits were significant for both genders, although men’s full recovery of peak force production was more expedient. For peak rate of force production, men again demonstrated a significant (15%) decline acutely (24 h postinjury) followed by a full recovery by day 2 (48 h postinjury). Women had negligible acute and residual deficits over the first 3 d postinjury followed by full recovery at day 4 (96 h postinjury). Of particular interest is the finding that women demonstrated no significant deficit in the rate of force production postinjury. This finding is perplexing given the fact that women suffered significant deficits in peak force production. This inconsistency warrants further investigation.
A quantifiable threshold for determining risk of injury is not presently known. Although empiric evidence has suggested that any strength deficit = 10% is considered clinically significant, and athletes are recommended to refrain from provocative exercise of the affected area (1). If we apply the aforementioned criteria to the results of our study, we would observe an increased risk for both genders 24– 48 h postinjury.
The magnitude and rate with which a muscle can produce force has clinical applications to both injury and reinjury prevention (4,17,18,32). Muscle microinjury frequently occurs after novel or unaccustomed muscular exertion of high intensity, frequency, and duration (15,26). In athletics, muscle microinjury is common during the pre- and early season, as well as end of season tournaments when athletes’ engage in repetitive sessions of high-intensity exercise with very little recovery. Microinjury is commonly perceived as a “muscle soreness,” rather than a physical impairment or musculotendinous injury (14). Athlete’s who sustain a muscle microinjury are frequently encouraged to “work it out” before practice and/or competition (14). We feel it is important for sports medicine practitioners, athletes, and coaches alike to realize that a muscle microinjury can produce significant functional limitations, and if not adequately assessed, recognized and treated may increase the risk for repeated microinjury, especially if an athlete continues to compete.
In the acute (24 to 48 h) and postacute (>48 h) stages of a muscle microinjury, muscle function can be diminished due to increased pain perception, intramuscular swelling, and damaged neural and mechanical apparati (1,12,14,15,19). Effective intervention strategies for treating and preventing muscle microinjury do not presently exist, and more extensive research needs to be conducted to identify efficacious intervention strategies.
Early intervention and prevention strategies are important for minimizing both acute and residual effects of muscle microinjury, as well as facilitating the tissue healing process. The goal of acute management is to control inflammation and pain (1,14). Recommended physical agents include cryotherapy (i.e., ice), transcutaneous electrical nerve stimulation (TENS), nonsteroidal antiinflammatory drugs (NSAIDs), and analgesic balms (10,14–16,26). The goal of postacute management is to restore normal function (1,14). Recommended treatments include sports massage, stretching, and submaximal resistance exercise (10,14,29). Additionally, a well-designed sport-specific training and conditioning program has been suggested to prevent and/or reduce the magnitude of muscle microinjury (2,29).
Our results reveal that muscle force generating capabilities of physically active men exceed that of women both before and after muscle microinjury. Myokinetic deficits were most pronounced acutely, between 24 and 48 h postinjury, followed by a near complete recovery at day 4 (96 h postinjury). Both genders suffered acute and residual deficits for peak force production, whereas only men showed significant acute deficits for peak rate of force production. We recommend that athletes, both male and female, refrain from strenuous exercise at least 48 h postinjury, or until force-generating capabilities normalize. More research needs to be conducted to substantiate these findings.
The authors would like to acknowledge the efforts of Darren Dutto for developing the computer program that was used to reduce our data in this investigation. No author or related institution has received financial benefit from this study.
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Keywords:© 2000 Lippincott Williams & Wilkins, Inc.
PEAK FORCE; PEAK RATE OF FORCE PRODUCTION; BICEPS BRACHII MUSCLE; ISOKINETIC EXERCISE