In recent years, there has been a resurgence of research interest in high-intensity interval training and sprint interval training (SIT). Studies usually set out to understand the cellular regulation of training adaptations and to investigate the implementation of training practices to improve health status of various populations (6). However, most of the research in these areas included only young, male subjects, with females and middle-aged people being underrepresented.
It should not be taken as a certainty that the findings from studies including only men will apply equally to women. Men have higher maximal skeletal muscle strength and power compared with women (30) and higher maximal power output during sprinting (16,19). However, the superior performance does not lie entirely with men because men fatigue more quickly than women during controlled isometric contractions of single muscle groups (25,35) and during sprinting, whereas women recover faster during short rest periods between repeated sprinting bouts (16,19).
Male advantage when producing maximal muscle force and power is in part due to the higher relative muscle mass (8), larger muscles (28), and larger fiber cross-sectional areas (CSAs) in men than women (18). Any sex-related comparisons for muscle force and power should therefore normalize values to muscle size (“normalized” force and power), sometimes termed as “muscle quality” in the literature (22,30). However, it is not only muscle mass that exhibits a sex-related difference, but also the contractile and metabolic characteristics. For instance, men have been reported to have higher concentrations of glycolytic enzymes and faster rates of contraction and relaxation than women (35). Men may use relatively more carbohydrates during submaximal aerobic exercise than women (45), shift to anaerobic metabolism at lower relative intensity during incremental exercise (39), and during maximal sprinting, higher glycolytic contributions were reported in men compared with women (17). Skeletal muscle characteristics such as these that affect energetics may also confer sex differences in fatigability and may influence adaptations to SIT.
Few studies directly examine sex-related differences in adaptations to SIT. We recently reported sex differences for the changes to maximal rate of oxygen uptake (V̇o2max) and body fat after 12-week SIT (5). Focusing on peak power output, Esbjörnsson-Liljedahl et al. (18) showed that women increased their power output more than men after 4-week SIT, but another study looking at adaptation to just 6 SIT sessions reported similar gains in peak power for men and women (4).
Increased power output could in theory be due to changes to neural activation, but the results from Esbjörnsson-Liljedahl et al. (18) allude to proportionally larger gains in vastus lateralis fiber CSA (particularly type IIx) in women than in men as a mechanism. This would suggest that women have a greater hypertrophic adaptation to SIT than men, but there is little evidence to this effect because muscle-specific hypertrophy has been largely overlooked in studies of SIT. Hypertrophy follows a net increase in anabolic signaling over time and, in this regard, Fuentes et al. (2012) found no sex-related differences in their anabolic responses to a single SIT session (20). Another study, however, reported approximately 150% higher rates of muscle protein synthesis 48 hours after 3 weeks of SIT in men than women (40). This is in conflict with the previous report of higher increases in type IIx muscle fiber CSA in women than men (18), although it should be noted that type IIx fibers typically account for approximately 10% of vastus lateralis fibers (44). Irrespective of the conflicting reports, neither of these cross-sectional studies of acute training adaptation examined changes to muscle size after several weeks of SIT. Measurements of total body lean mass by dual energy X-ray absorptiometry (DXA) are contradictory in this area. Heydari et al. (24) observed a 2% significant increase after 12-week high intensity interval training in young, overweight men, whereas Trapp et al. (using the same protocol but over 15 weeks) saw no change in total body lean mass in young women (21). These studies taken together suggest that the muscle hypertrophic response to SIT may be sex-specific, but the contradictory results highlight the need for further evidence.
Thus, the aim of this study was to compare sex-related differences in muscle size, knee extensor torque-velocity relationship, and fatigability (occurring after 60 maximal voluntary concentric knee extensions), and their adaptation to 12-week SIT. It was hypothesized that 12 weeks of cycling SIT would promote knee extensor hypertrophy, increased torque, and fatigue resistance. Based on the limited available data indicating larger gains in type IIx fiber CSA in women than men, women were hypothesized to have a greater hypertrophic adaptation compared with men. Assuming no change to the muscle quality, it was further hypothesized that women would increase maximal torque more than men in line with the greater hypertrophic adaptation.
Experimental Approach to the Problem
To determine the effect of SIT on knee extensor hypertrophy, torque production, and fatigue resistance, 16 men and 15 women were recruited from the general population through advertisement in local and national newspaper articles, a local gym, and campus advertisement. The subjects reported to the laboratory and completed measurements of knee extensor CSA (magnetic resonance imaging, MRI), total body composition (DXA), and knee extensor torque and fatigue (unilateral knee extension dynamometry). The subjects then completed 12 weeks of SIT in a local gym or in the laboratory before returning to repeat the measurements.
The study conformed to the latest revisions of the Declaration of Helsinki (47) and was approved by the ethics committee at Manchester Metropolitan University. Volunteers provided written, informed consent before participation. Those with a history of cardiovascular, neuromuscular, or metabolic disease were excluded as well as people who had suffered a leg fracture within the past 2 years. Subjects who were involved in competitive sports or cycled for more than 15 minutes per day for 3 or more days per week were also excluded. The included subjects were from the same group as previously reported in Bagley et al. (5). Included subjects ranged from 20 to 69 years. Although not reported in the present article, the mean V̇o2max values (43 and 34 ml·kg·min−1 for men and women, respectively) previously reported for the study group (5) were at around the 60th percentile of population-based values (2,10,46). A total of 31 subjects completed the 12-week SIT intervention and the primary outcome measurements for this study were changes to: quadriceps muscle cross-sectional area (CSAQ), knee extensor maximal isometric and concentric torque, and fatigue resistance. Subject characteristics are shown in Table 1.
Table 1. -
Subject characteristics and muscle size in men (n
= 16) and women (n
= 15) before and after 12-week SIT.*†
||Time effect (p)
||Sex effect (p)
||Sex × time interaction (p)
||40.8 ± 3.2
||40.9 ± 3.9
||32 ± 2
||34 ± 2
||3.5 ± 0.2
||3.6 ± 0.2
||2.0 ± 0.1
||2.4 ± 0.1
||43.7 ± 2.1
||45.8 ± 1.7
||34.3 ± 2.3
||40.8 ± 2.4
|Total body mass (kg)
||80.0 ± 2.2
||79.2 ± 2.1
||62.5 ± 2.6
||62.1 ± 2.5
||1.75 ± 0.02
||1.66 ± 0.01
|Body Mass Index (kg·m−2)
||25.9 ± 0.9
||25.8 ± 0.9
||21.8 ± 0.7
||21.8 ± 0.7
|Body Fat (%)
||22.5 ± 1.4
||21.3 ± 1.4
||30.4 ± 1.4
||29.9 ± 1.3
|Total body lean mass (kg)
||60.4 ± 1.6
||61.1 ± 1.6
||39.1 ± 0.9)
||39.2 ± 1.0
|Leg lean mass (kg)
||21.6 ± 0.7
||21.0 ± 0.7
||13.6 ± 0.3
||13.0 ± 0.4
|Right thigh lean mass (kg)
||5.0 ± 0.2
||5.1 ± 0.2
||3.4 ± 0.1
||3.5 ± 0.2
||86.2 ± 3.3
||89.7 ± 3.1
||56.7 ± 1.9
||60.0 ± 1.6
*SIT = sprint interval training; CSAQ = quadriceps muscle cross-sectional area.
†Data are shown as mean ± SEM. Knee Extensor CSA (cm2) is measured in 15 men and 10 women due to equipment maintenance.
Total body and leg lean mass were assessed by DXA (Lunar Prodigy Advance; GE Medical; enCore version 10.50.086) using the same procedures as previously reported by our group (5), and thigh lean mass was also recorded (33). The test-retest variation in measurement for this equipment has been determined in our laboratory as 1% (unpublished).
Magnetic resonance imaging was used to measure peak CSAQ using a T1-weighted turbo 3D sequence (256 x 256 matrix, repetition time 40 ms, echo time 16 ms) on a 0.25-T scanner (G-scan; Esaote, Genoa, Italy) with the subject supine and hips and knees fully extended. The scanning coil was positioned over the thigh of the dominant leg and contiguous transverse-plane slices of 6-mm thickness were collected with no gap between slices. Images were analyzed using OsiriX imaging software (OsiriX medical imaging; OsiriX, Atlanta, GS) by manually tracing the quadriceps muscles and avoiding any visible fat deposits in the muscle. Slices at 24-mm apart were analyzed and the slice with the highest quadriceps anatomical CSA was recorded. Analyses were performed by the same investigator. Using the same equipment and measurement techniques, our laboratory previously reported a coefficient of variation of 0.43, 0.35, 0.30, and 0.31% for repeated measurements of vastus lateralis, rectus femoris, vastus medialis, and vastus intermedius muscles, respectively (14). The scanning procedure was repeated after 12 weeks of SIT.
The knee extensor torque-velocity relationship and fatigue resistance were assessed in 16 men and 15 women using unilateral extensions on a Cybex Norm Dynamometer (Cybex; division of Lumex Inc., Ronkonkoma, New York, NY). Subjects were seated upright (hip angle of 85°) with straps secured firmly around the upper body and the hips to limit extraneous body movements. The torque lever was strapped 2 cm above the ankle malleolus of the dominant leg (as determined by the subject) and the center of knee rotation was aligned with the point of rotation of the dynamometer lever arm. A brief warm-up included 6 isokinetic contractions at 180°·s−1 using approximately 60–70% of maximal effort. The maximal voluntary isometric torque (MVC) was assessed 3 times with 60 seconds of rest between efforts at a knee angle of 90°. The highest torque value was recorded. After a 3-minute rest, isokinetic torque was assessed over 2 efforts separated by 60 seconds of rest between efforts at 60, 120, 180, and 240°·s−1 in a random order, blinded from the subject, with a 60-second rest between velocities. Each trial started with the leg flexed as far as possible and subjects made 2 maximal efforts at each velocity through the full range of movement until the leg reached full extension. The peak torque occurring at any point during the concentric contraction was recorded. A rest of 30 seconds was given between maximal efforts and strong verbal encouragement was given throughout. The data obtained from MRI scanning were then combined with the data from the isokinetic torque production to measure “normalized torque,” that is, torque produced per cm2 of CSAQ, to give an indication of muscle quality before and after SIT:
Isokinetic torque as a percentage of isometric torque produced was assessed by:
Knee extensor fatigue resistance was assessed after a 3-minute rest. The test started with the knee flexed as far as possible and subjects performed 60 maximal-effort isokinetic contractions over 2 minutes (one every 2 seconds, as timed by a metronome), moving through the full range of knee extension at a velocity of 120°·s−1 and returning to the fully flexed knee angle between contractions. The highest torque produced during the first 3 contractions was recorded as the highest contraction torque during the test and in all cases the lowest torque was produced during the final contraction and this was recorded as the lowest torque. The fatigue index was calculated using the formula:
In this instance, a higher value indicates greater fatigue resistance, i.e., the percentage of muscle torque output maintained after 60 contractions relative to the first contraction.
The isometric MVC ICC using these techniques is 0.854 with isokinetic variation ICC of 0.819 at 60°·s−1, 0.810 at 120°·s−1, 0.850 at 180°·s−1, and 0.836 at 240°·s−1.
Sprint Interval Training
Subjects completed an incremental cycling test to establish the workload at the maximal rate of oxygen uptake (V̇o2max), as previously described (5) and this was used to determine the SIT workload. Sprint interval training was completed on cycle ergometers (Cateye, Japan). The training consisted of 2 minutes of warm-up at a self-selected moderate intensity. This was followed by 4 bouts of 20-second maximal-effort sprints at a workload (in Watts) that was set at a power output corresponding to 175% of the workload attained in the V̇o2max test at the initial laboratory visit. This target workload was increased every 2 weeks by 5%, reaching 200% of the workload attained in the V̇o2max test at the initial laboratory visit after 12 weeks (a standard incremental cycling test was used to determine the V̇o2max, as described previously ). Each of these bouts was separated by 2 minutes of very low intensity cycling (a workload of 20% of that attained in the initial V̇o2max test). This training protocol was chosen due to its brevity, as well as previous studies yielding significant changes in physiological measurements in short time periods (11,36). Thus, each training session lasted less than 10 minutes and only 80 seconds was completed at an intensity that would be expected to influence the primary outcome variables: knee extensor size, maximal torque, and fatigue resistance.
The first training session for each subject was supervised by the research team in the research laboratory and subjects received clear instructions on the use of the cycle ergometers and the training regimen. Subjects were then instructed to train 3 times per week for 12 weeks (36 sessions in total) using the ergonomic cycles (Cateye, Japan) that we provided in a local gym or at our laboratory. Subjects completed on average 33 (±2) sessions over 12 weeks, with men and women completing similar numbers of sessions (Table 2). Exercise instructors at the local gym were fully informed of the research and training protocols, and they were available to provide a safe training environment and to assist subjects if needed during training sessions. The exercise instructors were not involved in the data collection process or in the interpretation of data. Subjects maintained a training logbook to record workloads during training sessions and were otherwise asked to maintain their usual dietary and exercise habits throughout the intervention period.
Table 2. -
Knee extensor torque in men and women before and after 12-week SIT.*†
||Sex × time interaction
|Isometric MVC (Nm)
||314.32 ± 21.86
||322.98 ± 23.10
||193.33 ± 9.43
||204.20 ± 9.65
|Isometric MVC (Nm·cm−2)
||3.65 ± 0.21
||3.57 ± 0.24
||3.23 ± 0.09
||3.23 ± 0.11
||260.25 ± 13.78
||265.89 ± 16.45
||161.23 ± 9.64
||175.66 ± 6.45
||227.01 ± 12.27
||231.29 ± 13.42
||128.98 ± 6.34
||145.22 ± 4.03
||199.55 ± 8.70
||201.50 ± 11.03
||120.11 ± 5.05
||120.54 ± 3.04
||173.93 ± 8.22
||172.73 ± 9.91
||107.57 ± 3.93
||107.19 ± 3.25
|Fatigue index (%)
||52.40 ± 2.52
||56.42 ± 3.27
||55.92 ± 3.56
||64.80 ± 2.26
*SIT = sprint interval training; MVC = maximal voluntary isometric torque.
†Data are shown as mean (SEM). Men (n = 16) and women (n = 15).
The primary outcome measurements were: peak CSAQ; torque measured at the different velocities; and fatigue index after 60 maximal-effort concentric contractions. A secondary outcome measurement was lean mass measured by DXA. The differences between before and after 12-week SIT were calculated and sex-related differences in adaptation were compared. All data were tested for normality of distribution using the Kolmogorov-Smirnov test. Independent-samples t-test was used to examine sex-related differences in the number of training sessions completed (Sex effect, Table 1) and baseline sex-related differences in all recorded measurements. If no sex difference was found at baseline, a 2-factor repeated-measures analysis of variance (ANOVA) was used to assess sex differences in training adaptation and between isokinetic velocities. If a baseline sex difference was found, a 2-factor repeated-measures analysis of covariance was used with baseline values as a covariate. In examining sex-related differences over the training intervention, the sex × time interaction effect refers to sex-related differences as a result of the training intervention (pretraining to posttraining intervention). Three-factor repeated-measures ANOVA was used to assess sex- and time-related differences in the force-velocity profile. Relationships between measurement outcomes and subject age were examined using partial correlation coefficients controlling for sex. The data were analyzed using SPSS (v.20 IBM) and statistical significance was accepted at p ≤ 0.05. Data are presented as mean ± SEM.
No significant correlations were found between subject age and the adaptations to SIT for any of the outcome variables (all p > 0.200).
Body Composition and Knee Extensor Muscle Size
Table 1 shows variables relating to body composition and skeletal muscle size in men and women. Total body lean mass was unchanged after training (p = 0.147), with no sex difference in this adaptation (sex × time interaction: p = 0.067). Analysis of leg lean mass and thigh lean mass from DXA scans showed no significant changes after training. However, the more detailed analysis of CSAQ from MRI showed a significant increase of 3.25 cm2 after SIT (p = 0.023), with both men and women increasing CSAQ similarly after training (sex × time interaction: p = 0.140).
Knee Extensor Maximal Torque and Fatigue Resistance
Men had higher isometric torque than women at baseline (Table 2), but both sexes generally showed similar percentage decline in torque as contraction velocity increased, with torque at all velocities (°·s−1) relative to isometric MVC being similar for men and women (sex × velocity interaction; p = 0.374) (Figure 1A). When normalizing torque to CSAQ, men had higher values than women at 180°·s−1 (Figure 1B), with sex-related difference in the decrease in torque per CSA of muscle approaching significance (sex × velocity interaction; p = 0.051). There were no significant sex-related differences in the torque-velocity profile after SIT (sex × time × velocity interaction; p = 0.425). The fatigue index (proportion of torque that remained after 60 maximal-effort concentric contractions) showed no sex-related differences at baseline (Table 2). Fatigue index improved overall by 4.8% after training (p = 0.048), with no sex differences in this adaptation (sex 3 time interaction; p = 0.127) (Table 2).
This SIT program included only 4 minutes per week of very high intensity exercise and led to significant increases to fatigue resistance and CSAQ, but no change to knee extensor concentric torque. The training effects were similar for men and women. These findings advance previous studies of physiological adaptation to SIT which typically focused on aerobic and metabolic adaptation in young adult men and women, separately.
There are 2 contradictory reports in the literature concerning the possible sex differences in hypertrophic responses to sprint interval or high-intensity interval training. Esbjornsson-Liljedhal et al. (18) reported that women increased type IIx fiber CSA more than men after 4 weeks of SIT, whereas another study reported lower skeletal muscle anabolic response to training in women compared with men (40), but this latter work was based only on acute responses and did not follow-up after a period of training. In this study, we found that men and women showed similar increases in CSAQ, which are agonist muscles during cycling (7). This could be expected because sprint interval exercise activates type I and IIx fibers similarly in men and women (17,41), suggesting motor unit recruitment and therefore the training stimulus received by the IIx fibers is similar for men and women.
A review of the literature indicated that the extent of hypertrophy may depend on the length of the training program: training 6 weeks or less did not cause significant changes to fiber CSAs (38). However, longer-term training of 7 weeks or more generally increased fiber CSAs, although it should be noted that studies in this area included only small sample sizes (n = 8–13) and none of them compared sex-related differences in adaptation (38). For example, a six-week SIT protocol caused a nonsignificant increase of fiber CSA by 6–12% in 11 untrained men (mean age = 23 ± 5 years) (1). However, 8-month SIT increased fiber CSA by 8–16% in 13 athletes (8 men and 5 women, mean age = 17 ± 1 year), although sex comparisons were not possible due to small sample sizes and the women in the study were trained in sprinting before the intervention, whereas the men were not (12).
Total body lean mass measured by DXA was unchanged by SIT, with no sex difference in this adaptation (Table 1). The leg lean mass and thigh lean mass measured using DXA also showed no significant changes after training. It is not clear why the DXA detected no significant changes with training, whereas the MRI clearly showed significant hypertrophy of the quadriceps muscles. The problem is not due to lack of consistency of repeated scans because Kiebzak et al. (29) observed a 1% variance in lean mass with repeated DXA scans on the same subject over consecutive days and in previous pilot work, we determined the test-retest variation to be 1% from our scanner over 4 weeks (unpublished). Disparity between MRI and DXA for measuring muscle size has been reported previously, with DXA also failing to reveal the full extent of muscle loss with aging (32,33). So, it is possible that the DXA is not suitable for detecting these relatively small changes to muscle tissue.
The higher values in men compared with women across the concentric torque-velocity relationship are mainly due to the larger muscle mass of men, but there was a clear trend for the torque per muscle CSA to be higher in men than in women and this was significant at 180°·s−1. Torque decreased with increasing knee extension velocity (Figure 1B). These findings fit with previous estimations of torque per muscle CSA, sometimes described as “muscle quality” in the literature. Lindle et al. (30) observed in 346 men and 308 women aged 20–90 that men have a 9% higher concentric peak torque per muscle CSA than women. Similarly, Goodpaster et al. (22) observed approximately 17% higher muscle quality in older men compared with older women (mean age = 73 ± 3 years) during isokinetic contractions, suggesting that this sex difference is maintained throughout the lifespan. Previous studies examining sex differences after SIT in muscle strength characteristics have been significantly shorter, not lasting more than 4 weeks (4,18).
The increase in quadriceps muscle size in this study did not lead to gains in knee extension maximal torque in either men or women. A previous study that measured torque before and after SIT (repeated Wingate tests over 3 weeks) also found no significant changes to knee extensor MVC in 11 men or 9 women (3). It is possible that the mode of exercise training (cycling) might not have trained the neural control needed for isolated knee extensions, as was suggested when the converse was observed when knee extension training did not increase cycling power output (15).
Most of the studies into sex differences in muscle fatigue during controlled exercise of individual muscle groups used isometric contractions (25,26) and the results from such studies generally indicate that women have superior fatigue resistance compared with men (35,42). Some studies involving concentric contractions also suggest superior fatigue resistance of women compared with men (25,37,48). However, fatigability, measured in this study as the decline in torque after 60 maximal-effort unilateral moderate-velocity concentric knee extensions, was similar in men and women at baseline, with torque during the final contractions dropping to 55% of the first 3 contractions. A possible explanation for why our findings differ from other previous studies is that other studies tended to include young adults or older adults, whereas we included a range of young and middle-aged adults. Differences between studies in the velocity of contraction might also influence the fatigability and it is interesting to note that a study using maximal-velocity contractions at a load equal to 20% of the subject's MVC also reported similar fatigue in men and women during knee extension (42).
There are reports that fatigue characteristics of individual muscle groups are unaffected by sprint training in men (23), but no previous studies compared chronic training adaptation of men and women. Fatigue resistance improved after 12-week SIT in this study. When examining isokinetic contractions, previous work suggests a sex dimorphism in muscle fatigue, with men fatiguing significantly faster than women when velocity is controlled. However, when examining as a product of exercise intensity (i.e., a percentage of maximal power or 1 repetition maximum, not a controlled velocity), a sex difference is no longer observed (34). When matched for initial sprinting mechanical work, men and women see similar decline in muscle power after repeated sprint exercise (9,43), similar to isokinetic contractions which are matched to initial power output, suggesting that the mechanism for increased fatigue resistance in women seen when measuring power output in repeated sprint exercise is an initial higher power output in men (25). In this study, where training was normalized to cycling power at V̇o2max, it was observed that men and women increased their resistance to fatigue similarly after SIT when measured as a product of contraction velocity (sex × time interaction: p = 0.127). Taken together, this could suggest that differing prescription of exercise training (i.e., normalized to initial power output versus velocity or body mass etc.) may have sex-related differences in muscle fatigue. The physiological mechanisms underlying the training-induced improvement to fatigue cannot be identified from this study, but are likely to be associated with increases in mitochondrial concentrations (and therefore improvements in skeletal muscle metabolism) (31) and capillary density (13) that have been found after SIT and collectively improve muscle oxidative energy recovery during the brief rest intervals between contractions.
The design of the training program, being performed in a local gym or the research laboratory, gave exercise volunteers more control and although this is the case in real-life situations, it may confer less commitment or obligation to training compared with typical fully supervised laboratory-based programs and we were not able to directly record the power output completed by the study volunteers during training. It was not possible to control for physical activities outside the training program and dietary intake was not monitored throughout the training program. Instead, subjects were asked to maintain their usual patterns of food and drink consumption. Finally, there was no control for menstrual cycle variations, which potentially limits the interpretation of some aspects of the data relating to sex-related differences in adaptation. In this regard, however, all female subjects should equally have completed 3 full menstrual cycles before returning to the laboratory for posttraining measurements. Furthermore, evidence points toward there being no change in the muscle parameters measured in this study (27).
The novel aspects of this research were that we examined maximal knee extensor torque, size, and fatigue resistance in men and women ranging from young through to middle-aged adults before and after a period of cycling SIT. Although knee extension torque across a wide range of velocities did not change with training, fatigue resistance and CSAQ were improved in men and women. Practitioners can use these findings as evidence that SIT is a time-effective option to increase muscle size and resistance to fatigue.
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