Abstract

The effects of a 10-wk unilateral knee extension strength training (ST) program on peak power (PP) and peak movement velocity (PV), at given absolute (force load) and relative (same % of 1 repetition maximum) resistances (loads), were examined in 30 older men [64 yr (7 SD)] and 32 older women [62 yr (6 SD)]. PP increased significantly in both men and women at the same absolute (P < 0.001) and relative loads (P < 0.01) with ST. Men had a significantly greater increase in relative PP than women with ST at 60% (P < 0.01) and 70% (P < 0.001) of 1 repetition maximum when covarying for baseline differences and age. However, when each subject was tested at the same absolute load and when PP was normalized for the muscle volume of the trained knee extensors (i.e., absolute muscle power quality), women increased by 9% (P < 0.05), whereas men did not change. Both men and women increased their absolute PV (P < 0.001) but decreased their relative PV significantly with ST (P < 0.05). However, when baseline values and age were covaried, women had significantly less of a decrease in relative PV quality with ST than men (P < 0.01), although the difference was small. These normalized data suggest that ST-induced increases in PP depend on muscular hypertrophy in men, but not in women, providing further support for the hypothesis developed from our previous report (Ivey FM, Tracy BL, Lemmer JT, NessAiver M, Metter EJ, Fozard JL and Hurley BF. J Gerontol A Biol Sci Med Sci 55: B152–B157, 2000) that improvements in muscle function with ST result from nonmuscle mass adaptations to a greater extent in women than men.

sarcopenia is the loss of muscle mass with advanced age and is associated with dysfunction, poor health status, and the loss of muscle strength and power in older adults (17, 18). Muscle power accounts for a greater amount of the variance in physical performance than strength in older adults (3, 9) and deteriorates at a faster rate than strength with advanced age (2, 16, 21). Previous cross-sectional data suggest that this decline in peak muscle power with age is associated with muscle structure and function, tendon characteristics, and sarcopenia in specific muscle groups (20).

Previous reports on the effects of strength training (ST) on muscle power did not report how the training affected power per unit of the muscle involvement [muscle power quality (MPQ)], or peak velocity (PV) (5, 8, 12, 13, 15), the latter possibly being an important component of power and possibly functional abilities in the elderly. The expression of peak power (PP) and movement velocity normalized for muscle volume (MV) allows better understanding of potential mechanisms (e.g., hypertrophy and neuromuscular adaptations) for training-induced adaptations. It is also important when comparing groups who possess different amounts of muscle mass, such as men compared with women. For example, in a previous investigation from our laboratory, we found that muscle quality [one-repetition maximum (1 RM) strength/MV] increased significantly more in women than in men (11). This finding suggests that ST-induced increases in muscle strength in women are preferentially influenced by nonmuscle mass adaptations compared with men, thus providing support for the hypothesis that other indicators of improved muscle function with ST, such as muscle power and movement velocity, may be less dependent on muscle mass increases in women than in men. Moreover, expressing PV changes with ST relative to the MV involved in the movement would better isolate the influences of muscle power changes that are independent of muscle mass.

Furthermore, previous investigations (5, 8, 12, 13) reported PP as the highest average power obtained during multiple trials of a power test, as opposed to the highest power value attained during a single trial. The highest PP, i.e., the highest combination of force and velocity that occurs simultaneously during a single trial, might be a more accurate measure of the explosive capacity of the trained musculature than average (area under curve) power of a single trial. This is because average PP includes two phases of movement that represent reduced power. The first is at the beginning of the movement when one is trying to overcome inertial forces and the other is near the end of the movement when cocontraction of the antagonist muscle group produces a reduced force and velocity. Although some previous investigations did exclude data from the first and last 5% of the range of movement in the power tests (3, 8, 9), these studies still used the average power for a given trial and reported it as PP. Thus there is no information available, to our knowledge, on the effects of ST on PP. PP could conceivably have a different relationship to functional tasks and be affected differently by ST than average power.

In addition, several of the recent training studies that reported changes in leg muscle power with ST did not have an inactive control group to control for biological, methodological, or seasonal variations (3, 5, 8). An untrained contralateral limb is ideal for controlling for drifts in muscle mass or power assessments due to the effects of biological, methodological, or seasonal variation. It also has the advantage over a separate inactive control group by controlling for genetic differences between groups, differences in attention given to the training group compared with a separate control group, and differences in physical activity levels between two groups.

Thus the purpose of this investigation was to determine the effects of moderate-velocity ST on muscle power and movement velocity when normalized for the entire trained musculature involved in the movement (MPQ and movement velocity quality, respectively) at the same absolute and relative loads in middle-aged and older men and women. It was hypothesized that both absolute and relative PP of the knee extensors would increase with ST in both men and women, but peak MPQ and peak movement velocity quality, i.e., PV-to-MV ratio, would be increased to a significantly greater extent in women than in men, based on our previous data (11). We also hypothesized that relative (same % of 1 RM) PV would decrease in both men and women with ST, based on the force-velocity curve, but absolute (same load) PV would increase in both sexes with ST.

METHODS

Subjects.

Sixty-two previously inactive, relatively healthy men (n = 30) and women (n = 32) between 50 and 74 yr of age volunteered to participate in the study. All subjects underwent a phone-screening interview, received medical clearance from their primary care physician, and completed a detailed medical history before participating in this study. All subjects were nonsmokers, free of significant cardiovascular, metabolic, or musculoskeletal disorders that would affect their ability to safely perform heavy-resistance exercise. Subjects who were already taking medications for at least 3 wk before the start of the study were permitted into the study as long as they did not change medications or dosages at any time throughout the study. Two subjects were previously diagnosed with type II diabetes mellitus but were otherwise healthy. After all methods and procedures were explained, subjects read and signed a written consent form to participate in the study, which was approved by the Institutional Review Board of the University of Maryland, College Park. All subjects were continually reminded throughout the study not to alter their regular physical activity levels or dietary habits for the duration of the investigation, and body weight was measured weekly throughout the study to confirm the maintenance of energy balance.

Body composition assessment.

Body composition was estimated by dual-energy X-ray absorptiometry using the fan-beam technology (model QDR 4500A, Hologic, Waltham, MA). A total body scan was performed at baseline and again after the ST program. A standardized procedure for patient positioning and utilization of the QDR software was used. Total body fat-free mass (FFM), fat mass, and percent fat were analyzed by using Hologic version 8.21 software for tissue area assessment. Total body FFM was defined as lean soft tissue mass plus total body bone mineral content. The coefficients of variation (CV) for all dual-energy X-ray absorptiometry measures of body composition were calculated from repeated scans of 10 subjects who were scanned three consecutive times with repositioning. The CV was 0.6% for FFM and 1.0% for percent fat. The scanner was calibrated daily against a spine calibration block and step phantom block supplied by the manufacturer. In addition, a whole body phantom was scanned weekly to assess any machine drift over time.

Body weight was determined to the nearest 0.1 kg with subjects dressed in medical scrubs, and height was measured to the nearest 0.1 cm using a stadiometer (Harpenden, Holtain, Wales, UK). Body mass index was calculated as weight (kg) divided by height (m) squared.

Strength testing.

One-repetition maximum (1 RM) strength tests were assessed on the knee extension exercise before and after the ST program using an air-powered resistance machine (Keiser Sports/Health Equipment, Fresno, CA). This exercise was chosen because it could easily be tested in a standardized way by using objective criteria. The 1 RM test was defined as the maximal resistance that could be moved through the full range of motion with proper form one time. Approximately the same number of trials (6–8) and the same rest periods between trials (∼1 min) were used to reach the 1 RM after training as before training. Before the regular ST program, 1 RM testing, and power testing were performed, subjects underwent at least three familiarization sessions in which the participants completed the training program exercises with little or no resistance and were instructed on proper warm-up, stretching, and exercise techniques. These low-resistance training sessions were conducted to familiarize the subjects with the equipment, to help control for the large 1 RM strength gains that commonly result from skill (motor learning) acquisition during the initial stages of training, and to help prevent injuries and reduce muscle soreness after the strength testing protocol. The same investigator conducted strength tests for each subject both before and after training using standardized procedures with consistency of seat adjustment, body position, and level of vocal encouragement. When appropriate, straps and/or belts were used to stabilize the subject so that recruitment of outside muscle groups was minimized. The 1 RM was achieved by gradually increasing the resistance from an estimated submaximal load after each successful exercise repetition until the maximal load was obtained. A light system was used to indicate a successful attempt when the knee was extended past ∼165°. Only those trials that turned on the light were considered successful trials.

Training program.

The training program consists of unilateral (one-legged) training of the knee extensors of the dominant leg, three times per week, for ∼10 wk. Training was performed on a Keiser A-300 air powered leg extension machine. It allows ease of changing the resistance without interrupting the cadence of the exercise. The untrained control leg was kept in a relaxed position throughout the training program.

Subjects warmed-up on a bicycle ergometer for ∼2 min before each training session. After the three familiarization training sessions previously described, the training consisted of five sets of knee extension exercise. The protocol was designed to include a combination of heavy-resistance and high-volume exercise. The first set was considered warm-up and consisted of five repetitions at 50% of the 1 RM strength value. The second set consisted of five repetitions at the present five-repetition maximum (5 RM) value, which was initially estimated based on our previous data showing that it corresponds to ∼85% of 1 RM in most people. Adjustments were made as needed during each training session so that the resistance used would result in failure to complete a sixth repetition. The 5 RM value was increased continually throughout the training program to reflect increases in strength levels. The first four or five repetitions of the third set were performed at the present 5 RM value, and then the resistance was lowered just enough to complete one or two more repetitions before reaching muscular fatigue. This process was repeated until a total of 10 repetitions are completed. This same procedure was used in the fourth and fifth sets, but the total number of repetitions was increased in each set to 15 and 20 repetitions, respectively. This procedure allowed subjects to use near-maximal effort on every repetition while maintaining a relatively high training volume. The second, third, fourth, and fifth sets were preceded by rest periods lasting 30, 90, 150, and 180 s, respectively. The shortening phase of the exercise was performed in ∼2 s, and the lengthening phase took ∼3 s. Subjects perform supervised stretching of the knee extensors and hip flexors after each training session.

MV assessment.

To quantify quadriceps MV, computed tomography imaging of the trained and untrained thighs was performed (GE Lightspeed Qxi, General Electric, Milwaukee, WI) at baseline and during the last weeks of the 10-wk unilateral ST program. Axial sections of both thighs were obtained starting at the most distal point of the ischial tuberosity down to the most proximal part of the patella while subjects were in a supine position. Measurements of MV in the untrained leg served as a control for seasonal, methodological, and biological variation of MV, by comparing the changes in the control leg to the training-induced changes in the trained leg. Section thickness was fixed at 10 mm, with 40 mm separating each section, based on previous work in our laboratory by Tracy et al. (23). Quadriceps MV was estimated based on using a 4-cm interval between the center of each section. Each computed tomograph image was obtained at 120 kVp with the scanning time set of 1 s at 40 mA. A 48-cm field of view and a 512 × 512 matrix was used to obtain a pixel resolution of 0.94 mm. Two technicians performed analyses of all images for each subject using Medical Image Processing, Analysis, and Visualization software (National Institutes of Health, Bethesda). Briefly, for each axial section, the cross-sectional area of the quadriceps muscle group was manually outlined as a region of interest. The quadriceps cross-sectional area was manually outlined in every 10-mm axial image from the first section closest to the superior border of the patella to a point where the quadriceps muscle group is no longer reliably distinguishable from the adductor and hip flexor groups. The same number of sections proximal from the patella was measured for a particular subject before and after training to ensure within-subject measurement replication. Investigators were blinded to subject identification, date of scan, and training status, for both baseline and after-training analysis. Repeated measurement coefficient of variation was calculated for each investigator based on repeated measures of selected axial sections of one subject on 2 separate days. Average intrainvestigator CV was 1.7 and 2.3% for investigators one and two, respectively. The average interinvestigator CV was <4.3%. Final MV was calculated using the truncated cone formula as reported by Tracy et al. (23) and described by Ross et al. (19).

Muscle power and movement velocity testing.

Determination of peak knee extensor PP and movement velocity were performed on a customized Keiser pneumatic resistance knee extension (K410) machine (Keiser Sports/Health Equipment), specifically designed for muscle power assessment. The K410 machine is equipped with load cell force transducers and position sensors to detect rotary motion at the joint. The K410 hardware is connected to a personal computer and uses an industrial data-collection expansion card to digitize data at 400 times/s from the force sensors and position sensors. This speed is configured and set by the K410 software. Movement velocity assessment is derived from a crystal oscillator on the data collection board.

Before testing, seated blood pressure was monitored after 5 min of rest, and then a 1-min warm-up was performed on a stationary cycle ergometer. Subjects were then positioned in the K410 with the medial condyle aligned with the axis of rotation of the machine arm. Subjects were instructed to cross their arms across the chest, and a seat belt attached to the machine was then securely fastened around the waist to help isolate the knee extensor muscle group. Subjects were instructed to perform a knee extension with each leg unilaterally at a resistance of ∼30% of their measured 1 RM and at ∼50% of their maximal velocity, as a warm-up trial. After a 30-s rest period, subjects performed three power tests on each leg alternating between right and left at 50, 60, and 70% of their 1 RM, with a 30-s rest period between each of the three trials and 2-min rest periods between each increase in resistance. The tester offered standardized oral encouragement to each subject to extend his or her knee as quickly and forcefully as possible during each trial. The highest PP value of the three trials for each percentage of 1 RM and the highest PV attained during this same trial was selected. Although PV was selected from the same test trial as PP, it was measured separately from PP as the highest velocity obtained during the trial, independent of where PP was obtained. The entire procedure was repeated 48–72 h later, and the PP values at each resistance level for both baseline tests were averaged in an effort to establish a more stable baseline assessment. This test was repeated during the last week of the 10-wk unilateral ST program for the posttraining test. During this latter test, an attempt was made to find a load that could be replicated from baseline testing that represented 50 or 60% of the posttraining 1 RM for testing at the same absolute load. When a replicable load could not be found that fell at one of these relative loads (i.e., 50% or 60% of the posttraining 1 RM), the load that was used at 50% of the baseline 1 RM value was used for the posttraining same absolute load (regardless of the % of posttraining 1 RM that the load represents) during the posttraining test. The 70% of the posttraining 1 RM was compared with 70% of 1 RM at baseline for assessing the effects of training on PP and movement velocity at the same relative load. This relative load was chosen because it is the approximate load at which the highest PP was found at baseline and after training in our pilot data and from other investigations (3, 8). Data for each repetition were passed through a zero-phase forward and reverse digital filter designed using MATLAB version 6.0.5 (MathWorks, Natick, MA) to remove sensor noise before determining the PP and movement velocity. A low-pass, 10th-order Butterworth filter with a cutoff frequency of 10 Hz was used. A simple point-to-point search of the power and movement velocity data was conducted to determine the peaks because the resulting power and movement velocity curves are unimodal throughout a single repetition (Fig. 1).

Fig. 1.

Fig. 1.Raw and filtered power (A) and movement velocity (B) curves for 1 repetition of a knee extensor trial. The raw power and movement velocity were passed through a zero-phase forward and reverse digital filter to remove sensor noise before determining the PP and peak movement velocity. A low-pass, 10th-order Butterworth filter with a cutoff frequency of 10 Hz was used. A simple point-to-point search of the power and movement velocity data was conducted to determine the peaks because the resulting power and movement velocity curves are unimodal throughout the repetition.


Data analysis.

All statistical analyses as described below were performed using SAS software (SAS version 8.1, SAS Institute, Cary, NC). To determine whether the training programs had an effect on physical characteristics (body mass, body fat, FFM), strength (1 RM), quadriceps MV, PP, or PV, we conducted a mixed-model analysis of covariance, which was used to determine between-group differences (i.e., trained vs. untrained legs) after ST, when baseline values were covaried, to compare changes in MV, PP, and PV. Paired t-tests were used to determine within-group changes for each of these variables in the trained and untrained legs. Statistical power calculations revealed values between 0.70 and 0.80, with a significance level set at P < 0.05.

RESULTS

The physical characteristics of the subjects are shown in Table 1. Both men and women increased their 1 RM significantly (P < 0.001). However, men had a significantly greater increase in 1 RM than women when covarying for baseline differences and age (P < 0.05). There were no other significant changes in any of the physical characteristics shown in Table 1 for men or women with ST. There was a significantly greater increase in the trained leg than the untrained (control) leg for changes in 1 RM (P < 0.001), MV (P < 0.001), absolute PP (P < 0.05), relative PP (P < 0.01), relative MPQ (P < 0.01), and absolute PV (P < 0.05) in men and women. In addition, there was a significantly greater decrease in the trained leg than the untrained leg for relative PV in men (P < 0.05) but not in women. There was also a significantly greater increase in the trained leg than the untrained leg for absolute MPQ in women (P < 0.01) but not in men. In contrast, there was a significantly greater increase in the trained leg than the untrained leg for absolute movement velocity quality in men (P < 0.05) but not in women. Finally, there was a significantly greater decrease in the trained leg than the untrained leg for relative movement velocity quality in men and women (both P < 0.05). As expected, within-group analyses show that the untrained leg had a significant increase for 1 RM (P < 0.01), based on the well-established cross-education effect (Table 2). However, there were no other significant changes in the untrained leg (Table 2), indicating that the untrained leg serves as an appropriate control for all other variables.

Table 1. Physical characteristics at baseline and after ST in men and women

Men (n = 30)
Women (n = 32)
BaselineAfter STBaselineAfter ST
Age, yr64 (7)62 (6)
Height, cm174.4 (6)162.7 (5)
Weight, kg82.1 (9.2)82.3 (9.6)72.1 (12.1)71.9 (13.2)
BMI, kg/m227.0 (2.9)27.2 (3.0)27.2 (4.4)27.3 (4.8)
Body fat, %27.1 (5.1)26.7 (4.8)39.2 (4.7)38.5 (5.1)
Fat-free mass, kg59.7 (6.0)60.1 (5.6)43.5 (5.9)43.8 (6.3)
1 RM, n274 (77)353 (91)*138 (57)185 (63)*

Values are means (SD). ST, strength training; BMI, body mass index; 1 RM, knee extension 1-repetition maximum.

*Significantly different than baseline (P < 0.001).

Significantly greater change than women when covarying for baseline differences and age (P < 0.05).

Table 2. Untrained (control) knee extensor 1 RM, peak movement velocity, muscle volume, muscle quality, peak power, and movement velocity quality at baseline and after ST in men and women

Men (n = 30)
Women (n = 32)
BaselineAfter STBaselineAfter STa
1 RM, N262 (13)289 (16)a177 (10)186 (10)*
Absolute peak power1 (W)405 (21)417 (23)225 (17)212 (17)
Absolute peak movement velocity,b rad/s5.6 (0.2)5.8 (0.2)4.9 (0.2)4.9 (0.2)
Relative peak movement velocity,c rad/s4.7 (0.2)4.5 (0.1)4.1 (0.1)4.1 (0.1)
Muscle volume,d cm31,692 (50)1,697 (51)1,073 (39)1,079 (38)
Muscle quality,e (N/cm3) × 10−11.6 (0.1)1.7 (0.1)1.2 (0.1)1.5 (0.1)
Absolute muscle power quality,b (W/cm3) × 10−12.5 (0.1)2.5 (0.1)2.1 (0.1)2.1 (0.1)
Relative muscle power quality,c (W/cm3) × 10−12.4 (0.1)2.5 (0.1)2.0 (0.1)2.0 (0.1)
Absolute movement velocity qualityb (rad·s−1·cm−3) × 10−33.5 (0.1)3.5 (0.1)4.7 (0.2)4.8 (0.2)
Relative movement velocity qualityb (rad·s−1·cm−3) × 10−32.8 (0.1)2.7 (0.1)4.4 (0.3)4.8 (0.3)

Values are means (SE). rad, Radians.

aSignificantly greater than baseline (P < 0.01).

bThe same absolute resistance at both baseline and after ST.

c70% of 1-RM at baseline and 70% of the improved 1 RM after ST.

dMuscle volume of the knee extensors.

e1 RM/muscle volume.

There was a significant increase in absolute PV in both men and women (P < 0.001). Table 3 shows that there was a significant decrease in relative PV with ST in both men (P < 0.01) and women (P < 0.05). There were no differences in the changes between men and women for absolute PV or relative PV. Both men and women showed a significant increase (P < 0.001) in MV with ST, but men had a significantly greater increase in MV (P < 0.001) than women when covarying for baseline values and age (Table 3). There was a significant increase in absolute MPQ with training in women (P < 0.05), but not in men (Table 3). However, there were no significant differences between men and women in the changes in absolute MPQ. Both men and women showed a decrease in relative movement velocity quality (P < 0.001) with ST (Table 3). Men had a significantly greater decrease in relative movement velocity quality than women when covarying for age and baseline differences (P < 0.01), but this difference was too small to likely be physiologically meaningful.

Table 3. Knee extensor peak movement velocity, muscle volume, and muscle quality at baseline and after ST in men and women

Men (n = 30)
Women (n = 32)
BaselineAfter STBaselineAfter ST
Absolute peak movement velocity,a rad/s5.5 (0.2)5.9 (0.1)g4.7 (0.2)5.3 (0.2)g
Relative peak movement velocity,b rad/s4.6 (0.1)4.0 (0.1)f4.2 (0.1)3.9 (0.1)e
Muscle volume,c cm31,713 (44)1,878 (52)g,i1,126 (43)1,219 (44)g
Muscle quality,d N/cm3 × 10−11.6 (0.1)1.9 (0.1)g1.2 (0.1)1.5 (0.1)g
Absolute muscle power quality,a (W/cm3) × 10−12.5 (0.1)2.5 (0.1)2.2 (0.1)2.4 (0.1)e
Relative muscle power quality, (W/cm3) × 10−12.4 (0.1)2.4 (0.1)2.1 (0.1)2.1 (0.1)
Absolute movement velocity quality,a (rad·s−1·cm−3) × 10−33.3 (0.1)3.2 (0.1)4.4 (0.2)4.5 (0.2)
Relative movement velocity quality, (rad·s−1·cm−3) × 10−32.7 (0.1)2.2 (0.1)g,h3.9 (0.1)3.3 (0.1)g

Values are means (SE).

aThe same absolute resistance both at baseline and after ST.

b70% of 1-RM at baseline and 70% of the improved 1-RM after ST.

cMuscle volume of the knee extensors.

d1 RM/muscle volume.

eSignificantly different than baseline (P < 0.05).

fSignificantly different than baseline (P < 0.01).

gSignificantly different than baseline (P < 0.001).

hSignificantly greater change when compared the other sex when covarying for baseline differences and age (P < 0.01).

iSignificantly greater change when compared the other sex when covarying for baseline differences and age (P < 0.001).

Table 4 shows the baseline and after training values in men and women for both the trained and untrained legs for 50, 60, and 70% of 1 RM. In men, there were significant increases in relative PP at 50% (P < 0.001), 60% (P < 0.001), and 70% (P < 0.05) of 1 RM in the trained leg but not in the untrained leg, whereas in women there were significant increases in relative PP at 50% (P < 0.01) and at 70% (P < 0.05) in the trained leg, and significant decreases in the untrained leg at 50%, 60%, and 70% of 1 RM (P < 0.05). These data confirm that there is no cross-education effect for PP with ST. With regard to relative PV at 50, 60, and 70% of 1 RM at baseline and after ST in men, there were significant decreases in PV with ST in the untrained leg at 50, 60, and 70% of 1 RM (P < 0.05). There were significant decreases in PV at 50% (P < 0.01), 60%, and 70% (both P < 0.001) with ST in the trained leg. In women, there was a significant decrease in PV at 70% of 1 RM in the trained leg (P < 0.01) and at 60 and 70% of 1 RM (both P < 0.05) in the untrained leg. Between-group comparisons indicate that there was a significantly greater increase in relative PP in the trained leg in men than women at 60% (P < 0.01) and 70% of 1 RM (P < 0.001) when covarying for baseline differences and age (Table 4).

Table 4. Relative peak power and peak movement velocity at 50%, 60%, and 70% of 1 RM at baseline and after ST in men and women

Peak Power, W
Peak Velocity, rad/s
BaselineAfter STBaselineAfter ST
Men
Trained leg
    50% 1RM453 (23)493 (25)c6.3 (0.1)5.9 (0.1)b
    60% 1RM452 (24)488 (24)c,d5.5 (0.1)5.0 (0.1)c
    70% 1RM430 (22)456 (18)a,e4.7 (0.1)4.0 (0.1)c
Untrained leg
    50% 1RM432 (22)434 (23)6.3 (0.2)6.0 (0.1)a
    60% 1RM438 (23)442 (23)5.6 (0.2)5.3 (0.1)a
    70% 1RM413 (21)416 (21)4.9 (0.2)4.5 (0.1)a
Women
Trained leg
    50% 1RM262 (21)281 (20)b5.4 (0.2)5.2 (0.2)
    60% 1RM259 (22)271 (19)4.8 (0.2)4.5 (0.2)
    70% 1RM244 (15)256 (13)a4.2 (0.1)3.9 (0.1)b
Untrained leg
    50% 1RM262 (23)247 (22)a5.5 (0.3)5.3 (0.2)
    60% 1RM261 (23)245 (23)a4.9 (0.2)4.7 (0.2)a
    70% 1RM240 (18)228 (18)a4.3 (0.2)4.1 (0.2)a

Values are means (SE).

aSignificantly different than baseline (P < 0.05).

bSignificantly different than baseline (P < 0.01).

cSignificantly different than baseline (P < 0.001).

dSignificantly greater change than women when covarying for baseline differences and age (P < 0.01).

eSignificantly greater change than women when covarying for baseline differences and age (P < 0.001).

Figure 2 shows that there were also significant increases in the trained leg with ST in absolute PP from baseline in both men and women (both P < 0.001) when subjects were tested at the same absolute resistance (load). This load represented 61 ± 2% of 1 RM at baseline and 50 ± 2% of the improved 1 RM after ST in men, and 63 ± 2% of 1 RM at baseline and 52 ± 2% of the improved 1 RM after ST in women. There was no significant difference in absolute PP increases between men and women when covarying for baseline differences and age (Fig. 2).

Fig. 2.

Fig. 2.Knee extensor absolute peak power (PP) assessed at the same absolute resistance at both baseline and after strength training (ST) in men and women. There was a significant increase in absolute PP after ST in both men and women (*P < 0.001), but there were no significant differences between men and women with regard to changes in absolute PP.


DISCUSSION

The results of the present study demonstrate for the first time, and in support of our hypothesis, that moderate-velocity ST significantly increases knee extensor PP in women, but not in men, when tested at the same absolute load and when normalized for the entire volume of trained musculature (i.e., absolute muscle power quality). When the data are normalized under these same conditions, and tested at the same relative (% of 1 RM) load, both men and women reduce their movement velocity significantly with training (as expected, due to the force velocity relationship), but men show significantly greater reductions than women. However, the magnitude of difference is too small to likely be physiologically meaningful. Nevertheless, the ST program did increase absolute and relative PP in both men and women, when not normalized for MV. In addition, both men and women significantly increased their absolute peak movement velocity but significantly reduced their relative peak movement velocity with training, when not normalized for MV. The ST program did not significantly change relative MPQ in men or women.

The finding of a 9% training-induced increase in muscle power quality in women, but no change in men, supports the hypothesis generated from a previous report from our laboratory (11) that women may not rely on muscle hypertrophy as much as men to improve muscle function with training. In that report, we suggested that one explanation for the greater increase in ST-induced muscle quality in women than men could be the preferential influence on nonmuscle mass adaptations to ST in women. However, we were unable to find any investigations that addressed the specific mechanisms responsible for this finding with muscle quality or with the finding in our present study with MPQ. The fact there was no difference between the changes in absolute PP, but there were differences in the changes in MV between men and women with ST, suggests a training adaptation in women that is less dependent on muscle hypertrophy. It has been reported previously that ∼40% of the increase in muscular power with ST is due to muscular hypertrophy, and other factors determine the remaining 60% (7). One of these factors is possibly some type of neuromuscular adaptation that compensates for the reduced capacity of women to undergo muscle hypertrophy with ST, compared with men, resulting in a compensatory increase in power per unit of muscle. Support for this hypothesis comes from previous data showing that increases in power and strength with ST in older adults can be strongly influenced by neural adaptations (10). This may be due to a greater reduction in the coactivation of the antagonist muscle groups, which has been reported to occur to a greater extent in women than men during the first 2 mo of ST (10). However, there are no data from the present study to support any specific mechanisms for sex differences in MPQ. Further data in support of sex differences in the contribution of muscle hypertrophy to changes in muscle function with ST come from a recent investigation by Bamman et al. (1), who compared ST-induced changes in strength and myofiber hypertrophy between men and women at different stages of training. They observed similar rates of strength increases for men and women during the first few weeks of training followed by no further increase in strength upon subsequent training in women, whereas men showed a continuous increase in strength along with greater myofiber hypertrophy throughout 26 wk of training (1). There are no time course data in the present study from which to speculate any particular mechanisms for explaining our finding of a 9% increase in absolute MPQ in women and no changes in men.

Our finding that both men and women significantly increased their absolute PV with ST when using relatively low loads (∼50% of 1 RM after training) may have important implications for functional abilities. This conclusion is based on recent data suggesting that selected functional ability performance in older adults may be more dependent on movement velocity at lower external loads than at high loads (4). Previous investigations examined the effect of different training velocities on PP (3, 8) but did not assess the contribution of velocity to enhanced power, which may have more important implications for functional abilities.

The use of the untrained leg also adds a unique contribution to the existing literature on this topic. Our data show that like MV, but unlike strength, there is no cross-education effect on absolute, relative PP, MPQ, movement velocity, or movement velocity quality. These data confirm its value as a control for normal drift in values due to variations in methodology, biology, season of the year, genetic differences between groups, or differences in attention between experimental and control groups.

Nevertheless, there are limitations with regard to the present investigation. Subjects in this investigation were trained by using a moderate-velocity training protocol (∼2 s during the shortening phase and ∼3 s during the lengthening phase). Although it could be argued that a higher velocity training protocol is likely to produce greater gains in power (8), we chose to investigate a protocol more commonly used, with an extensive track record for being safe and effective for producing improvements in all the major components of sarcopenia (i.e., muscle mass, strength, muscle quality, and power) (11, 14, 22). It is still not well established whether a high-velocity training program is well tolerated by older subjects (6). Finally, the subjects in this study were relatively homogenous with respect to age but not with respect to race. By self-report, there were 21 African Americans and one Asian American in this cohort, with remainder of subjects being Caucasian. However, race was not a significant covariate in our analyses. Finally, the age range was quite large (50–74 yr), and it is conceivable that the youngest subjects in the study may have slightly different training responses than the older ones, but age was included as a covariate in our analyses.

Future research needs to be done to examine how this novel method of measuring peak muscle power and normalizing it by the volume of the trained musculature compares to the more common measure of nonnormalized average PP. In addition, this technique needs to be applied to measure the PP in other movements, such as upper leg extension used in the leg press exercise, as previously reported with average PP (5, 8). Finally, investigations need to be done to determine the influence of genotypes and racial differences on peak muscle power changes with ST.

FOOTNOTES

  • The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This study was supported by research contract no. 1-AG-4-2148 and Grants AG-1620501 and AG-022791 from the National Institute on Aging.

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AUTHOR NOTES

  • Address for reprint requests and other correspondence: B. Hurley, Dept. of Kinesiology, Univ. of Maryland, College Park, MD 20742 (e-mail: )