Whey protein and high-volume resistance training in postmenopausal women

Abstract

Objectives

To examine the combined effects of whey protein supplementation and low intensity, high-volume resistance training in healthy postmenopausal women.

Design, setting and subjects

Postmenopausal women (n=12; age: 57 ± 4.7 years, weight: 75 ± 17.4 kg, height: 163 ± 5.5 cm, body mass index: 28.3 ± 7.0) consumed whey protein (4 × 10 gram aliquots) or placebo (maltodextrin) during unilateral resistance training sessions 2 days per week (Monday, Thursday) and consumed the opposite beverage during training the other side of the body on alternating days (Tuesday, Friday) for 10 weeks. Participants performed 3 sets at 30% baseline 1-repetition maximum (1RM) to volitional muscle fatigue for 4 exercises (leg curl, biceps curl, leg extension, triceps extension). Prior to and following training, assessments were made for upper and lower limb lean tissue mass (dual energy x-ray absorptiometry), muscle thickness of the elbow and knee flexors and extensors (ultrasound) and muscle strength (1RM leg curl, biceps curl, leg extension, triceps extension).

Results

There was a significant increase over time for muscle strength (biceps curl, leg extension, triceps extension; P = 0.006) and muscle thickness (elbow flexors and extensors; P = 0.022) with no differences between whey protein and placebo.

Conclusions

High volume resistance training is effective for improving some indices of muscle mass and strength in postmenopausal women, but the strategic ingestion of whey protein during training sessions does not augment this response.

Introduction

Females typically experience accelerated muscle loss after menopause (1) which has a negative effect on strength (2, 3). The age-related decline in muscle mass and strength is partially due to an attenuated response to anabolic stimuli (e.g., resistance training and dietary protein), known as aging anabolic resistance (4, 5). Therefore, interventions which augment the physiological response to resistance training and dietary proteins are needed to increase aging muscle health.

Mechanical stimuli from resistance training promotes numerous adaptations in skeletal muscle, many of which may help prevent or reverse age-related declines in muscle mass and strength (3, 6, 7). The American College of Sports Medicine recommends that resistance training should be performed at training intensities ≥ 70% 1RM to achieve muscle accretion (8). However, postmenopausal women may find it difficult to consistently train at high intensities due to potential co-morbidities (i.e. arthritis) associated with aging (4). Two recent studies have shown that low intensity (30% 1RM), high-volume leg resistance training (performing 3-4 sets to volitional fatigue) increases the rates of muscle protein synthesis, muscle size and strength in young males (n =15-18; 21 ± 1 yr; refs. 9, 10). Furthermore, this mode of training produced similar increases in muscle protein synthesis (≤ 4 hours post-exercise) and muscle hypertrophy, after 10 weeks of training (3x/week), compared to high intensity training (80-90% 1RM to volutional fatigue; 9). Therefore, based on these preliminary findings in young adults, it is plausible that low intensity, high-volume resistance training may be an effective intervention to increase muscle mass and strength in postmenopausal women.

The addition of whey protein to low intensity, high-volume resistance training may further increase the rates of muscle protein synthesis which could lead to greater muscle mass and strength over time in postmenopausal women. Whey protein has a high essential amino acid profile (i.e. leucine) (11), which are quickly absorbed leading to rapid amino acid delivery to skeletal muscles (12) and increases the rates of muscle protein synthesis following resistance training (13). Therefore, the purpose of this study was to determine the effects of whey protein combined with low intensity, high-volume resistance training on muscle mass and strength in postmenopausal women. It was hypothesized that low intensity, high-volume resistance training would increase lean tissue mass and strength and the addition of whey protein would further augment these gains compared to placebo.

Materials and Methods

Participants

Seventeen females, who verbally confirmed they were postmenopausal (i.e. defined as having their last menstrual cycle ≥ 1 year prior to the start of the study) and were not performing supervised resistance training for at least 3 months prior to the start of the study were enrolled. Non-resistance trained postmenopausal women were selected to potentially maximize the physiological adaptations from resistance training. Participants indicated that they performed mild-intensity physical activity 1-3 times per week (i.e. walking, gardening) prior to the start of the study. Participants were required to fill out a Physical Activity Readiness Questionnaire which assessed their readiness to participate in the resistance training program and included questions related to heart conditions, angina at rest or during physical exercise, as well as balance and bone or joint problems that may affect exercise performance. Participants were excluded if they had previously taken medications that affect muscle biology (i.e. glucorcorticoids, hormone replacement therapy), suffered from severe osteoarthritis, had consumed ergogenic aids (i.e. creatine supplements) for ≤ 6 weeks prior to the start of the study, if they were vegetarians, if they were smokers or if they had pre-existing kidney or liver abnormalities. Participants were instructed not to change their diet or engage in additional physical activity that was not part of their normal daily routine during the study period, to refrain from food and drink for one hour post-exercise so that a valid estimate on the effects of protein supplementation could be made (water was permitted ad libitum) and not to consume non-steroidal anti-inflammatory drugs during the study, as these interventions could affect muscle protein synthesis (14) and potentially influence our outcome measures. The study was approved by the Research Ethics Board at the University of Regina. Participants were informed of the risks and purposes of the study before written consent was obtained.

Experimental Design

The study used a double-blind, repeated measures, within-subject (placebo control) design where postmenopausal women were randomized to consume whey protein or placebo during unilateral (dominant side of the body was also randomized as all participants were right hand dominant) resistance training 2 days per week (Monday, Thursday) and consume the opposite beverage while training the other side of the body on alternating days (Tuesday, Friday) for 10 weeks. This unique design allowed for the direct comparison of whey protein vs. placebo within the same participant which increased our statistical power and internal validity.

At baseline and after the study, the primary dependent variables assessed included: (1) upper and lower limb lean tissue mass (dual energy x-ray absorptiometry; DXA), (2) muscle thickness of the elbow and knee flexors and extensors (ultrasound) and (3) unilateral muscle strength (1-repetition maximum leg curl, biceps curl, leg extension, triceps extension). A secondary dependent variable was habitual dietary intake. Participants filled out a 3-day food diary at baseline and after 10 weeks of training to determine whether total energy (kcal) and macronutrient intake changed over time.

Supplementation

Whey protein (40 grams; ISOWhey BreezerTM lemon iced tea flavoured powder, Interactive Nutrition; manufactured by NutrMix Laboratories under current Good Manufacturing Practises and Natural Health Products certifications) and placebo (30 grams of cornstarch maltodextrin and 10 grams of lemon iced tea flavoured sucrose) were identical in energy content (180 Kcal), taste, texture, volume and appearance. Protein and placebo were provided to each participant in separate plastic bags with detailed supplement instructions. Participants consumed the protein or placebo in the presence of an exercise supervisor (i.e. certified personal trainer) to ensure 100% compliance. The whey protein dosage of 40 grams was chosen as this quantity increases the rates of muscle protein synthesis following resistance training in older adults compared to lower protein dosages (15). Participants consumed 25% of their beverage (i.e. 10 grams of protein or placebo) following each exercise (4 exercises in total per training day). We chose this ‘pulse’ supplementation strategy for three reasons: (1) protein ingestion (~ 10 grams) every 15 minutes during resistance training increases the rates of muscle protein synthesis (16), (2) protein ingestion (10 grams) immediately following resistance training improves muscle hypertrophy over time in aging adults (17) and (3) ingesting 40 grams (bolus) of whey protein following resistance training sessions may not be feasible for older adults (4).

Resistance Training Program and Muscle Strength

Prior to the start of supplementation and training, each participant had their unilateral 1RM strength assessed (right side muscle group followed by left side muscle group) on the leg curl, biceps curl, leg extension and triceps extension resistance training equipment (Pulse Fitness Systems Inc, Winnipeg, Manitoba, Canada). Following 5-minutes of cycling on a stationary cycle ergometer at a self-selected intensity, participants performed two warm-up sets in order: 1 set of 10 repetitions using a weight determined by each participant to be comfortable and 1 set of 5 repetitions using increased weight. Two-minutes following the warm-up sets, weight was progressively increased for each subsequent 1RM attempt with a 2-minute rest interval. The 1RM was achieved in 6 sets or less. Five minutes of rest separated each 1-RM assessment between different muscle groups. All measurements were performed by the same experienced researcher. Following determination of baseline 1RM strength, but prior to the start of supplementation and training, participants familiarized themselves with the machine-based resistance training equipment, under direct supervision of a certified personal trainer. We chose to use machine-based resistance training equipment because they are considered safer and easier to learn than free weights (18) and the use of machine-based equipment lead to greater improvements in machine-based strength tests (19). During the 3 familiarization resistance training sessions, participants were properly shown how to breathe, use the equipment, and perform repetitions to volitional fatigue (defined as the inability to perform the concentric or eccentric phase of a muscle contraction) using 30% baseline 1RM for each exercise.

At least 4 days following the last familiarization session, participants performed 3 sets at 30% baseline 1RM to volitional fatigue for each exercise in order (i.e. leg curl, biceps curl, leg extension, triceps extension) for 10 weeks. We chose this duration of training because Mitchell et al. (10) found a significant increase in muscle accretion and strength when 3 sets of exercise were performed at 30% 1RM to volitional fatigue in young males over 10 weeks. Participants were instructed to perform the concentric phase in 2 seconds, pause for 2 seconds and then perform the eccentric phase in 2 seconds (i.e. 6 seconds of time under tension) for each muscle contraction. There was a 2 second pause between the concentric and eccentric movements to help reduce the stretchreflex of muscle shortening (20) and potential momentum which may have influenced the results. The 30% 1RM load remained constant for each participant throughout the study and the number of repetitions performed progressed over time on an individual basis. Participants trained their right extremities on Monday and Thursday and left extremities on Tuesday and Friday. Participants maintained daily training logs where average training volume per session (weight x sets x repetitions) was determined for each participant and monitored by the personal trainer following each resistance training session. Only complete muscle contractions were recorded and used for training volume calculations.

Body Composition

Limb bone mineral-free lean tissue mass was measured by dual-energy x-ray absorptiometry (Hologic Wi System, Christie Group, Manitoba, Canada) in array mode. Before scanning, participants were required to take off all removable objects containing metal (i.e. jewellery, glasses, clothing with buttons, and/or zippers). Scans were performed with participants lying in a supine position along the scanning table’s centerline longitudinal axis. Feet were taped together at the toes (i.e. phalanges) to immobilize the legs while the hands were maintained in a pronated position within the scanning region. All scans were performed by the same Nuclear Medicine Technologist.

Muscle Size

Muscle thickness of the elbow flexors (biceps), elbow extensors (triceps), knee flexors (hamstrings) and knee extensors (quadriceps) was measured using B-Mode ultrasound (Aloka SSD-500 Tokyo, Japan). To measure elbow flexor and extensor muscle thickness, a small mark was drawn on the lateral side of each arm to indicate 65% of the distance down from the acromion process to the olecranon process (21). For knee flexor and extensor muscle thickness, a small mark was drawn on the lateral side of each leg to indicate 70% of the distance down from the greater trochanter to the lateral epicondyle of the tibia (22). Muscle thickness measurements were extrapolated from the monitor screen by measuring the distance from the bottom of the subcutaneous adipose layer to the surface of the humerus for the elbow flexors and extensors and to the surface of the femur for the knee flexors and extensors. Two muscle thickness measurements were taken at each site and averaged to give a muscle thickness value for that site. For each muscle thickness measurement precise markings on the skin were taken using overhead transparency film to ensure that identical sites were measured on each occasion. All measurements were performed by the same experienced researcher.

Dietary Intake

Habitual dietary intake was recorded prior to and immediately following supplementation and training to determine whether dietary intake changed over time. Participants used a 3-day food booklet to record what they eat for 2 weekdays and 1 weekend day. Participants were instructed to record all food items, including portion sizes consumed for the 3 designated days. The Interactive Healthy Eating Index (Center for Nutrition Policy and Promotion, United States Department of Agriculture) was used to analyze 3-day food records. Each food item was entered and the program provided a total energy consumption average over the 3 days as well as energy from carbohydrates, fats, and proteins individually.

Statistical analysis

A 2 (Limbs: whey protein side vs. placebo side) x 2 (Time: pre vs. post) x 4 (exercise task or muscle group: leg curl or knee flexors, leg extension or knee extensors, biceps curl or elbow flexors, triceps extension or elbow extensors) repeated measures ANOVA was used to determine differences between sides of the body for muscle thickness and unilateral strength. A 2 (Limbs: whey protein side vs. placebo side) x 2 (Time: pre vs. post) x 2 (Muscle region: arm vs. leg) repeated measures ANOVA was conducted for lean tissue mass. Post hoc analyses of simple effects were used where significant interactions were found. Repeated measures ANOVA was used to assess changes in dietary intake over time. To determine if differences in training volume impacted the results, paired t-tests were used to assess differences in training volume between the right and left side of the body and between upper and lower muscle groups over 10 weeks of training. All results are expressed as means ± standard deviation. Statistical analyses was performed using SPSS version 18.0 for Windows XP (SPSS Chicago, IL). Significance was set at P < 0.05.

Results

Of the 17 participants who were initially enrolled, 12 completed the study (age: 57 ± 4.7 years, weight: 75 ± 17.4 kg, height: 163 ± 5.5 cm, body mass index: 28.3 ± 7.0). Four participants withdrew due to time constraints and one participant withdrew due to health complications not related to the study. Of the 12 participants who completed the study, nine had resistance training experience while 3 participants were resistance training naive. There were no adverse effects reported from the resistance training, protein or placebo. The average 30% 1RM loads used over the 10 weeks of training were: Leg curl (Protein: 27.4 ± 5.2 kg, Placebo: 27.5 ± 5.1 kg), Biceps curl (Protein: 8.2 ± 1.7 kg, placebo: 8.6 ± 1.3 kg), leg extension (Protein: 26.7 ± 6.7 kg, Placebo: 27.7 ± 7.0 kg), and Triceps extension (Protein: 5.2 ± 0.9 kg, Placebo: 5.2 ± 0.9 kg). Participants performed 36.6 ± 4.7 of the required 40 training sessions (91.6 ± 11.7% compliance).

There was a significant main effect of exercise task (P = 0.001) and time (P = 0.001) and a significant exercise task x time interaction (P = 0.006) for strength. Post hoc analysis showed that muscle strength increased over time for the biceps curl (P = 0.035), leg extension (P = 0.024) and triceps extension (P = 0.001), with no change for the leg curl (P = 0.918; Table 1).

Table 1.

Muscle strength measurements (1-RM) for the biceps curl, leg extension, triceps extension and leg curl in postmenopausal women before and after 10 weeks of supplementation and resistance training

	Protein (n=10)			Placebo (n=10)
Muscle Group	Pre	Post	%	Pre	Post	%
Biceps Curl	18.2 ± 3.9	24.0 ± 5.0*	34.5 ± 32.6	19.0 ± 4.4	22.5 ± 4.1*	22.6 ± 30.1
Leg Extension	59.7 ± 15.3	65.2 ± 9.9*	7.6 ± 11.2	61.0 ± 16.1	66.2 ± 13.5*	9.3 ± 30.1
Triceps Extension	11.0 ± 1.7	20.5 ± 4.2*	87.7 ± 24.4	11.0 ± 1.7	20.2 ± 5.2*	83.8 ± 28.7
Leg Curl	61.8 ± 11.9	60.1 ± 7.4	0.7 ± 17.0	61.5 ± 12.2	62.5 ± 7.1	1.9 ± 13.7

Values are mean (kg) ± standard deviation. % = change over time (post mean-pre mean/pre mean x100%); *Significantly different after training (P < 0.05); Note: Two participants did not complete all strength measures at all time points

There was a significant main effect of muscle group (P = 0.001) and time (P = 0.009) and a significant muscle group x time interaction (P = 0.006) for muscle thickness. Muscle size of the elbow flexors (P = 0.035) and extensors (P = 0.006) increased over time with no change for the knee flexors (P = 0.443) or extensors (P = 0.244) (Table 2).

Table 2.

Muscle thickness measurements (cm) for the elbow and knee flexor and extensor muscle groups in postmenopausal women before and after 10 weeks of supplementation and resistance training

	Protein (n=12)			Placebo (n=12)
Muscle Group	Pre	Post	%	Pre	Post	%
Elbow Flexors	2.41 ± 0.48	2.58 ± 0.63*	3.2 ± 9.7	2.47 ± 0.52	2.63 ± 0.44*	12.0 ± 16.5
Elbow Extensors	3.25 ± 0.76	3.70 ± 0.60*	17.2 ± 24.3	3.25 ± 0.42	3.68 ± 0.48*	15.2 ± 15.0
Knee Flexors	4.43 ± 0.66	4.60 ± 0.64	3.0 ± 6.6	4.59 ± 0.68	4.58 ± 0.57	1.7 ± 10.9
Knee Extensors	3.23 ± 1.00	3.38 ± 0.80	6.4 ± 13.7	3.26 ± 1.00	3.35 ± 0.85	2.2 ± 14.8

Values are mean (cm) ± standard deviation. % = change over time (post mean-pre mean/pre mean x100%); *Significantly different after training (P < 0.05)

There was a significant main effect of muscle region (P = 0.000) for lean tissue mass (n=11), where leg lean tissue mass was greater than arm lean tissue mass. Changes in lean tissue mass were similar (P = 0.029) between protein and placebo for the arm (Protein: Pre 1847.3 ± 419.0 g, Post 1920.5 ± 419.4 g: Placebo: Pre 1808.9 ± 341.9 g, Post 1853.0 ± 337.9 g) and leg (Protein: Pre 6067.7 ± 975.8 g, Post 6228.9 ± 1082.1 g; Placebo: Pre 6162.7 ± 923.5 g, Post 6186.4 ± 1080.3 g).

There were no differences in total training volume between protein and placebo for biceps curl (Protein: 36099.1 ± 6858.2 kg; Placebo 37353.3 ± 10218.8 kg), leg extension (Protein: 47453.5 ± 16653.4 kg; Placebo: 49056.8 ± 17025.4 kg), triceps extension (Protein: 13677.2 ± 5324.1 kg; Placebo: 14458.4 ± 3701.6 kg) and leg curl (Protein: 56299.1 ± 13833.2 kg; Placebo: 51997.5 ± 15836.7 kg).

The average number of repetitions performed per set over the 10 weeks of training was similar between protein and placebo (Biceps curl: Protein 33.7 ± 7.8 reps; Placebo 33.4 ± 7.8 reps; Leg extension: Protein 13.4 ± 2.5 reps; Placebo 13.3 ± 2.6 reps; Triceps extension: Protein 20.5 ± 8.0 reps; Placebo 21.6 ± 5.3 reps; Leg curl: Protein 15.8 ± 4.8 reps; Placebo 14.6 ± 4.5 reps). Independent of protein or placebo, postmenopausal performed more repetitions for the upper body exercises (Biceps curl: 32.2 ± 6.0 reps; Triceps extension: 20.2 ± 5.8 reps; P = 0.000-0.016) compared to the lower body exercises (Leg extension: 13.3 ± 2.5 reps; Leg Curl: 15.2 ± 4.3 reps; P = 0.214).

There was a significant increase in dietary carbohydrate intake over time (P = 0.015), with no other differences in diet composition (Table 3).

Table 3.

Kilocalories (Kcal) and macronutrient (grams) per day for postmenopausal women before and following 10 weeks of nutritional supplementation and training

	Pre	Post	%
Kcal	1423.4 ± 270.9	1627.2 ± 258.1	19.2 ± 35.3
Carbohydrates	50.0 ± 42.3	185.3 ± 29.4*	34.4 ± 54.3
Fat	65.0 ± 43.9	60.4 ± 19.9	18.5 ± 75.1
Protein	73.1 ± 19.4	79.0 ± 18.3	12.7 ± 32.3

Values are means ± standard deviation; % = change over time (post mean-pre mean/pre mean x100%); Pre and post data is based on the average of 3 days; * Indicates significantly greater after training (p<0.05).

Discussion and conclusion

This study examined the effects of whey protein during high-volume resistance training in postmenopausal women. Contrary to our hypothesis, whey protein during training did not augment the training induced gains in muscle mass or strength. A main contributing factor of muscle loss with aging is an attenuated response to amino acids (4) which may precipitate a higher protein requirement following resistance training to achieve an anabolic environment for muscle growth (2). We considered this theory and prescribed the highest dosage of whey protein (40 grams) shown to increase the rates of muscle protein synthesis in older adults (15) using a pulse supplementation strategy when combined with a safe and practical resistance training program. However, the repeated ingestion (4 x 10 gram aliquots) of whey protein during high-volume resistance training sessions did not further increase muscle mass or strength in postmenopausal women; suggesting that our pulse supplementation strategy may have been ineffective. For example, West et al. (23) showed that a bolus ingestion of whey protein (25 grams) following an acute bout of resistance training increased the rates of muscle protein synthesis to a greater extent than pulse ingestion of whey protein (2.5 grams every 20 minutes) after training in young adults. Furthermore, whey protein (40 gram bolus) following lower-body resistance training increased the rates of muscle protein synthesis compared to lower protein doses (i.e. < 20 grams) in older men (15). Interestingly, 10 grams of whey protein had no effect on muscle protein synthesis following exercise, suggesting that the anabolic response to low protein doses (i.e. 10 grams) is diminished with aging (15). Therefore, it is possible that our pulse supplement strategy (4 x 10 gram aliquots), albeit 40 grams of whey protein in total, affected our muscle and strength results in postmenopausal women.

Despite the lack of substantial benefits from whey protein, a unique and very important finding of this study is that 10 weeks of low intensity, high-volume resistance training to volitional fatigue increases muscle strength (elbow flexors and extensors, knee extensors) and upper body muscle thickness (elbow flexors and extensors) in postmenopausal women. From a health promotion and knowledge translational perspective, these results are important as the reduction in muscle strength with aging decreases the ability to perform activities of daily living (24) and improvements in muscle size may lead to greater functionality (25). Furthermore, this study provides evidence that low intensity, high-volume resistance training is a safe (no adverse effects were reported) intervention for postmenopausal women. These results indirectly support the findings of Mitchell et al. (10) who showed that young healthy men who performed 3 sets of leg extension exercise at 30% 1RM to volitional fatigue for 10 weeks experienced a significant increase in muscle accretion and strength. Previous work from the same laboratory showed that resistance training performed at 30% 1RM to volitional fatigue (24 ± 3 repetitions) prolonged the muscle protein synthetic response compared to the same exercises performed at 90% 1RM (5 ± 1 repetitions) (9). Although it is difficult to compare results across studies which use different methodology and population cohorts, resistance training performed at 30% 1RM to volitional fatigue appears to be an effective intervention to improve indices of muscle accretion and strength in young adults and postmenopausal women. We have previously shown that postmenopausal women (n=39; age: 56.4 ± 3.7 years, weight: 74.5 ± 10.8 kg, height: 164 ± 7.4 cm) who consumed ibuprofen (400mg), creatine monohydrate (~ 7 grams) or placebo (cornstarch maltodextrin) during short-term, whole-body resistance training (9-12 weeks, 3x/week, 3 sets of 10 repetitions to muscle fatigue for leg press, chest press, lat pull-down, shoulder press, leg extension, leg curl, triceps extension, biceps curl, calf press, back extension) experienced changes in whole-body lean tissue mass (-0.8 ± 3.9%), leg press strength (29.9 ± 23.8%) and chest press strength (42.0 ± 52.9%) (26, 27) which are comparable to postmenopausal women in the present study (limb lean tissue mass: 2.8 ± 6.7%; muscle strength: 30.6 ± 39.9%) who ingested whey protein and carbohydrate (placebo) during low-load, high-volume limb resistance training. However, a direct comparison between low-load, high-volume resistance training and traditional resistance training (i.e. 3 sets of 10 repetitions to muscle fatigue), independent of nutritional or pharmaceutical interventions, is needed to determine which modality of resistance training is more beneficial for aging muscle mass and strength.

While the mechanistic actions explaining the positive effects of low intensity, high-volume resistance training remains to be elucidated, muscle fiber recruitment patterns may be involved. Hennemann’s ‘size principle’ indicates a hierarchy of muscle fiber recruitment, with Type I fibers being recruited first followed by Type IIa and then Type IIx (28). Type II muscle fibers contribute to strength and muscle hypertrophy to a much greater degree than Type I fibers (29). Speculation exists that the lighter loads performed to volitional fatigue, independent of training volume, may pre-fatigue Type I muscle fibers leading to earlier recruitment of Type II fibers. Mitchell et al. (10) recently observed an equal hypertrophic response of Type I and Type II muscle fibers from low intensity (30% 1RM), high-volume resistance training in young adults; suggesting that earlier recruitment of Type II may contribute to muscle accretion and strength during this type of training.

Postmenopausal women performed more repetitions per set over 10 weeks of training for upper body muscle groups (bicep curl, triceps extension) compared to lower body muscle groups (leg extension, leg curl) which may help explain the significant increase in upper body muscle accretion and strength. It is unclear why upper body muscle groups responded more favorably to resistance training than lower body muscle groups; however, there is evidence that lower body muscle groups may be more negatively affected with aging than upper body muscle groups. We previously showed that muscle thickness and strength of the knee flexors and extensors were reduced more with aging than the elbow flexors and extensors in healthy older men (30). Furthermore, Lynch et al. (31) found that arm and leg muscle mass and strength decreased with age but the decrease was significantly greater in the leg muscles of postmenopausal women, possibly because of alterations in muscle contractile properties, connective tissue, or architectural components. Potentially, the greater decline in muscle physiology of lower body muscle groups may help explain the blunted response to resistance training compared to upper body muscle groups that we observed in postmenopausal women. Therefore, increasing the duration (i.e. > 10 weeks) and intensity of resistance training (i.e. > 30% 1RM) may be required to produce significant muscle hypertrophy in lower body muscle groups in postmenopausal women.

In addition to our pulse supplementation strategy, there were several other possible limitations which may have influenced our outcome measures. First, to achieve 80% statistical power, 17 postmenopausal women were required. Unfortunately, 12 completed the study. This reduction in statistical power could have resulted in Type II error. Second, we chose to use a 10 week resistance-training program as Mitchell et al. (10) found a significant increase in muscle hypertrophy and strength when 3 sets of exercise were performed at 30% 1-RM to volitional fatigue for 10 weeks in young males. Results of the present study indicated that 10 weeks of high-volume resistance training increased muscle strength of the elbow flexors, elbow extensors and knee extensors and muscle size of the elbow flexors and elbow extensors but had no effect on knee flexor strength or muscle size of the knee flexors or knee extensors. Therefore, a longer training period (> 10 weeks) may have been needed to increase muscle size and strength in all muscle groups assessed. Third, our unilateral training design potentially involved cross-education, where the training of one muscle group can lead to neurally mediated strength gains in the untrained contralateral muscle group (32). However, since both sides of the body performed 20 training sessions over 10 weeks using the same exercises, and the side of the body was randomly allocated, the potential inter-limb effects of cross-education should be minimal. Furthermore, recent data using a unilateral design found no correlation between strength gains in the left vs. right leg (r=0.33) when performing the same amount of resistance training at different training intensities (i.e. 30% vs. 80% 1-RM to volitional fatigue) (10). Fourth, all postmenopausal women consumed whey protein which limits our ability to make conclusions about the independent effects of whey protein on muscle mass and strength. For example, Yang et al. (15) showed that whey protein (40 grams) immediately following an acute bout of unilateral leg resistance training increased the rates of muscle protein synthesis in the non-exercised leg in older adults. Therefore, it is possible that whey protein during training of one side of the body influenced the rates of muscle protein synthesis in the opposite side (placebo) of the body in our postmenopausal women. Furthermore, the muscle protein synthetic response to resistance training is elevated for 24 hours post-exercise when dietary protein is consumed in young adults (33). Potentially, muscle groups trained on the placebo days may be influenced by whey protein consumption on opposite training days. However, it is unknown whether this 24 hour anabolic window to dietary protein occurs in postmenopausal women. Fifth, habitual dietary protein intake may have masked the effects of whey protein supplementation. Postmenopausal women were consuming, on average, 78 grams of protein per day or approximately 0.95/kg/day of dietary protein during this study. The current recommended dietary allowance (RDA) for adults > 50 years of age is 0.8g/kg/day (2). Potentially, women in the current study may have been consuming sufficient protein, independent of the whey protein supplement, which could have influenced our results. Sixth, methodological issues may have decreased our ability to detect small changes in lean tissue mass over the 10 weeks of training. DXA measures lean tissue mass in the entire upper and lower limb. Exercises performed in the resistance training program primarily targeted the muscles proximal to the elbow and knee. Muscle groups distal to the elbow and knee would be minimally recruited and therefore could have diluted any small increase in muscle growth of the proximal muscle groups. Finally, no measure of muscle protein kinetics was made in this study.

In conclusion, whey protein supplementation during low intensity, high-volume resistance training did not augment the training induced increases in muscle mass or strength in postmenopausal women. Low intensity, high-volume resistance training appears to be a safe and effective intervention to improve some measures of muscle size and strength in postmenopausal women, particularly for the upper limbs. Future research should examine the safety and effectiveness of long-term, low intensity, high-volume resistance training on aging muscle and bone biology and tasks of functionality in males and females.

Acknowledgements: The authors would like to thank Dr. John Barden and Dr. Paul Bruno for their feedback on the study design. The study was supported by the University of Regina.