A calf ergometer for exercise testing and training of patients with peripheral arterial disease and calf claudication.

INTRODUCTION

Many patients with atherosclerotic peripheral arterial occlusive disease (PAD) have pain in leg muscles when they walk due to the inadequate blood and oxygen delivery to exercising leg muscles (10). This painful intermittent claudication during walking occurs most often in the calf (triceps surae) muscles (20). The standard-of-care conservative treatment for claudication in ambulatory PAD patients is progressive walking exercise training (10); other effective exercise modes for vascular rehabilitation include leg or arm cycle ergometry (22), pole-striding (13), stair-climbing (11), and resistance training (9). However, there is no standard for less ambulatory PAD patients with severe comorbidities with potential to ambulate. Gardner et al. (6)reported that, out of 905 veteran PAD patients evaluated for exercise research, 190 (20%) were excluded due to exercise intolerance from factors other than claudication, e.g., severe coronary artery disease, dyspnea, poorly controlled hypertension, cancer, or renal/liver disease. Other conditions will also impair walking such as cardiomyopathy, pain, unilateral lower-extremity amputation, stroke/hemiparesis, and severe deconditioning. Walking generally increases metabolism to 2-4 METs--beyond the safe limits for many of these patients. Therefore, substantial numbers of marginally ambulatory PAD patients will neither be candidates for walking training, nor cycle ergometry, pole-striding, stair-climbing, or resistance training. They, however, might benefit from a more tolerable alternative mode such as isolated calf exercise training.

Single-leg calf training has potential advantages over walking which include direct targeting of the claudicating calf muscles, low skill requirements, and low cardiopulmonary and metabolic stress making it more tolerable for patients with severe walking exercise intolerance. Calf training may also be used as a preparatory activity for functional walking. Disadvantages may include the lack of central cardiopulmonary training effects and functional dissimilarity to walking.

General calf exercise can be performed on existing weight training equipment such as leg presses and seated calf exercise machines, as well as seated heel raise weight placed on the thighs/lap and standing heel raises with full or partial bodyweight. However, tight controls over many exercise parameters such as comfortable body support, isolation of the triceps surae muscle group, controlled range of motion, and measurement of work and power are lacking. An isokinetic dynamometer (e.g., KinCom, Biodex, Cybex, Lido, BTE, etc.) could also be used, but the instrumentation is expensive, large, and nonportable, and is impractical for use with training groups of users. A variety of pedal ergometers have been developed for research testing of calf musculature (5,18), but each is a unique and expensive experimental instrument. Wang et al. (21) interfaced a hinged footplate with a leg cycle ergometer with steel rods, but load application is imprecise. The Stresst'er[R] (Stu-ert Medical Devices, Ltd., Frinton-on-Sea, Essex, UK) (2) is a clinical calf ergometer developed and marketed for diagnostic testing of PAD patients, but is no longer commercially available, hence the need for developing a practical clinical calf ergometer.

Calf ergometry has been utilized in physiological research for many years (3). More recently, calf ergometry has been used to evaluate hemodynamic, metabolic, and functional responses of calf muscles in PAD patients (1,2,4,12,14-17).

Calf exercise has only been recently used for exercise training of PAD patients. Wang et al. (21) trained 14 PAD subjects with individual-leg plantar flexion exercise (4 bouts of 4-minute intervals at 80% of maximal work rate three times per week for eight weeks). Compared to a nontraining control group, the training group significantly increased plantar flexion peak oxygen uptake (VO2peak) and power output by 24% and 44%, respectively, and treadmill VO2peak significantly increased 12%. Eleven of 14 patients no longer reported leg pain limitations during treadmill testing at VO2peak. Also, Helgerud et al. (8) reported that prior calf exercise training in PAD patients facilitated subsequent treadmill training effects compared to treadmill training alone, suggesting that calf muscle training primes PAD patients for cardiovascular adaptations when applying whole body exercise.

Our research involving exercise testing and training of PAD patients required a calf ergometer and since none was commercially available, we developed our own. The purpose of this manuscript is to describe our calf ergometer that has both research and clinical/therapeutic applications.

DEVELOPMENT

Our clinical research team wanted the calf ergometer to have the following capabilities:

1. to allow unilateral plantar and dorsiflexion motions only in order to isolate the triceps surae muscles, i.e., to stress the primary muscle group affected by intermittent claudication in PAD patients

2. to allow calculation of power output in small increments up to about 25 watts (W), in the range of expected exercise performances of patients with PAD

3. to be mechanical only, i.e., non-electronic or computer-interfaced for simplicity and ease of repair

4. to be independent of the user's foot size, i.e., for ease of use, so mechanical adjustments were not necessary between users

5. to be independent of the user's body mass, i.e., non-weight-bearing, so users of varying body mass can perform the same exercise tasks at equivalent levels of work and power

6. to be portable for easy moving and storage, i.e., to be practical in clinics or laboratories with limited space

7. to be adaptable and comfortable for users with special needs, i.e., to accommodate PAD patients with different comorbidities and abilities

8. to be durable for repetitive exercise training, i.e., to withstand frequent and extended uses with minimum maintenance during exercise testing and training for clinical and research purposes.

After consultation with a machinist at our medical center, all of the above design criteria were met and the calf ergometer was designed and constructed. (See Figures 1-3.) We decided to mimic some of the physical characteristics of the Stresst'er[R], but we employed a simple pulley and weight system instead of a mechanical spring resistance. Like the Stresst'er[R], our ergometer attaches to the foot of a standard clinical treatment table/plinth (Motorized Hi-Lo Treatment Table, Model TG2002, Tri W-G Group, Valley City, ND), so the ergometer is operated with the user in semi-recumbent position. A bolt protruding from the underside of the treatment table fits into a hole in the end of the ergometer frame that fits under table (see "Bolt" in Figure 1), preventing the ergometer from pushing away from the table during exercise. The user's legs and hips are supported by the padded treatment table. A large foam wedge (UCS, Lincolnton, NC) under the user's back and head elevates the upper body 18[degrees] above horizontal, allowing the user to see and control the exercise more easily. A small pad (folded towel) between the table and the hamstrings flexes the knee 5[degrees].

The ergometer consists of a rectangular Teflon[R]-coated aluminum foot pedal (10.0 x 33.5 cm). An assistant secures the user's foot (wearing a slipper with rubberized sole) onto a foot pedal with a two-inch-wide Velcro[TM] strap. A curved heel-rest and the friction between the sole and the footrest support the distal portion of the leg. The pedal's axis of rotation is aligned with the ankle joint of the typical adult user (near the malleoli) (6 cm proximal to the sole of the foot, 7 cm anterior to the posterior surface of the heel). The distance from the axis of rotation to the distal end of the foot pedal, and hence, the radius of the inscribed circle is 26.4 cm. The distal tip of the foot pedal attaches to a plastic-coated steel cable, so that ankle plantar flexion causes the steel cable to pull through a series of four low-friction pulley wheels (3.5 cm diameter, with sealed bearings). The cable lifts an aluminum cage, similar to an elevator, vertically a maximum of 8.0 cm. Ten 2-kg coated lead weights (2.2 x 6.2 x 15.2 cm) can be loaded into the cage that provides the resistance to active plantar flexion. Although the distance traveled by the distal end of the foot petal is 16.0 cm, the simple pulley configuration constrains the vertical displacement of the cage to 8 cm, decreasing the necessary height of the ergometer. The cage does not swing freely, but is guided in its vertical motion by four steel posts (0.5 cm thick x 44.5 cm tall) sliding through low-friction Teflon[R]-coated bushings attached to the frame.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Concentric contraction of the triceps surae muscles causes plantar flexion of the ankle from neutral position (0[degrees] plantar flexion) to 30[degrees] plantar flexion, limited by a fixed mechanical stop under the pedal. At that moment, a low-voltage LED light mounted on the ergometer flashes, visually alerting the user that he has reached the end of concentric plantar flexion that raised the resistive load. (The light is powered by a 9-V battery mounted on the frame and is activated by a contact switch near the mechanical stop.) The user then eccentrically plantar flexes and lowers the load to the base. Loads can vary from 2 to 20 kg, producing 2.7-45.2 W power output. An external electronic quartz metronome (Model SQ50, Seiko S-Yard Co., Ltd, Japan) provides additional audiovisual cues to assist the user in maintaining a chosen cadence, usually 60-100 bpm.

Patients are instructed to move only at the ankle joint and to avoid substitution of other muscles for the triceps surae muscles. For example, the pelvis can be laterally tilted to move the extended leg more distally. This can be accomplished by hiking the contralateral hip by contracting the quadratus lumborum muscle. Also, if the patient flexes the knee, plantar flexes the ankle against no resistance, and then extends the knee, the pedal can be pressed without active plantar flexion. These substitutions are more likely during strenuous calf exercise, and can be reduced through proper instruction and stabilization with a strap across the knee of the exercising leg.

The overall dimensions of the calf ergometer are 63.5 cm wide x 94.0 deep x 101.0 cm tall. The total mass is 18.68 kg (unloaded). The frame is constructed of 5.7-[cm.sup.2] aluminum fractional T-slotted framing material with four support legs arranged in a pyramidal pattern at the base with locking casters. Triangular connectors at joints are held by T-slot bolts. During use, the frame is rigid. Plastic end caps seal the ends of the aluminum frame. Height is adjustable for connection with treatment tables of differing heights. Our treatment table has top surface dimensions of 35.0 cm tall x 45.7 cm wide x 203.2 cm long.

EVALUATION AND VALIDATION

Work and power output calculations are straightforward (19), as users perform no extraneous work to support themselves, lifting a leg, or any other action that would add to the work done. The work is therefore evaluated by the standard formula of force x distance. As the pedal movement is rotational, the effective force is inversely proportional to the radius at which it is applied, but, as the distance traveled at the pressure point is proportional to the same radius, the radius cancels out in the formula, i.e., the work and power output are independent of the size of the foot.

The force varies as the pedal is moved, but this variation is substantially linear as it is derived from the distal end of the foot pedal pulling the cable nearly linearly over a short distance (18.6 cm). When the load is 2 kg, the average moment is 2 kg x 9.80665 m x [s.sup.-2] x 0.264 m = 5.17791 Nm. The force at a given radius r is therefore 5.17791 x [r.sup.-1]. The travel (i.e., distance or excursion) at the same radius r is 2[pi]r x [theta] x [360.sup.-1] where [theta] is the angle of rotation in degrees. For 30[degrees] as specified here, the travel is 0.5236r. The work done for this load and travel is 5.17791 x [r.sup.-1] x 0.52360r = 2.71115 J x [stroke.sup.-1]. At a cadence of 60 bpm, each stroke is accomplished in one second, so 2.71115 J x [stroke.sup.-1] = 2.71115 J x [s.sup.-1] = 2.71115 W power output. (See Table below.) Power output is proportional to the cadence; therefore, for cadences other than 60 bpm, multiplying the power output at 60 bpm in W by the cadence in bpm x 60 [bpm.sup.-1] gives the correct power output out for that cadence. The simplest form of the formula for power output in W at varying loads and cadences is as follows: (load in kg) x (cadence in bpm x 60 [bpm.sup.-1]) x 1.35558 m.

CLINICAL APPLICATION

Calf Exercise Testing. We use a progressive load-incremented protocol to test muscular endurance of the triceps surae muscles of PAD patients. We find this protocol more likely to elicit the desired symptom-limited end-point of the test (maximum tolerable claudication) than a constant-load protocol in which the optimal load to induce maximal claudication within a reasonable time limit of 10-20 minutes is unknown. Our testing protocol involves a constant cadence of 60 bpm with resistive load increasing by 2 kg every 2 minutes from 2 to 20 kg (2.7 to 27.1 W), or until claudication forces the patient to stop. After each 2-minute stage, additional weights are added manually to the stack in the cage. The patient is requested to exercise as long as possible during this protocol while claudication intensity is expected to increase from 0 (absent) to 1 (onset), 2 (moderate) to 3 (intense) to 4 (maximal) (7). The test is usually symptom-limited with the endpoint being a pain rating of 4 as subjectively judged by the patient. Twelve-lead ECG waveforms, rhythm, heart rate, and all symptoms are monitored continuously under direct medical supervision. Brachial blood pressure is recorded every two minutes. If both legs are symptomatic, left and right legs are tested separately. Cardiac precautions should be strictly followed, as the elderly population of patients with PAD usually have multiple risk factors for coronary artery disease and stroke, frequently including hypertension, dyslipidemia, and diabetes. After termination of exercise, a recovery period of 15 minutes is provided for claudication symptoms to dissipate and cardiovascular responses to return to pretest levels.

Calf Exercise Training. The calf ergometer lends itself to progressive resistance exercise training with whatever protocol desired by the clinician or researcher. Average work and power output per bout, session, or time period can be tracked quantitatively to assess progress. For interval training, exercise bout and recovery durations may be recorded and tracked over time. During a lengthy exercise bout, the weight stack can be easily secured with a Velcro[TM] strap.

Subsequent publications from the authors will present results from exercise and training with this calf ergometer. The ergometer described in this manuscript is not commercially available. It was designed and constructed with resources including research grant funding at a large VA medical center with substantial technical expertise on station. Clinicians or researchers could build a similar one at a cost of approximately $200, depending on the materials and features required. Other exercise equipment, as previously described, could also be adapted for these purposes.

ACKNOWLEDGEMENT

This project was supported by grant # B3644P from the VA Rehabilitation Research and Development Service.

REFERENCES

(1.) Amirhamzeh, M. M., H. J. Chant, J. L. Rees, L. J. Hands, R. J. Powell , and W. B. Campbell. A comparative study of treadmill tests and heel raising exercise for peripheral arterial disease. Eur J Vasc Endovasc Surg 13:301-305, 1997.

(2.) Cameron, A. E. P., A. Porter, S. Rosser, A. E. C. F. DaSilva, and L. M. DeCossart. The Stresst'er ergometer as an alternative to treadmill testing in patients with claudication. Eur J Vasc Endovasc Surg 14:433-438, 1997.

(3.) Cathcart, E. P., G. M. Wishart, and J. McCall. An ergometer adaptable for either hand- or foot-movements. J Physiol 58:92-97, 1923.

(4.) Cheetham, D.R., M. Ellis, A. H. Davies, and R. M. Greenhaigh. Is the Stresst'er a reliable stress test to detect mild to moderate peripheral arterial disease? Eur J Vasc Endovasc Surg 27:545-548, 2004.

(5.) Francescato, M. P., and V. Cettolo. Two-pedal ergometer for in vivo MRS studies of human calf muscles. Magn Reson Med 46:1000-1005, 2001.

(6.) Gardner, A. W., L. A. Killewich, P. S. Montgomery, and L. I. Katzel. Response to exercise rehabilitation in smoking and nonsmoking patients with intermittent claudication. J Vasc Surg 39:531-538, 2004.

(7.) Gardner, A. W. Peripheral arterial disease. In: ACSM's Exercise Management for Persons with Chronic Diseases and Disabilities. J. L. Durstine (Ed.) Champaign, IL: Human Kinetics, 1997, pp. 64-68.

(8.) Helgerud, J., E. Wang, M. P. Mosti, O. N. Wiggen, and J. Hoff. Plantar flexion training primes peripheral arterial disease patients for improvements in cardiac function. Eur J Appl Physiol. DOI 10.1007/s00421-009-1011-z, published online 24 Feb 2009.

(9.) Hiatt, W. R., E. E. Wolfel, R. H. Meier, and J. G. Regensteiner. Superiority of treadmill walking exercise versus strength training for patients with peripheral arterial disease. Implications for the mechanism of the training response. Circulation 90:1866-1874, 1994.

(10.) Hirsch, A. T., Z. J. Haskal, N. R. Hertzer, C. W. Bakal, M. A. Creager, J. L. Halperin, et al. ACC/AHA guidelines for the management of patients with peripheral arterial disease (lower extremity, renal, mesenteric, and abdominal aortic): a collaborative report from the American Associations for Vascular Surgery/Society for Vascular Surgery, Society for Cardiovascular Angiography and Interventions, Society for Vascular Medicine and Biology, Society of Interventional Radiology, and the ACC/AHA Task Force on Practice Guidelines--summary of recommendations. J Vasc Interv Radiol 17:1383-1398, 2006.

(11.) Jones, P. P., J. S. Skinner, L. K. Smith, F. M. John, and C. X. Bryant. Functional improvements following stairmaster vs treadmill exercise training for patients with intermittent claudication. J Cardiopulm Rehabil 16:47-55, 1996.

(12.) Laing, S. P., and R. M. Greenhalgh. Standard exercise test to assess peripheral arterial disease. Br J Med 280:13-16, 1980.

(13.) Langbein, W. E., E. G. Collins, C. Orebaugh, C. Maloney, K. J. Williams, F. N. Littooy, et al. Increasing exercise tolerance of persons limited by claudication pain using polestriding. J Vasc Surg 35:887-893, 2002.

(14.) Manning, B. J., G. McGreal, H .Crowley, H. P. Redmond, and J. A. O'Donnell. A prospective comparison of pedal ergometry with conventional treadmill testing in the investigation of lower extremity pain. Ir J Med Sc. 170:169-171, 2001.

(15.) McCully, K. K., C. Halber, and J. D. Posner. Exercise-induced changes in oxygen saturation in the calf muscles of elderly subjects with peripheral vascular disease. J Gerontol 49:B128-134, 1994.

(16.) McPhail, I. R., P. C. Spittell, S. A. Weston, and K. R. Bailey. Intermittent claudication: an objective office-based assessment. J Am Coll Cardiol 37:1381-1385, 2001.

(17.) Mohler 3rd, E. R., G. Lech, G. E. Supple, H. Wang, and B. Chance. Impaired exercise-induced blood volume in type 2 diabetes with or without peripheral arterial disease measured by continuous-wave near-infrared spectroscopy. Diabetes Care 29:1856-1859, 2006.

(18.) Quistorff, B., S. Nielsen, C. Thomsen, K. E. Jensen, and O. Henriksen. A simple calf ergometer for use in a whole body MR scanner. Magn Res Med 13:444-449, 1990.

(19.) Stu-ert Medical Devices, Ltd. Distributor information: The Stresst'er[R]. No date (c1997).

(20.) Wang, J. C., M. H. Criqui, J. O. Denenberg, M. M. McDermott, B. A. Golomb, and A. A. Fronek. Exertional leg pain in patients with and without peripheral; arterial disease. Circulation 112:3501-3508, 2005.

(21.) Wang, E., J. Hoff, H. Loe, N. Kaehler, and J. Helgerud. Plantar flexion: an effective training for peripheral arterial disease. Eur J Appl Physiol 104:749-756, 2008.

(22.) Zwierska, I., R. D. Walker, S. A. Choksy, J. S. Male, A. G. Pockley, and J. M. Saxton. Uppervs lower-limb aerobic exercise rehabilitation in patients with symptomatic peripheral arterial disease: a randomized controlled trial. J Vasc Surg 42:1122-1130, 2005.

Stephen F. Figoni, PhD, RKT, [1] William K. Atkinson, [2] Charles F. Kunkel, MD, MS, [1,3] A. M. Erika Scremin, MD, [1,3] Oscar U. Scremin, MD, PhD [1,3,4]

[1] Physical Medicine and Rehabilitation Service, VA Greater Los Angeles Healthcare System (VAGLAHS), West Los Angeles Healthcare Center (WLAHC), Los Angeles, CA; [2] Nuclear Medicine Service; Research and Development Service, VAGLAHS; [3] Physical Medicine and Rehabilitation Division, David Geffen School of Medicine, UCLA; [4] Department of Physiology, David Geffen School of Medicine, UCLA

AUTHOR CORRESPONDENCE:

Stephen F. Figoni, PhD, RKT

PM&RS (117), VAWLAHC

11301 Wilshire Blvd.

Los Angeles, CA 90073

Phone: [phone]

FAX: [phone]

E-mail: [e-mail]

Table 1. Resistive loads and resultant power outputs
for cadences of 60, 70, 80, 90, and 100 bpm.

                          Power Output (W)
Load
(kg)    60 bpm   70 bpm   80 bpm    90 bpm   100 bpm

2         2.7      3.2      3.6       4.1      4.5
4         5.4      6.3      7.2       8.1      9.0
6         8.1      9.5     10.8      12.2     13.6
8        10.8     12.7     14.5      16.3     18.1
10       13.6     15.8     18.1      20.3     22.6
12       16.3     19.0     21.7      24.4     27.1
14       19.0     22.1     25.3      28.5     31.6
16       21.7     25.3     28.9      32.5     36.1
18       24.4     28.5     32.5      36.6     40.7
20       27.1     31.6     36.1      40.7     45.2